By Geology we mean the study of the rocks, and we must at the outset explain that the term rock is used by the geologist without any reference to the hardness or compactness of the material to which the name is applied; thus he speaks of loose sand as a rock equally with a hard substance like granite.

Rocks are of two kinds, (1) those laid down mostly under water, (2) those due to the action of fire.

The first kind may be compared to sheets of paper one over the other. These sheets are called beds, and such beds are usually formed of sand (often containing pebbles), mud or clay, and limestone, or mixtures of these materials. They are laid down as flat or nearly flat sheets, but may afterwards be tilted as the result of movement of the earth's crust, just as you may tilt sheets of paper, folding them into arches and troughs, by compressing their ends. Again, we may find the tops of the folds so produced wasted away as the result of the wearing action of rivers, glaciers, and sea-waves upon them, as you might cut off the tops of the folds of the paper with a pair of shears. This has happened with the ancient beds forming parts of the earth's crust, and we therefore often find them tilted, with the upper parts removed.

The other kinds of rocks are known as igneous rocks, which have been melted under the action of heat and become solid on cooling. When in the molten state they have been poured out at the surface as the lava of volcanoes, or have been forced into other rocks and cooled in the cracks and other places of weakness. Much material is also thrown out of volcanoes as volcanic ash and dust, and is piled up on the sides of the volcano. Such ashy material may be arranged in beds, so that it partakes to some extent of the qualities of the two great rock groups.

The production of beds is of great importance to geologists, for by means of these beds we can classify the rocks according to age. If we take two sheets of paper, and lay one on the top of the other on a table, the upper one has been laid down after the other. Similarly with two beds, the upper is also the newer, and the newer will remain on the top after earth-movements, save in very exceptional cases which need not be regarded by us here, and for general purposes we may regard any bed or set of beds resting on any other in our own country as being the newer bed or set.

The movements which affect beds may occur at different times. One set of beds may be laid down flat, then thrown into folds by movement, the tops of the beds worn off, and another set of beds laid down upon the worn surface of the older beds, the edges of which will abut against the oldest of the new set of flatly deposited beds, which latter may in turn undergo disturbance and renewal of their upper portions.

Again, after the formation of the beds many changes may occur in them. They may become hardened, pebble-beds being changed into conglomerates, sands into sandstones, muds and clays into mudstones and shales, softdeposits of lime into limestone, and loose volcanic ashes into exceedingly hard rocks. They may also become cracked, and the cracks are often very regular, running in two directions at right angles one to the other. Such cracks are known as joints, and the joints are very important in affecting the physical geography of a district. Then, as the result of great pressure applied sideways, the rocks may be so changed that they can be split into thin slabs, which usually, though not necessarily, split along planes standing at high angles to the horizontal. Rocks affected in this way are known as slates.

If we could flatten out all the beds of England, and arrange them one over the other and bore a shaft through them, we should see them on the sides of the shaft, the newest appearing at the top and the oldest at the bottom, as shown in the table. Such a shaft would have a depth of between 10,000 and 20,000 feet. The strata beds are divided into three great groups called Primary or Palaeozoic, Secondary or Mesozoic, and Tertiary or Cainozoic, and the lowest Primary rocks are the oldest rocks of Britain, which form as it were the foundation stones on which the other rocks rest. These may be spoken of as the Pre-Cambrian rocks. The three great groups are divided into minor divisions known as systems. The names of these systems are arranged in order in the table and on the right hand side the general characters of the rocks of each system are stated.

With these preliminary remarks we may now proceed to a brief account of the geology of the county.

Sectional diagram

Sectional Diagram

This cross section shows what would be seen in a deep cutting nearly E. and W. across England and Wales. It shows also how, in consequence of the folding of the strata and the cutting off of the uplifted parts, old rocks which should be thousands of feet down are found in borings in East Anglia only 1000 feet or so below the surface.

In Cornwall there is a succession of nodes of granite rising to the surface, a continuation westward of the mass of Dartmoor. It has surged to the surface in four large masses continued westward by the Scilly Isles. These granitic masses have upheaved the superincumbent beds of stratified rocks, partly melting them. These distinct nodes are: the Bodmin moors, the St Austell elevation, the Carn Menelez, and the Land's End district. Smaller masses of granite occur in the double heights of Godolphin and Tregonning, St Michael's Mount, Carn Brea and Carn Marth, and Castel-an-Dinas.

The Cheesewring

The Cheesewring

The Elvans are dykes of quartz-porphyry which issue from the granite into the surrounding slates, and are often mistakenly supposed to be a bastard granite.

The granite in its upheaval has strangely altered and contorted the superposed beds. There are as well intrusive veins of igneous rocks. In the Lizard district is serpentine, a compact, tough rock often of a green colour, lending itself to a high polish, and forming magnificent cliffs with a special gloss and colour, as well as maintaining on the surface a special flora.

The prime feature in Cornish geology is the upheaval of the granite, distorting, folding back, and altering the superincumbent beds.

In the north-east of Cornwall from a line drawn from below Launceston, on the Tamar, to Boscastle the rocks belong to the culm measures of North Devon. All the rest of the peninsula, except the protruding granite and the serpentine of the Lizard, pertains to the Devonian series of sedimentary rocks, in which the first signs of life appear; consisting largely of clay-slate, locally known as Killas, alternating with beds of red or grey grit and sandstone. Although these slaty rocks must be some thousand feet in thickness, they have been so broken up and turned over by the convulsions of the earth that their chronological sequence cannot easily be determined. In these convulsions they have been rent, and through the rents have been driven hot blasts that have deposited crystalline veins, or injections of trap and other volcanic matter, altering the character of the rock through which they have been driven. By the Menheniot Station on the G.W.R. is a hill of serpentine thrown up at one jet, and now largely quarried for the sake of the roads.

The culm measures already alluded to consist of black shales and slates with seams of grit and chert, much undulated through enormous lateral pressure. The granite, the lowest and most ancient formation of all, was itself consolidated under vast pressure from above, and was not in a molten condition when forced to the surface. Had it been so, it would have resolved itself into lava. It was cold when upheaved, tearing apart the superincumbent stratified sedimentary rocks, which disappeared from the summits, and on all sides about these upheavals were twisted, contorted, thrown back, and fissured.

Atmospheric effect and natural gravitation is constantly carrying the soil from the upper land, from the hills into the bottoms, and consequently it is in the latter that we find the richest land, best calculated to repay the toil of the agriculturist. On the high moors there is little depth of so called "meat earth," below which is clay and grit, hard and unprofitable, commonly called the "calm" or the "deads." But adjoining the granite is the wash from it of its dissolved felspar, the china-clay that furnishes the inhabitants of the St Austell district with a remunerative and ever-growing industry, of which more presently.