CHAPTER V MISCELLANEOUS ROCKS

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There are a few rocks which do not fit into any of the three groups described, such as concretions, geodes, meteorites, etc., and they are gathered together here. There is also one type of rock, which really belongs among the minerals, but is likely not to be so recognized at first glance, and that is the material filling veins. These last are sometimes designated “vein rocks,” but are really massive deposits of one, two or more minerals, and should be referred to the minerals when found.

Concretions

In the sedimentary rocks there frequently occur inclusions of a nature different from the surrounding rock. In shape they are usually rounded, nodular, spherical, discoidal, ovate, flattened, elongated or ring-shaped, or combinations of the foregoing, making often curious and fantastic forms. In size they range from a fraction of an inch in diameter to several feet through. When broken, they may show a nucleus, around which more or less concentric layers have formed, or neither nucleus nor concentric structure may be visible. The layered structure of the surrounding rock in some cases continues right through the nodular mass. These structures are called concretions, and their formation in all cases is at least due to similar reactions.

In general the concretions differ from the surrounding rock in composition, but are usually composed of some one of its impurities, of lime in the clays or silica in limestones, of iron oxide in sandstone, etc. They seem to have originated as a result of the solution of the minor mineral, and then its redeposition around some center or nucleus. In many cases the nucleus is organic, such as a leaf, a shell, a bone, etc., so that when the concretion is split, in its center will be found the perfect imprint of the leaf, or the shell of a mollusk, or a bone of a higher animal, sometimes a whole skeleton. Again the nucleus may be inorganic like a grain of sand; and in still other cases no nucleus can be found, though there was probably one in the beginning. What has happened is somewhat like the case of accessory minerals in igneous and metamorphic rocks. A layer of sediment was laid down, including in it, here and there, something foreign to the run of the rock. Later when the water leaches through this rock, impregnated with lime for instance, it comes to the point where a leaf is decomposing. The products of the leaf decomposition are different from what is already present in solution, and may precipitate some of the lime in that neighborhood. As long as leaf decomposition continues the precipitation in that region will continue and increase the size of the concretion. This sort of action accounts for many of the concretions, especially those about organic remains. In some other cases where there is no nucleus, as the flint in chalk, what has taken place is that the small amounts of silica in the lime have been dissolved, and then around some center has constantly been added more and more non-crystalline silica until a mass of flint has accumulated. There may be a considerable variety of ways to account for different concretions, but in all cases solutions of one mineral have come in contact with solutions of a different kind, and precipitation about a center has resulted.

Clay stones
Pl. 68

Of all the concretions these are perhaps the commonest, being found in the clays of all types and in many regions. They are made of lime and precipitated around some nucleus of foreign matter. The shapes vary widely, usually discs, flattened ovals or even rings, in most all cases however flattened. This is indicative of the water moving though the clay more freely in some layers than others. Often clay stones occur so abundantly that two or more have grown together making fantastic shapes, sometimes resembling animals, and all sorts of fancied but unrelated objects. As the clay stones have grown the clay has not been pushed aside, but has been incorporated within the concretion; so that when a concretion is dissolved in acid, it yields not only the lime, which is its reason for being, but also a large amount of clay.

Claystones are found in clays most anywhere, usually occurring in certain layers and being absent from others.

Lime concretions

These are found mostly in shales which carry a high percentage of clay as impurities, and are characteristic of the older geological formations, especially ancient sea bottoms. They are likely to have as a nucleus some shell, fish bone, or a leaf, which when the concretion is split, reveals a wonderfully preserved portion of an animal or a plant, which was buried millions of years ago. The lime concretion is closely related to the claystone, and is really a claystone which has been buried so long that the surrounding matrix has changed to a shale instead of remaining clay.

One of the most famous localities for these lime concretions is Mazon Creek, Illinois, where thousands of these concretions have been picked up and split to study the organic remains included. The commonest objects found are fern leaves, like the one on Plate 68. But about once in a thousand times they inclose a spider or insect, and once in ten thousand times the skeleton of an amphibian, which is of especial interest, as here have been thus found the remains of the very earliest of the land animals. These remains were inclosed in these concretions during the coal age, probably 50,000,000 years ago, and once inclosed all the hard parts have been as well preserved after that long interval, as they were immediately after being inclosed in the concretion. Lime concretions range from less than an inch in diameter to several feet through. They are not confined to shales, but sometimes occur in sandstones, in this case also usually having as a nucleus either a shell, or the bone, or bones, of some animal.

They are likely to be found anywhere in the limestone belt, from the Appalachian Mountains to the Rocky Mountains, or in the Great Basin, or on the Pacific Coast. Often they have been mistaken for turtles and other objects. A good many of the cases where the head or body of animals “petrified with all the flesh” are reported, it is one of these concretions which has a shape sufficiently like the part described, for the imagination to construct the rest.

Septeria
Pl. 69

Septeria are lime concretions, which, after they had formed, have shrunk and developed a series of cracks running through them in all sorts of directions, and since then the cracks have been filled with various minerals, such as calcite, dolomite, and siderite. These make a series of veins which intersect the concretion, in a sort of network. Septeria are mostly of considerable size, ranging from six inches in diameter to several feet through. They are characteristic of the shales of ancient sea bottoms, especially those of Devonian age in New York, and Pennsylvania, and those of Cretaceous age in Wyoming, Montana and the Dakotas.

Flint concretions

The silica in limestones is often segregated into nodular masses of varying sizes, to make concretions of flint. Such masses have grown in the limestone, and, while growing, have either pushed away, or dissolved the adjacent limestone, so that the flint nodule is pure silica. They are especially characteristic of the chalk beds, and of ancient limestones which formed on the floor of the sea, like the Helderberg Limestone of New York, Pennsylvania, Ohio, etc. When thin sections are cut through these flints, and examined under the microscope, many remnants of the shells of plants and animals are still recognizable. A nucleus is seldom found, but in some cases there is a fossil in the nodule about which the concretion doubtless formed. The spicules of sponges, shells of diatoms, and of radiolarians seem to have contributed most of the material from which flint concretions are formed. In addition to the silica there are frequently inclosed in these nodules the horny jaws of various sea worms, and a host of spiny balls the relationships of which are still unknown.

Sandstone concretions

There are two types of sandstone concretions, first those which are cemented with lime, and second those cemented with iron oxide. The concretions bound by lime are especially characteristic of sandstones which were laid down as river deposits, either in the channels or on the flood plains, and also the sandy deposits resulting from wind deposition. In these cases the concretions will mostly be found to have formed around some organic nucleus, most frequently about a bone, or group of bones, of some ancient animal. In this country they are mostly found in the arid and semiarid sections of the West, where the present day wind erosion exposes the harder parts of bluffs, etc.

The second type of sandstone concretion is the one in which the cement is most often limonite, less often hematite. These concretions are less dense than the lime ones, and in some cases the limonite is only precipitated at a distance from the nucleus, which has resulted in the formation of a hollow shell, filled with loose sand. This is especially characteristic of certain concretions, found in a gravel or coarse sand in the region of Middletown, Del.

Oolites

In large bodies of water like the sea and some larger lakes we find concretions which have formed, or are still forming, about tiny grains of sand, which are still being moved about by the waves and currents. In such cases not only are great masses of concretions formed but they have very clearly marked the concentric layering, which shows that they have increased in size, sometimes more rapidly and sometimes more slowly. Where great masses of such concretions have formed the resulting rock appears like a great mass of small eggs, whence the term oolite. The cement may be any one of several substances, but lime, silica, and hematite are perhaps the most common. Here and there are found larger or smaller masses of this oolite. In some cases it would appear that the material was precipitated by the action of bacteria. Such for instance is probably the origin of the Clinton iron ore, a bed of oolitic hematite, extending from New York State all down the Appalachian Mountains to Alabama.

Pisolite
Pl. 69

When the concretions, formed in exactly the same manner as in the case of oolite, are of a size bigger than a pea, then the rock is known as pisolite.

Other Concretions

Though less abundant concretion may form from still other substances. Hematite has been mentioned, and when concretions are made of this material, either they have been deposited by bacteria, or were formed as limonite and the water of crystallization of this latter mineral driven off.

Manganese concretions are found on the floor of the ocean at maximum depths, and brought to the surface by dredging.

Geodes

Geodes are nodules, which, when broken open, are found to be hollow and the cavity lined with one or more minerals. They represent a special case of minerals in a cave. There was in the first place a cavity in the surrounding rock, usually of sand or clay. As the water leached through the surrounding rock, it became saturated with one or more minerals and then coming into the cavity, deposited the minerals, either as crystals, or as a non-crystalline mass, lining the cavity. Thus the inside is often a beautiful cluster of bristling crystals, or it may be simply layer on layer of chalcedony of any color. Before this process had gone so far as to completely fill the cavity, erosion had dislodged the mass, and it has been found. One usually recognizes that it is a geode by the fact that it is far too light to be a solid rock, and then it may be carefully broken. They are characteristic of certain formations; so that having accidentally broken the first one, others can be carefully opened to display the beauty of the interior. The geode illustrated on Plate 70 is lined with quartz crystals, but near by were found many others, some of which had chalcedony and some jasper as a lining. Such crystallined nodules are usually called geodes so long as they occur in a softer matrix so that they are easily dislodged, and until they reach a size of three or four feet in diameter.

Pebbles

When picked up either from brook beds, sea beaches, or the open plain, there are few forms of rock which tell a story of the past more completely than do pebbles; and any one, who enjoys reading a story written in form, structure and composition, will find in pebbles one of the most satisfying and at the same time testing exercises. The story may be complex or simple according to what has happened to the parent rock, and to that is added what happened since the pebble left the ledge where it was a part of a great mass. One must not forget to take into consideration where the pebble was found and the character of its associates. This sort of exercise is recommended to all interested in rocks. It will yield something upon first trying, and more on prolonged study; and the fullness with which it is done will test one’s knowledge of the meaning of rocks as nothing else will do. As a sample of this sort of exercise let us take the two pebbles illustrated on Plate 71.

The upper one is a common quartz pebble picked up in a New England brook bed. Such pebbles are common all over the country formerly covered by the glacial ice sheet. It is crystalline quartz, but the individual crystals are not distinguishable, and such quartz is typical as the filling of veins. It therefore goes back to a time when the rocks were fissured, probably in connection with the folding accompanying mountain making far to the north in Canada. Into the fissures thus formed seeped the water which had been leaching through the adjacent rocks, and it was saturated with silica which it had dissolved from those rocks. In the open fissure the quartz was deposited as crystals, which grew finally filling the fissure and crowding each other so that all the faces were obliterated. The quartz vein was complete, but it must have been far below the surface of the ground. Time must have passed, thousands of years of it, until, in the weathering away of the mountain system, the many feet of overlying rock were removed and this vein was brought to the surface. As the quartz is harder than the adjacent rocks, the vein soon projected as a ledge. The effect of changes of temperature in alternately expanding and contracting the rocks developed cracks, into which water worked its way, and then the breaking was hastened by the expansion which takes place when water freezes, and in exposed regions is so effective, because the freezing and thawing are so often repeated. Finally an angular fragment of quartz was dislodged and lay on the surface, resistant to the solvent power of the rain. In this case this happened just before the advance of the great ice sheet. When that came to the place where the fragment lay, it was picked up along with all other loose material and partly shoved in front of, but probably mostly carried frozen in the ice, and journeyed one, two, three hundred, perhaps a thousand miles. This took many years for the ice moved only a few feet a day. Finally however it came to the point where the ice melted as fast as it advanced, and our quartz fragment was dropped at the front of the ice sheet along with other great masses of till. Here there was abundant water, partly from the melting of the ice, and partly from the storms which must develop where there are such contrasts in temperature, as there would be over the ice, on one hand, and over the bare land in front of the ice on the other hand. A torrent picked up our fragment and started it on a second journey, banging against other stones as it rolled along down the stream bed, every time it struck another stone bruising the corners which soon became rounded. Thus from time to time during high water the quartz fragment, becoming rounder every time it moved, journeyed down stream, until it came to the point where the stream emptied into a lake. Here the current was checked and the stone dropped to the bottom along with other larger stones to make the delta at the mouth of the stream. There it lay as long as the lake existed, and would be lying now, but that in New England a tilting movement of the land tipped the north end of the lake up and the water all ran out. Then the stream began to flow over its own delta and in time of freshet tore a channel down through the old delta carrying the pebble still further down, until it came to the level stretch which represented the old lake’s bottom and there it dropped the pebble in its bed. And there it was found and picked up to become the pebble which told the above story of its life, and to repeat it as often as anyone will look at it with a seeing eye.

The second pebble is quite a different one. It was picked up in a gravel bank along a railroad cut, just at the foot of Mt. Toby in Massachussetts, and the writer has used it many times to test his students, to see if they could read the story which it tells.

It consists of two sorts of rock, the one, angular fragments of a hornblende schist, the other, a fine-grained granite filling all the spaces between the fragments of schist, even in cracks less than a quarter of an inch wide. The schist is the older rock and in its first appearance represents a deposit of mud (clay and sand) on the floor of the ocean, well out from the shore, and somewhere off to the east of Mt. Toby, perhaps ten miles, perhaps more, from the place where it was found. This was back in early PalÆozoic times, millions of years ago.

This deposit was buried by further layers of sediment on the sea bottom and cemented into a shale. Then during a mountain making period the region was folded, and the sediments were altered by the combined pressure and heat, our layer of rock becoming a hornblende schist. After that happened considerable time must have passed, but just how much is not indicated by the pebble, before another period of disturbance took place, during which this deep seated schist was faulted, and shattered to fragments along the line of breaking. This accounts for the angular fragments. Then into the fissure thus formed was pressed a molten magma, which while liquid enough to flow and be squeezed into every opening could not have been very hot; for not even the corners of the schist fragments are melted or altered, so as to appear any different from the mass of the schist. The molten magma cooled rather slowly, making a fine-grained granite. This must all have taken place far below the surface, or the magma would have cooled into a felsite or dense lava.

Again a long time must have elapsed, while the rock overlying our piece was eroded away, so it could come to the surface. Just about the time it did come to the surface, the Connecticut Valley was formed by a great block, 95 miles long by fifteen to twenty miles wide, dropping down six or eight thousand feet (probably not all at once but by one or two hundred feet at a time) between two north and south faults. This took place in the Triassic Period. Of course the streams then began to wash sand and stones of all sizes into the hole. Our pebble was one of these. While still an angular fragment, lying perhaps ten miles east of the Connecticut Valley, a stream started it moving, and as it rolled along the brook bed, it was battered and rounded to its present shape, and finally tumbled over a waterfall to the bottom of the great hole, which had been formed as described above. Here with other stones it formed part of a coarse gravel, coarsest near the sides of the hole, and finer toward the middle; for the material was further distributed in the bottom of the valley. Our stone stayed pretty near the side and was soon buried beneath hundreds of feet of similar material. The leaching water dissolved enough iron rust so that this acted on the lower layers as a cement and bound the whole mass into a conglomerate.

Here for some millions of years our pebble rested, while above it was piled sand and gravel and a couple of sheets of lava, until the hole was filled, and our pebble was near the bottom of the mass. Later movements of the land raised the whole region, fully six thousand feet, and erosion went on for other millions of years. The conglomerate and sandstone wore away faster than the metamorphosed rocks on either side of the filled valley, so that a new valley, the present Connecticut Valley, came into existence.

When our pebble finally came near to the surface on the side of Mt. Toby (a mound of conglomerate which somehow was protected and wore down a little less rapidly than the conglomerate on either side of it), it was just about the time of the glacial period. The great ice sheet went over the mountain removing all the loose material and some more of the solid conglomerate. This brought our pebble to the surface, but too late to be moved by the ice. However as soon as the ice left the Mt. Toby region, the rains fell, and in the further weathering of the conglomerate, the cement holding our pebble in place was dissolved and it was freed. At once a tiny brook started it rolling down the side of the mountain, a brook so small that when the pebble reached the foot of the slope it did not have power to carry it further. Here there gathered a fan-shaped mound of such pebbles, known as an alluvial fan. It rested here not over a couple of thousand years, when the Central Vermont R. R. cut a groove through the fan, using the material for ballast, and here the pebble was found and brought home.

Meteorites

Meteorites can hardly be called common, but there is always a chance of finding one, and their interest is so great, that none should escape because unrecognized.

Meteorites are visitors to the earth from space, and they bring to us knowledge of the composition of planets and solar systems, other than our own. It is of interest to note, that while they have brought to us some combinations of elements which do not occur in the earth, still they have not brought any element with which we were not already familiar. They are popularly known as “falling” or “shooting stars,” though of course they are not stars, but only small masses of matter which are entirely invisible until they come inside our atmosphere.

In space there are many small (compared with the size of the earth) chunks of matter, each pursuing its solitary way around the sun, or wandering through space along paths entirely unrelated to the sun. From time to time one of these passes near enough to the earth, so as to be influenced by its attraction, and then comes rushing toward it at tremendous speed, 20 to 30 miles per second. As soon as it comes into the atmosphere, even the very attenuated atmosphere, a couple of hundred miles above the surface, friction heats the surface of the meteor until it glows, and by that light we see the so-called shooting star, often with a trail of luminous matter streaming out behind. Of course in using this term “shooting star,” we understand the meteor is no star, for they are bodies as big as our sun, shining at distances billions of miles away.

As the meteor rushes through the atmosphere it may all burn up, no large fragment reaching the earth’s surface. The luminous matter streaming out behind is material which has melted and dripped off the main mass. As this oxidizes and cools, that part which did not become gaseous will finally fall to the earth as fine dust. When however a meteor actually falls to the earth, its surface is still hot, though probably there has not been time enough for much heat to be transmitted to the interior. At any rate they do not show any alteration due to this cause. On landing and sometimes before they land meteors break into two or more pieces. When found the surface always shows the effects of the heat generated by the friction of passing through the air, the surface being smoothed, and covered with stream lines and melted out pits and hollows, and the outer surface consisting of a thin crust, making an appearance, which once seen, can hardly be mistaken.

There are two types of meteorites, those made wholly or largely of iron with some nickel, and appearing like great chunks of iron, and those which are stony and resemble a granite boulder. In collections the first sort, i.e. iron meteorites, are most abundantly represented, because most easily recognized when found. They consist of masses of iron and nickel with small amounts of other elements, ranging in size from the Cape York meteorite, which fell in northern Greenland in 1894 and was later brought by Peary to the American Museum, and weighs some 36 tons, down to small grains as small as a grain of wheat. The largest one which has fallen in the United States was the Willamette meteorite weighing some 15 tons, and falling 19 miles south of Portland, Oregon. These and all iron meteorites have the iron in crystalline form which is readily seen if the meteorite is cut, and the surface thus made polished, then etched with acid, which is put on and quickly washed off. Every meteorite has its particular pattern, as illustrated on Plate 72, and by these patterns can be identified. Meteorites have a high value and are eagerly sought by certain large institutions and collectors. Since the crystalline structure is so characteristic of each fall, when a new meteorite is found, it is usually cut in two, and one part retained by the finder or some institution; while the other part is cut into small pieces, an inch or two on a side and a quarter of an inch thick, but each large enough to show the characteristic pattern. These are distributed largely by sale to other collectors. Thus a great meteorite collection consists of a few large meteorites and a great many small portions of other meteorites.

The second type of meteorite is the stony meteorite. Where meteorites have been located as they fell and recovered, the majority of them were of this type, so that probably more than half of the meteorites which fall are of the stony type. However when the stony meteorite is exposed to weathering it takes only a very short time before the surface is eroded off and then such a meteorite looks like any other boulder and probably most of them fail to be recognized, and so have been lost. Because they have so much greater variety, they are in many ways of greater interest than the iron type.

It is desirable that every one have his eye out for meteorites, and when found it is desirable that the fact should be reported to some one of the great institutions which collect them, such as the National Museum in Washington, or the American Museum in New York. Each one should be on record even if it is desired to keep it in a private collection.

Fossils

In the sedimentary rocks one is apt to find remains of some of the animals and plants that lived at the time the rock was forming. While the soft parts of animals decompose rapidly, shells and bones are likely to be buried in the sediments, and if the conditions have been favorable, these remains may be preserved more or less perfectly. All through the millions of years that sedimentary rocks have been forming in the sea, in lakes, on river flood plains and in wind swept deserts, there was an abundance of life, as much as there is today; and our knowledge of that life is derived from these buried fossil remains, so that fossils have a great historic interest.

However as there have lived and died several times as many different kinds of animals as live today, the study of fossils becomes a separate subject, which cannot be treated in this book. Should any collector of rocks and minerals come upon fossils, he is opening a new field, and it will be necessary to turn to other sources for their identification. General books on this subject are scarce, but one or two are given in the literature list.

A List of the Elements, the Abbreviations Used for Them, and Their Atomic Weight, Which Is Approximately the Number of Times Heavier They Are Than Hydrogen.

Name Oxygen = 16
Aluminium, Al 27
Antimony, Sb 122
Argon, Ar 40
Arsenic, As 75
Barium, Ba 137
Beryllium, Be 9
Bismuth, Bi 209
Boron, B 11
Bromine, Br 80
Cadmium, Cd 112
CÆsium, Cs 132
Calcium, Ca 40
Carbon, C 12
Cerium, Ce 140
Chlorine, Cl 35
Chromium, Cr 52
Cobalt, Co 59
Columbium, Cb 93
Copper, Cu 64
Dysprosium, Dy 162
Erbium, Er 167
Europium, Eu 152
Fluorine, F 19
Gadolinium, Gd 157
Gallium, Ga 70
Germanium, Ge 63
Glucinum, Gl 9
Gold, Au 197
Hafnium, Hf 179
Helium, He 4
Holmium, Ho 165
Hydrogen, H 1
Indium, In 115
Iodine, I 127
Iridium, Ir 193
Iron, Fe 56
Krypton, Kr 84
Lanthanum, La 139
Lead, Pb 207
Lithium, Li 7
Lutecium, Lu 175
Magnesium, Mg 24
Manganese, Mn 55
Mercury, Hg 201
Molybdenum, Mo 96
Neodymium, Nd 144
Neon, Ne 20
Nickel, Ni 59
Nitrogen, N 14
Osmium, Os 190
Oxygen, O 16
Palladium, Pd 107
Phosphorus, P 31
Platinum, Pt 195
Potassium, K 39
PrÆseodymium, Pr 141
Protoactinium, Pa 231
Radium, Ra 226
Radon, Rn 222
Rhenium, Re 186
Rhodium, Rh 103
Rubidium, Rb 85
Ruthenium, Ru 102
Samarium, Sm 150
Scandium, Sc 45
Selenium, Se 79
Silicon, Si 28
Silver, Ag 108
Sodium, Na 23
Strontium, Sr 88
Sulphur, S 32
Tantalum, Ta 181
Tellurium, Te 128
Terbium, Tb 159
Thallium, Tl 204
Thorium, Th 232
Thulium, Tu 169
Tin, Sn 119
Titanium, Ti 48
Tungsten, W 184
Uranium, U 238
Vanadium, V 51
Xenon, Xe 131
Ytterbium, Yt 173
Yttrium, Y 89
Zinc, Zn 65
Zirconium, Zr 91

Table of Geologic Time

Eras
Periods and their Duration in Millions of Years Important Physical Events Important Organic Events
Cenozoic
Quaternary
Recent Youthful land forms having high relief formed. Dominance of man.
Pleistocene Epoch 2M.Y. Period of glaciation; four great ice advances. Heidelberg, Neanderthal, and CrÔ-Magnon man; extinction of large mammals.
Tertiary
Pliocene Epoch 10M.Y. Continuing world-wide land elevation. Intermigration of North and South American mammals. Transformation of ape to man.
Miocene Epoch 18M.Y. Cordilleras, Alps, Himalayas formed. Widespread vulcanism-basalt flows in northwestern United States. Culmination of modern types of mammals. Apes appear in Old World.
Oligocene Epoch 10M.Y. Land dominant; seas marginal. Carnivores and ungulates develop into importance.
Eocene Epoch 20M.Y. Extensive sedimentation; seas marginal. Dawn of the dominance of mammals. Reptiles subordinate.
Cretaceous 65M.Y. Widespread epicontinental seas. Laramide revolution at close of period—Rocky Mountains formed. Climax and culmination of reptiles, especially dinosaurs; first flowering plants and grasses.
Mesozoic
Jurassic 38M.Y. Continent emergent; shallow seas on western North America. Rise of birds and flying reptiles, first modern trees.
Triassic 35M.Y. Continent emergent; seas marginal. Rise of dinosaurs, cycads, and ammonites.
Paleozoic
Permian 35M.Y. World-wide continental uplift and mountain building. Widespread glaciation. Extinction of most Paleozoic fauna and flora. First modern insects.
Pennsylvanian 48M.Y. Continent alternately rising and sinking. Great coal-forming forests, of ferns and seed-ferns.
Mississippian 35M.Y. Low lands and widespread submergence. Culmination of crinoids, numerous sharks.
Devonian 40M.Y. Widespread submergence, local vulcanism. First known land animals, first forests.
Silurian 28M.Y. Widespread submergence, local deserts. First lung fishes and scorpions, abundant corals.
Ordovician 65M.Y. 60% of North America below sea. Climax of invertebrate dominance, first vertebrate.
Cambrian 105M.Y. Widespread submergence. First abundant invertebrate fauna, trilobites dominant.
Proterozoic 700±M.Y. Long periods of granite intrusion, sedimentation, and mountain building. Bacteria and seaweeds present. Most invertebrates probably present, but remains are lacking.
Archeozoic 800±M.Y. World-wide intrusive igneous activity; some sediments. Blue-green algae present, primitive one-celled plants and animals probably present.
                                                                                                                                                                                                                                                                                                           

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