SUGGESTED ITINERARIES FOR PILGRIMAGES TO STUDY EARTH FEATURES

The chief value of the laboratory studies discussed in the preceding appendices is as a preparation for observations made in the field—the laboratory par excellence of the geologist. The pilgrimages whose itineraries are here suggested have been planned especially for impressing by observation the lessons of this book. Such journeys are best interrupted at a relatively small number of localities which, because already studied in some detail, are specially adapted to serve as centers for local excursions. These localities will in most cases be the great scenic places to which tourists resort, or the seats of universities near which specially detailed explorations have been often made.

Within the United States a few local geological guides have been published, and the Geologic Folios published by the United States Geological Survey are already available for a number of such centers. For one long geological pilgrimage we are fortunate in having a carefully prepared guide, namely, from New York to the Yellowstone National Park and back, with a side trip to the Grand CaÑon of the Colorado. Except for the side trip this route, in large measure, corresponds with one here chosen, and for the return journey especially the student is referred to it for information (Geological Guide Book of the Rocky Mountain Excursion, edited by Samuel Franklin Emmons. Comte Rendu de la CongrÉs GÉologique Internationale, 5me Session, Washington, 1891, 1893, pp. 253-487, map and plates 13, figs. 32).

Our journey is begun at New York City, which is built about the deeply submerged channels of an estuary choked with glacial deposits, though the channel may be followed as a deep caÑon across the continental shelf to its margin (252,[4] pl. 17 B). New York City is also upon the margin of the glaciated area, the outer terminal moraine of which is well represented on Long Island (298). Across the Hudson in New Jersey is the great Coastal Plain which meets the oldland in a well-defined margin (159, 246, 247). A local geological guide of the vicinity of the metropolis has been written by Gratacap (Geology of the City of New York, Greater New York. Brentanos, New York, 1904, pp. 119, pls. and map).

Traveling by the New York Central Railway, we follow up the Mohawk outlet of the glacial lakes Iroquois and Algonquin (334), first skirting upon the east the great sills of intrusive basalt known as the Palisades, with their markedly columnar jointing and intersections by numerous faults. Above Peekskill we enter the picturesque narrows of the river (174), cut in the hard crystalline rocks of the Highlands. Entering the Mohawk Valley, we pass Syracuse with limestone caverns and well-oriented joints widened by solution through the agency of the descending ground water (181, pl. 6 B). A branch line to the southwest reaches the vicinity of Cayuga Lake and Ithaca, where are well-oriented joints which have controlled the drainage directions, and there is also a typical strath (55, 87, 428).

To Niagara Falls at least a day should be allotted for the “gorge ride” by trolley car, thus making the complete circuit of the brink of the gorge with interruptions and local studies at all important points (352-366, pl. 23 A). From Niagara Falls over the Michigan Central Railway we reach Detroit on the present outlet of the upper Great Lakes as well as of the later Lake Algonquin (334). From this city as a center a trip is made by electric railway to Ypsilanti and Ann Arbor, across the bottoms of the early glacial lakes from the first Maumee to Warren (330-333). The strong Whittlesey beach is encountered at the little station of Ridge Road, and one of the Maumee beaches on Summer Street in Ypsilanti. The city of Ypsilanti is built upon a terrace (165) of the Huron River, and another terrace in the same series is crossed by the electric line. In an excursion of a few miles down the river, passing meanders (164-165) and ox-bow lakes (165, 415), is found an interesting case of stream capture near the little village of Rawsonville (175. See Isaiah Bowman, Jour. Geol., Vol. 12, 1904, pp. 326-334).

Continuing our journey from Ypsilanti over a high moraine (312), Ann Arbor is reached, built upon the level plain of outwash with fosses sometimes separating it from the moraine (281, 314). Upon the campus of the university are great bowlders of jasper conglomerate and jaspilite, which were transported from the north by the continental glacier (305). Across the river from the Michigan Central station and behind the little church is a delta formed in one of the glacial lakes Maumee and here opened in section (168). West of the city is a great valley which was the former course of the Huron River when thus diverted by the continental glacier lying to the eastward of Ann Arbor—border drainage (see Ann Arbor folio by the U. S. G. S., and, further, R. C. Allen and I. D. Scott, An Aid to Geological Field Studies in the Vicinity of Ann Arbor, George Wahr, publisher, Ann Arbor).

Returning to Detroit (M. C. Ry.), the great Sibley quarries in limestone near Trenton may be visited. They display perfect jointing, numerous fossils, and especially well-glaciated surfaces interrupted by deep troughs and showing striÆ of several glaciations (304). From Detroit the journey is continued by steamer to Mackinac Island in the strait connecting Lakes Michigan and Huron, passing on the way through the peculiar delta of the St. Clair River (431), and coming in view of the notched headlands, which are a monument to the post-glacial uplift of the glaciated area (250, 341). A day is spent at Mackinac Island and St. Ignace in order to study with some care these uplifted strands of the late glacial lakes (341-344). Chicago may now be reached either by steamer or by rail, and in its vicinity we may see the elevated beaches and the ancient outlet of Lake Chicago (331-332, 347, pl. 22 A. See Chicago Folio, U. S. G. S.). By the Chicago and Northwestern Railway the area of recessional moraines and intermediate outwash plains, and later that of the drumlins, are crossed in journeying to Madison, Wisconsin. By examination of the maps on pages 308 and 317 in connection with the larger scale atlas sheets of the United States Geological Survey (Janesville, Evansville, and Madison sheets), this car journey can be made most instructive in gaining familiarity with the characteristic glacial features, and this study is continued to special advantage in excursions about Madison as a center (316-317, 407). This is the more true since at numerous localities in the vicinity of Madison the well-striated glacier pavement is exposed for comparison of the striÆ as regards direction with the axes of the several types of glacial features.

An especially instructive excursion may be made by carriage in a single day to the “driftless area” some twelve miles west of the city. Before reaching it we cross in alternation a series of recessional terminal moraines (pl. 17 C) and outwash plains, and near Cross Plains encounter the partially dissected upland with its arborescent drainage and even sky line (298, 300-301, 312-313, pl. 16 A and B). Typical shore formations (233, 241, 242) are studied to advantage about Lake Mendota in a walking trip to and beyond Picnic Point, where are found the best ice ramparts (431-434. See Buckley, Trans. Wis. Acad. Sci., Vol. 13, pp. 141-162, pls. 18).

Our journey is now continued over the Chicago and Northwestern Railway to Devils Lake near Baraboo, where we cross a salient of the driftless area, within which lies Devils Lake, imprisoned in a former valley of the Wisconsin River, since diverted to another course as a result of the glacial invasion (312-313). The valley here is a former narrows in hard quartzite (466), which towers above the lake in unstable chimneys (300), such as the Devils Tower, but such remnants are not found on the other side of the moraine, being there replaced by rounded rock shoulders. Just north of the lake the marginal moraine which blocks the valley is so characteristic as to merit special study (pl. 17 C). Only a few miles northward along the railway from Devils Lake is Ableman, where, exposed in a high cliff, the hard purple quartzite with beautiful ripple marks to reveal its plane of sedimentation (pl. 11 A) dips vertically, and is overlain by horizontally bedded yellow sandstone. The marked angular unconformity which is thus displayed is further made evident by a basal layer of conglomerate (463) in the sandstone (51-53). Here also are deposits of loess along the river, which display their vertical joint surfaces (207). An excellent geological guide to this interesting district and that of the neighboring “Dalles” of the Wisconsin River has been written by Salisbury and Atwood (The Geography of the Region about Devils Lake and the Dalles of the Wisconsin, etc., Bull. 5, Wis. Geol. and Nat. Hist. Surv., 1900, pp. 151, pls. 38, figs. 47).

If we have taken a conveyance at Devils Lake for Ableman, we may continue in the same manner to Kilbourn, where begin the picturesque Dalles of the Wisconsin River—here a young gorge cut in sandstone, because the Wisconsin was diverted from its old valley to border drainage at the edge of the driftless area (300, 321). The side caÑons of the river, through their abrupt zigzags, reveal the control of their courses by the joint system (224). In the journey up the rapids by steamer to inspect the Dalles, we observe many beautiful examples of cross bedding in the sandstone (37).

From Kilbourn we continue our journey to Minneapolis over the Chicago, Milwaukee, and St. Paul Railway, and near Camp Douglas are over a peneplain, out of which rise prominent monadnocks (171). At La Crosse the Mississippi River is reached, flowing beneath bluffs of sandstone which are capped by loess (207). The meanderings and the numerous cut-offs of the Mississippi may be observed to the left (415). Lake Pepin is a side-delta lake blocked by the deposits of the Chippewa River (419).

From Minneapolis an excursion is made to Fort Snelling to view the young gorge of the Mississippi, cut by the Falls of St. Anthony for a distance of about eight miles in manner similar to that of the seven miles of Niagara gorge (354), and to compare this narrow gorge with the broad valley of the Warren River which drained Lake Agassiz (327). Somewhat farther up the Warren River are examples of saucer lakes (416).

From Minneapolis the journey may be continued by the Great Northern Railway to Livingston, Montana, thus crossing between the stations of Muscoda and Buffalo the bed of Lake Agassiz and its marginal beaches (325-328. For local geology of Minnesota consult C. W. Hall, Geology of Minnesota, Vol. 1, Minneapolis, 1903).

The Yellowstone Park is entered from Livingston (Livingston Geological Folio, U. S. G. S.) and departure from it made at the relatively new Union Pacific terminal at the southwest margin. The regular trip through the Park includes visits to the several geyser basins (191-194), Obsidian Cliff (33, 463), the CaÑon of the Yellowstone, etc. Good climbers can make a side trip from near the Mammoth Hot Springs to the top of Quadrant Mountain, the remnant of a “biscuit cut” upland (372), and there study the nivation process (368, Yellowstone National Park Folio, U. S. G. S.).

The trip from the Park to Salt Lake City, over the Union Pacific Railway, passes through the Red Rock Pass, the former outlet of Lake Bonneville (423), into the desert of the Great Basin ( Chaps. XV and XVI). Great Salt Lake is a saline lake or sink with an interesting record of climatic changes (198, 401). The front of the Wasatch Range, in view and easily reached from Salt Lake City, is deeply scored by the horizontal shore terraces of Lake Bonneville (198, 199), and these terraces are extended at every reËntrant by barrier beaches of great perfection. In the Pleistocene period mountain glaciers in part occupied the valleys of this range, though they did not always extend as far as the mountain front. Big Cottonwood CaÑon, which realizes this condition, and the neighboring Little Cottonwood CaÑon, from whose front its glacier spread into an expanded foot (264), thus show for comparison in a single view the V and the low U sections respectively (172, 376). Here are also alluvial fans (213) and recent faults which intersect them.

From Salt Lake City the return to New York may be made by the Denver and Rio Grande Railway across deserts and through the Royal Gorge, the caÑon of the Arkansas River. A full itinerary of the points of geological interest along this route, and continued to Chicago, Washington, and New York, is supplied in much detail in the guide of the geological excursion to the Rocky Mountains above cited. This the traveling geologist should not fail to study. Some references to points along this journey will be found on preceding pages of this book (219-220, High Plains; 170, Allegheny Plateau in West Virginia; 176, water gap of Harper’s Ferry; 176-177, 184-186, side trip up the Shenandoah Valley to Luray Caverns and Snickers Gap; 251, Chesapeake Bay).

Instead of returning directly from Salt Lake City, the traveler, if he has sufficient time at his disposal, may extend his journey southwestward across the Great Basin to Los Angeles. A branch line from this route leaves the Vegas Valley and passes within reach of the famous Death Valley (201) to Tonopah (79) and the Owens Valley (77-78, 92), where are many surface faults dating from the earthquake of 1872 and other less recent disturbances. Returning to the junction point, the route continues across the Colorado and Mohave deserts to Los Angeles. From Los Angeles as a center the exceptionally interesting terraces, caves, and stacks of an uplifted coast are to be seen to best advantage near Pt. Harford (Chap. XIX). The islands of San Clemente and Santa Catalina may also be reached from Los Angeles (239, 248, 249, 250, 256, 257, pls. 5 B, 7 A, 12 A). The return to the East, if made by the Santa Fe Railway, permits of a visit to the Grand CaÑon (174, 443) from the station of Williams. From that point eastward the geology of the route is fully covered in Emmons’ Guide to the Rocky Mountain Excursion already cited.

For the benefit of those who are privileged to travel in Europe, and the number increases yearly, a pilgrimage is suggested which may easily be made to correspond with plans laid out on the basis of historical, artistic, and scenic points of interest. The only popular guide of a general nature written for geologists traveling abroad appears to be a brief but valuable little paper by Professor Lane (The Geological Tourist in Europe, Popular Science Monthly, Vol. 33, 1888, pp. 216-229). The publishing house of GebrÜder BorntrÄger in Berlin is now publishing a quite valuable series of geological guides dealing with special districts and written by well-known authorities (Sammlung Geologischer FÜhrer). Of this series some thirteen numbers have already been issued. Many other valuable local guides of a geological nature are the Livrets Guides of the International Geological and Geographical Congresses, and the similar pamphlets supplied in connection with annual meetings of national or provincial geological societies.

Passengers on steamships sailing from the harbor of New York pass out over a deeply submerged caÑon (252) largely filled with glacial deposits, through the Narrows (174), and in sight of Sandy Hook, a modified spit (238, 240). To the left are seen the great morainic accumulations at the border of the glaciated area on Long Island (298). In the course of the trans-Atlantic voyage a much-rounded iceberg may be encountered (291), though this is much more apt to occur upon the northern routes from Quebec, and late in the season. Upon entering the English Channel the land on both coasts rises in steep cliffs, where are found all the common shore features well developed (Chap. XVIII). The German steamships pass in sight of Heligoland, that last remnant of wave erosion (236).

While traveling in Europe, the student should consult a map of the glaciated area (299), and so learn to recognize its peculiarities, and carefully mark its marginal moraine (311) and other strongly marked features.

If the British Isles are visited and the more rugged areas are selected, one may study the cirques and other characteristic features due to the presence of mountain glaciers about Snowdon (Chap. XXVI). More mature stages of the same processes are to be found in the Scottish Highlands and the Inner Hebrides, but especially upon the Island of Skye (Fig. 492). A very valuable aid to excursions in this district is Baddeley’s Scotland (part I, Dulau, London) and Sir Archibald Geikie’s Explanatory Notes to accompany Bartholomew’s Geological Map of Scotland (map and notes in cover, Edinburgh, 1892, pp. 23).

It is from Oban, the “Charing Cross of the Highlands”, that one should start out upon the summer steamers in order to reach both Skye and Staffa, the latter with fine basaltic columns (463), and Fingal’s Cave. In sailing to Skye one passes upon either shore of the narrow fjords many relics left in the dissection of volcanoes (139-143 and Sir A. Geikie, Ancient Volcanoes of Great Britain, Vol. II); also rocky islands and skerries marking submergence (252), and the coast terraces which register a later uplift (250). Skye is a complex of many intrusive and volcanic rocks of such markedly different colors as to appear as tints in the landscape. In the Cuchillin Hills of dark green rises the massive gabbro (462) cut by cirques into the jagged pinnacles of horns and comb ridges (373); while lower down and to the east are rounded domes of rhyolite (463) abraded beneath the glaciers and of a delicate salmon tint. Still lower and to the westward are flat mesas composed of horizontal layers of black basalt under a rich carpeting of the brightest verdure. Eastward across the channel are seen the purplish walls of an ancient sandstone. The jagged gabbro core of the island thus represents a fretted upland (372) and is now the training ground of the Alpinist (Abraham, Rock Climbing in Skye, Longmans, London, 1908), while nestled in one of the bottoms of a U-valley is Loch Coruisk, a typical rock-basin lake (412), its shores of hard rock planed and scored.

From Skye we may go to study the remarkable thrusts (45) on the north shore of Loch Maree, a marked lineament, and one directed at right angles to that on the course of the Caledonian Canal connecting Loch Linne with Loch Ness. This northeast wall of Loch Maree is a strikingly rectilinear fault represented by an escarpment, up which we climb to find at the top the crushed and fluted thrust planes of movement dipping southeastward at a flat angle. Here also are beautiful rock-basin lakes, lying in hollows molded beneath the continental glacier. On our way from Skye we have passed up Loch Carron, a sea loch or fjord (252), and along the strath at its head known as Strathcarron (428).

Returning now to Oban, it is but a short trip by steamer up Loch Linne to Fort William along the striking lineament (226) which continues to Loch Ness and beyond (Fig. 492), and thence by rail to Glen Roy and the neighboring glens of Lochaber (322-325).

From Paris as a starting point, we may visit in a most picturesque region the beautifully preserved craters of extinct volcanoes in the Auvergne of Central France (105, 124, 145), which district is entered from Clermont-Ferrand. Here are found the characteristic puys, steep lava domes of viscous lava (105), which figured largely in the early controversies of geologists concerning the origin of rocks.

The rest of our pilgrimage will be so planned as to enter the noble river Rhine at its mouth (Fig. 493), ascend its course to its birthplace in the snows of Switzerland, and after further exploration of the features of this fretted upland, traverse northern and central Italy so as to make our departure for America by the southern route. Entering then upon this course in the Low Countries, we have first the opportunity of observing the characteristics of a great delta with natural levees artificially strengthened as dikes (165-168). Here also are found dunes of beach material which has been raised by the wind into a great rampart near the shore (209-211). Such a wall of dune sand is well displayed at the bathing resort at Scheveningen near the Hague (421). The flood plain of the Rhine (162-165) may be studied in a journey up the river to the university town of Bonn, from whence a day’s excursion should be devoted to the relics of volcanoes known as the Seven Mountains (H. von Dechen, Geognostischer FÜhrer in das Siebengebirge, Bonn, 1861). As a preparation for this trip and others in the volcanic Eifel higher up the river, a visit should be made to the mineral and rock collections of the Poppelsdorfer Schloss at the University. In the volcanic Eifel are found some of the most interesting of crater lakes (405), the largest being Lake Laach with its somewhat peculiar volcanic ejectamenta and its picturesque abbey (see von Dechen, Geognostischer FÜhrer zu der Vulkanreihe der Vorder-Eifel, etc., Bonn, 1886. Consult also Lane, A Geological Tourist in Europe, l.c.).

Continuing our course up the river from Bonn, we soon enter the gorge of the Rhine cut in an uplifted peneplain (169, 171, 174). From Coblenz, where the Moselle enters the Rhine, a side trip may be made up this tributary river past Zell with its entrenched meanders (173) to the ancient Roman city of Treves. Above Bingen on the Rhine we leave behind us the narrow gorge and rapid current of the river and continue over the broad floor at the bottom of a rift valley (403), lying between the forest of Odin and the Black Forest on the east and the “Blue Alsatian Mountains” far away to the west. At the margins of this plain are beds of loess with their characteristic joint structures and inclusions (207), and in the higher hills on either hand a wealth of intrusive igneous rocks.

At the entrance of the Neckar River to this broad plain is nestled the picturesque castle and university town of Heidelberg, a convenient center for excursions (Julius Ruska, Geologische StreifzÜge in Heidelbergs Umgebung, etc., NÄgele, Leipzig, 1908, pp. 208, map). At Strassburg (Schwarzwaldstrasse 12) is located the German Chief Station for Earthquake Study, with a particularly large set of modern seismographs. In the university cabinet is also one of the largest and most representative mineral collections in Europe. For excursions in the neighborhood consult Benecke, Sammlung Geognostische FÜhrer, Vol. 5, Elsass, 1900.

From Strassburg we may go by the Black Forest Railway to the Hegau with its volcanic plugs (140), each surmounted by a picturesque castle. We enter next the broadly extended piedmont apron site, above which Lake Constance still remains as a border lake (399). Outwash aprons (314), moraines (311), and drumlins (317) are each in turn encountered. Still continuing our course up the Rhine from Bregenz, we enter the fretted upland (372) of the Alps, mountains composed of great folds and thrusts about a core of intrusive rock (Rothpletz, Sammlung Geologische FÜhrer, Vol. 10, 1902, Thrusts in the Alps between Lake Constance and the Engadine). Some fourteen miles above Chur we pass the terrace produced by successive landslides (414), known far and wide as the Flimser BergstÜrz. The further assent of the cascade stairway of this glacier-carved valley brings us to the Furka Pass, from which point magnificent views of the fretted upland are obtained. At the KÄnzli, a mile from the hotel, one may view the nÉvÉ of the Rhone Glacier, which may also be easily visited.

We have now followed a great river from its mouth in the sands of Holland to its source in the snows of the higher Alps. Passing over the divide and descending to Gletsch, we may observe the lower end, or foot, of the Rhone glacier and the crevasses and sÉracs (391) on the steep descent of this radiating glacier (383, 386). The response which glaciers make to climatic changes is here well illustrated by the recession of the glacier front from near the hotel (its position in the ’50s of the nineteenth century) to its present position about a mile farther up the valley.

The characteristics of a glaciated mountain valley may be further illustrated by climbing to the Grimsel Pass, which is scratched and striated (377, 385), and then descending the valley of the Aar to Meyringen (377). Near the Grimsel Hospice are the characteristic rock basin lakes (412), and upon the Aar Glacier to our left were carried out the epoch-making researches of Louis Agassiz, the founder of the glacial theory for explaining the drift. We encounter some thirteen rock bars (377). Just before reaching Meyringen we pass the last of these, the Gorge of the Aar, cut by the stream through limestone.

Interlaken (419) may be made the center for additional excursions up the Lauterbrunnen Valley, with its prominent albs (376) and its ribbon fall of the Staubbach (378). By the Jungfrau Mountain railway we may now ascend partly in tunnels of the rock to the Ewigeismeer, and look down upon the nÉvÉ and bergschrunds of the Great Aletsch Glacier (370, see Baltzer, Sammlung Geologische FÜhrer, Vol. 10, Bernese Oberland, 1906). Returning to Interlaken by way of Grindelwald, one may study the foot of a radiating glacier, the Untergrindelwald glacier, with its tunnel and its milky and braided stream.

Crossing now the Alpine foreland to Villeneuve at the upper end of Lake Geneva and upon a well-developed strath (426, 428), we may look out upon the turbid waters extending far from the shore of the lake. Journeying to Geneva by steamer we note the gradual clearing of the water until at the outlet of the lake it is as clear as crystal. A walking trip from Geneva takes us to the Bois de la BÂtie, where the Arve with turbid waters meets this clear stream (427).

The railroad to Chamonix ascends another cascade stairway (376), affords views of complexly folded sedimentary rocks (43), and at Chamonix itself the mer de glace supplies opportunities for the study of moraines (386, 393) and glacial movement (390-392). To experienced Alpinists the summit of Mount Blanc offers a remarkably extended outlook over the fretted upland of the Alps (pl. 18 A). From the station of LeFayet below Chamonix, one may ascend to the DÉsert de la PlatÉ, where are Schratten in limestone due to solution (188).

Crossing by one of the passes to the valley of the Rhone at Martigny we may reach Zermatt, to-day the climbing center of the Alps. From the subordinate cirques surrounding this village descend the Gorner, Findelen, St. Theodul, and other components of this radiating glacier. A black tooth of rock, the Matterhorn, towers above the other peaks and shows to greatest advantage this feature of glacial sculpture (374), while the Gorge of the Gorner is a severed rock bar like that of the Aar (377). Either on foot or over the mountain railway we may ascend to the Gorner Grat, a subordinate comb ridge (373) which affords one of the most magnificent and instructive views of radiating glaciers.

From Brig, farther up the Rhone Valley, an excursion is made to the Eggishorn Hotel, a center for study on and about the Great Aletsch Glacier (329, 371, 385, 388, 395, 410). The easy ascent of the Eggishorn is rewarded by a view almost directly downward upon the ice-dammed MÁrjelen Lake (329, 411).

From Brig one may make his entry into Italy, either over the picturesque Simplon route afoot or by diligence, or else beneath it through the railway tunnel. By an alternation of short steamboat and rail trips the journey is continued in a direction transverse to the longer axes of the border lakes Maggiore, Lugano, and Como, and later southward to Milan. In leaving the village of Como we pass over heavy morainic deposits on the apron borders of the expanded-foot glacier (383, 385) which once occupied the valley above. On the journey from Milan to Venice, over the fertile plains of Lombardy, the similar accumulations about Lake Garda (414) are first encountered at the little station of Lonato and left behind at Somma Campagna (Tornquist, Sammlung Geologische FÜhrer, Vol. 9, Northern Italy, 1902).

The city of Venice is built upon pile foundations in the lagoon behind the barrier beach known as the Lido (242, 428-429). From here we may reach the Karst country by way of Trieste, some of the more interesting and typical features being found near Divaca (187-189, 422, pl. 6 A). In a different direction from Venice by way of Belluno we enter the Dolomites with their patterned relief and battlemented towers (228, 445).

Additional centers for geological excursions on the route to our point of departure from Italy are Rome and Naples. At the Italian capitol and in its neighborhood we may study the volcanic Campagna with its beds of tuff (105) and its crater lakes (405. See Sir A. Geikie, The Roman Campagna, Landscape in History and other Essays, Macmillan, 1905, pp. 308-352; also Deecke, Sammlung Geologische FÜhrer, Vol. 8, Campagna, 1901). From Rome it is an easy journey to the cataract of Tivoli with its deposits of travertine (184). In the opposite direction from Rome across the Campagna rise the Alban Hills, ruins of a composite cone with several crater lakes on the sites of former vents. On the summit of the encircling crater rim, like the Monte Somma of the Vesuvian Mountain now a crescent only, is located the chief Italian station for earthquake study.

From Naples we may reach in short excursions and study with some care still active volcanic mountains. To the east is Mount Vesuvius (94, 97, 122, 124, 127-137), which was in grand eruption in April, 1906. Westward from Naples are the Campi Phlegraeii, or burning fields, with many craters. Of these Astroni offers a fine example of a large-cratered cinder cone (105). In the same vicinity are Monte Nuovo (96) and the Solfatara (97), the latter a type of volcano which no longer erupts lava, but in its place emits carbon dioxide and other gaseous emanations (Grotto del Cane). The starting point for excursions in the PhlegrÆan fields is Pozzuoli with its Temple of Jupiter Serapis (254-255), reached from Naples by an electric line which pierces the wall of an immense crater (Posilippo) composed of fine yellow volcanic ash known as Pozzuolan.

From Naples steamers make short excursions to Sorrento with its deep ash deposits, and to Capri with its blue grotto (257-258). Herculaneum (139) and Pompeii (122), buried during the eruption of 79 A.D., are on the line of the Circum-Vesuvian Railway.

Steamships to New York from Naples call at Gibraltar, the land-tied island par excellence (241). Most steamships of the southern route pass through or near the volcanic islands of the Azores, and certain boats touch at Algiers, from which a line of railway gives access to Biskra on the borders of the Desert of Sahara.

Throughout these pilgrimages the traveler should be on the alert to note not only the agent responsible for the features which come under his observation, but, especially where this is the common sculpturing agent of running water, he should not fail to notice the stage of the erosion cycle which is represented (Chapter XIII).