Page images

and England. At that period the climatic conditions were probably such as are now experienced in Northern Siberia. Eventually, however, these conditions gradually gave way,—the Tundra-fauna began to retreat, until by and by it was supplemented by a no less characteristic Steppe-fauna, the range of which seems to have been as extensive as that of the former. The Arctic lemming, Arctic fox, reindeer, musk-ox, and glutton of the Tundras were now replaced by the jerboa, pouched marmot, tailless hare, little hamster rat, and other forms, the common denizens to-day of the Steppes of Eastern Russia and Western Siberia. It is certain, then, that a dry Steppe-climate has prevailed at no distant date, geologically speakings throughout Central and Western Europe. Thus we may be sure that dust-storms must formerly have been as common in France and Belgium and the regions lying to the east as they are now in Russian and Asiatic Steppes. It was during the prevalence of such climatic conditions, as geologists think, that the wide-spread flood-loams of the Glacial Period were so largely re-assorted and remodified by deflation, and the lossic accumulations assumed their present aspect and distribution.

Mention has been made of the fact that marshes and lakes occur now and again in the hollows amongst sand-dunes. They are met with likewise amongst dust-deposits. Thus pools and large and small sheets of water sometimes dapple the surface or extend over broad areas of the wind-swept Steppes. Such basins, loubtless, are partly due to the unequal distribution or ^ieaping-up of fine sand and dust. In some cases, however, they seem to have been caused by the unequal removal of superficial materials.

In fine, then, we conclude that wind-erosion is most effective in dry, desert regions. Its influence is, no doubt, world-wide; but as an active agent in levelling the land—in cutting, carving, undermining, and removing rock—wind plays the dominant part in desiccated lands. We note, further, that the forms assumed by rocks subject to wind-erosion are largely determined by geological structure and the nature of the rocks themselves, just as in temperate latitudes feeble structures and relatively soft rocks are the first to yield. Lastly, we recognise that certain windblown accumulations have a world-wide distribution, and occur under all conditions of climate. Sand-dunes may be met with wherever incoherent deposits of sufficiently fine grain are exposed to the action of the wind. Dust, on the other hand, is pre-eminently a product of relatively dry regions and of deserts— wherever, indeed, the land is naked or only partially clothed with vegetation, dust is formed, and may be swept up and transported by the wind.

[ocr errors]








IN Chapter VII. it was pointed out that subterranean action had played a most important part in the production of certain surface-features. In particular it was shown that depression of the surface has frequently taken place as a result of that action. We have now to consider another kind of action altogether, which, although by no means so important as that just referred to, nevertheless now and again causes the surface in certain regions to subside. Rocks, as we have seen, are very variously acted upon by water —a few are readily soluble, but the great majority are not. The most important of the soluble rocks are rock salt, gypsum, and limestones of every kind. These are all more or less easily removed by meteoric water. Rock salt is so very soluble that it is seldom >r never found cropping out at the surface; any surace-exposure in temperate lands would rapidly disappear. It is only in dry and rainless tracts, therefore, that rock salt can exist as a superficial accumulation. Gypsum is more readily dissolved than limestone, but both rocks become eaten into at the surface, and, according to circumstances, are more or less rapidly washed away. This process of dissolution, it is needless to add, is not confined to the surface. Meteoric water penetrates the ground, and circulates through the crust to considerable depths. After pursuing a shorter or longer course, it reappears at the surface as springs, the waters of which are more or less abundantly charged with dissolved mineral matter, according to the nature of the rocks through which it has passed. In this way enormous quantities of soluble materials are brought up from below; in short, wholesale chemical erosion goes on underground. It follows that in regions where soluble rocks enter largely into the framework of the land the surface must in time subside slowly or suddenly. The copious outpouring of brine-springs gradually reduces beds and sheets of rock salt, and the overlying strata sink down and thus produce depression at the surface. And the same result is brought about by the dissolution of gypsum, limestone, and dolomite. Sometimes the surface slowly subsides, but now and again it collapses suddenly, producing earthquakes, accompanied by much fracturing and shifting of the rocks. Thus it is believed that the earthquakes which disturbed

[graphic][ocr errors]

the Visp-Thal in Valais during the summer and autumn of 1855 were the result of the caving-in of the rocks consequent on the dissolution and removal of gypsum, for the springs of that district bring to the surface annually over 200 cubic metres of the mineral in solution. Similarly, it can hardly be doubted that many of the larger and deeper depressions of the surface which appear in regions of calcareous rock are the result of sudden collapse due to the removal of material by underground water.

As rock salt and gypsum do not enter largely into the composition of the crust, they are less important from our point of view than limestones. The latter not only attain in many cases a much greater thickness, but they are far more widely distributed, and extend over much broader areas of the earth's surface. It is in regions of calcareous rocks, therefore, where underground water plays the most prominent rSle, and where its action in modifying surface-features is best displayed. In a former chapter reference has been made to the fact that in countries occupied by limestone, the drainage is of ten largely or even wholly conducted underground. The rocks are so penetrated in all directions by rifts, clefts, and tunnels, that the water which falls at the surface very soon disappears. Concerning the origin of these subterranean spaces there is not much difference of opinion. Geologists recognise that they have been worked out by chemical and mechanical water-erosion. But while some have maintained that the underground water

« PreviousContinue »