Löss occurs in many other countries, but it nowhere attains so vast a development as in China. In Europe we meet with it in the valley of the Rhine and in the low grounds traversed by the Danube, where, although it forms no enormous plains like those of Northern China, it nevertheless mantles the ground so as in some measure to conceal the older features of the land. The extensive sheets of black earth which cover the surface of the great plains of Southern Russia are also a variety of löss. The origin of the European deposits has been much discussed by geologists, but it seems to be now the general opinion that the materials of the löss were, in the first place, introduced into the low grounds chiefly by the flooded rivers and inundations of the Ice Age. Muddy water escaping from the glaciers of the Alps and other mountains, and from the terminal front of the great "inland ice" of Northern Europe, doubtless drowned wide areas, while torrents derived from the melting snows of extraglacial tracts must likewise have swept down large quantities of fine-grained sedi Thus, long after the periodical inundations of glacial times had diminished in extent and finally ceased, the lower reaches of the great valleys and the broad plains, formerly subject to floods, must have been more or less sheeted with sandy loams. We know now that Tundra- and Steppe-conditions have succeeded in Central Europe. Already towards the close of glacial times a well marked Tundra-fauna had spread south to the Alps and west into France 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 speaking, 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 lössic 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, doubtless, are partly due to the unequal distribution or heaping-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 desertswherever, indeed, the land is naked or only partially clothed with vegetation, dust is formed, and may be swept up and transported by the wind. CHAPTER XIII LAND-FORMS MODIFIED BY THE ACTION OF UNDERGROUND WATER DISSOLUTION OF ROCKS-UNDERGROUND WATER-ACTION IN CALCAREOUS LANDS-KARST-REGIONS OF CARINTHIA AND ILLYRIA -EFFECTS OF SUPERFICIAL AND SUBTERRANEAN EROSIONTEMPORARY LAKES CAVES IN LIMESTONE-CAVES IN AND UNDERNEATH LAVA-"CRYSTAL CELLARS -ROCK-SHELTERS 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 Rock salt is so very soluble that it is seldom water. or never found cropping out at the surface; any surface-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 need less 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. it is believed that the earthquakes which disturbed Thus |