282 



KNOWLEDGE. 



[Decembeb 1, 1897. 



THE HEART OF A CONTINENT. 



By GiiENviLLE A. J. Cole, M.R.I. A., F.G.S., Professor of 

 Geolo(/if in the Royal College of Science for Ireland. 



IN a previous article* we have seen how unstable the 

 edge of a continent appears to be, and how the line 

 along which the sea meets the land is usually one of 

 movement in the earth's crust. By upthrust along 

 lines of fracture, or by the growth of mountain-folds, 

 the land becomes raised above the sea ; or, on the other 

 hand, the oceanic basins deepen, the water retreats from 

 the surroundinu; shores, and the continents grow by 

 remaining inactive and at rest. 



Prof. Suess, I indeed, regards the formation of ocean- 

 basins as the true cause of continental land ; the subsidences 

 on the surface of a priniiPval globe first collected and 

 localised the water, and so rendered the earth fit for our 

 present air-breathing fauna. 



It would seem, then, that we might seek for peace in the 

 heart of a continent, far from those lines of weakness that 

 have determined its existence and its form. But here 

 again the lover of repose is soon doomed to disappointment. 

 In the first place, the heart of a continent gives us a 

 more terrible picture of the forces of denudation than is 

 obtainable by a visit to any ordinary coast. Along the 

 cliffs, say, of Dover, of Trotternish, or of Moher, occasional 

 storms may bring down tons 'of rock, which are reduced 

 to fragments, and are distributed along the shore by the 

 constant swirling of the waves. The evidence of the 

 catastrophe is thus in time removed ; the face of the lard 

 still looks out proudly on the sea ; and the new rock-surfaces 

 and the upri,t;bt walls give an air of freshness to a sea- 

 board stricken with decay. I'-ven when we consider the 

 ordinary sandbanks as products of the destruction of the 

 land, so much of their mass is hidden from us beneath 

 the water that we gain no adequate conception of the 

 transference of material that is going on. 



But in the heart of a continent it is a very different 

 matter. The products of the decay of the innermost 

 masses stream only very slowly towards the shores. 

 Enclosed or rainless areas exist, whence no escape is 

 possible, except for particles of fine dust, which may be 

 carried outward by the winds. The tendency is, then, for 

 the huge refuse-heaps of nature to remain en I'ridence, and 

 for the varied slopes of crag and brae to become buried in 

 their own detritus. 



In high altitudes, the processes of denudation are always 

 imposing, and at times truly catastrophic. Even in our 

 own islands, where the ridges and plateaus have long been 

 worn down below the heroic scale, we frequently see the 

 relics of larger action ; the glacial moraines and huge 

 fallen masses, among which we may walk unconcernedly 

 to-day, point to a time when the warfare of the atmo- 

 sphere against the earth was carried on under far more 

 effective conditions. 



At the foot of every mountain-peak, however self- 

 assertive it may be, cones of debris may be seen, their 

 apices lying m some groove or " stone-shoot " of the rock 

 above. From this they spread outward, in fan-like forms, 

 until they coalesce at their outer edges, and form taluses 

 that conceal the whole lower slope of the mountain. 

 Gradually, the growth of these taluses alters the stern 

 aspect of the landscape. Their summits creep back, higher 

 and higher, up the surface of the mountain-wall ; their 

 lower portions encroach on the valleys, and assume a 



* Knowledgf, Vol. XX. (1897), p. 208. 

 t " Das Autlitz der Erdc," Bd. I , p. 778. 



characteristic and slightly concave curve. In times of 

 frost, or whenever a storm is raging round the crests above 

 us, we may hear the stones bounding down the grooves 

 that guide them, we may see them leaping freely out 

 on to the talus, and we may note the disturbance that the 

 larger blocks produce, as they help the whole mass to 

 settle down in curves of greater stability and rest. 



Even under the Drei Zinnen, the most striking of the 

 Dolomite towers in Tyrol, this contrast of sheer crag and 

 sloping talus is at once perceptible ; and the taluses arc 

 clearly destined to be in time the predominant feature of the 

 country. The peak of the Matterhorn, again, has sweep- 

 ing skirts, as it were, formed by stones that have fallen 

 from its crest ; and many lesser summits are threatened 

 with burial under the products of their own decay. 



A fine example of the banking-up of taluses against a 

 whole mountain-wall occurs under the Schwarzhorn, at 

 the summit of the Fliiela Pass in Switzerland (Fig, 1). 

 Numerous cases will occur to the minds of travellers in 

 our own islands, such as the huge " screes " of the North 

 Country, the slope of Craig-y-llam, on which the road 

 is carried at the back of Cader Idris, or the long curving 

 sides of Glen Docharty, descending on Loch Maree in 

 Eoss-shire. The crags of Wastwater and the Pass of 

 Brander show earlier stages in the growth of talus-cones. 



Day by day, year by year, these cones slip outwards, 

 stones and sand and mud being carried away from their 

 lower edges, while they are added to above by the constant 

 denudation of the peaks. Frequently, a few heavy showers 

 alter the angle of repose, and dangerous landslides occur, 

 by which the face of the landscape becomes transformed in 

 a few brief hours. The oozy mass, a thick river of mud 

 and stones, fills up the picturesque ravines, obliterates the 

 details of the valley-floor, and settles across it as a level 

 stretch of alluvium, which the streams may now be power- 

 less to clear away. Everywhere in the heart of our continent 

 there is this tendency to the abolition of variety and detail 

 — to the production of a uniform alluvium (Fig. 2). In 

 time, just as the cones on a mountain-side coalesce to form 

 a continuous talus, so the flatter fans of detritus, emerging 

 from the main valleys themselves, will unite in the low- 

 lands, and will yield us the monotony of a plain. 

 In fact, as I have elsewhere stated,* " the whole plain 

 results from the uniting and overlapping of exceedingly 

 flat cones of detritus, which become less clearly bounded — 

 more unstable at their edges— as they leave the limits 

 of the hills." In this way we have the great plain of 

 Northern Italy, " formed by cone uniting with cone, delta 

 with delta, all spreading outward into the continental 

 valley of tl'.e Adriatic." 



It is probable that in some parts of Eussia and Siberia 

 this essentially inland and continental feature can be 

 studied to the best advantage. Prof. Pavlowf has recently 

 described the general characters of the deposits along 

 the basin of the Volga. The beds of " diluvium " cover 

 the slopes of the hills, and correspond in composition 

 with the rocks that form the neighbouring heights. 

 They are inclined at a very gentle angle to the plain, 

 and " are merely the products of disintegration of the 

 older rocks, products heaped up on the slopes, worked 

 over again and again by the action of the rains, and 

 rendered more and more fine-grained in proportion as they 

 are carried farther from their place of origin. These 

 deposits, resting on the more abrupt slopes of the high- 



* " Open-Ail- Studies," p. 127. 



t " Ouide des Kxcursions dii VIT. Cougits Geologique Inter- 

 national," 1897, Section XX., p. 11; and Plates A (Fig. 11 and D 

 (Fig. 2), 



