Page images

say much. Shallow caves (rock-shelters) are frequently met with in river-valleys, where one can see that they owe their origin to the under-cutting action of the water. More extensive are the caves often excavated by the sea. These necessarily vary in appearance with the character of the rocks in which they are excavated. The presence of a cave indicates some weak structure—some rock or rock-arrangement which has offered less resistance to the attack of waves and breakers. Vertical dikes of basalt, for example, are often so abundantly jointed, that they are broken up and removed more readily than the rocks they traverse, although the latter may consist of "softer" material, such as sandstone. The highly jointed basalt, notwithstanding its superior hardness, is easily shattered. The mere force of the waves combined with hydraulic pressure in some joints, and the compression and expansion of air in others, suffices to rupture and burst the weak structure, and with each drop of the wave large and small fragments may sometimes be seen falling from the roof and sides of the cave. The cave thus increases in height as the sea works its way inland, until not infrequently it communicates with the surface by a "blow-hole," through which in storms not only spray but spouts of water, and even gravel and larger stones, are ejected. Similar caves are frequently formed in well jointed sandstones and in many other kinds of rock. They are very common, for instance, in Orkney and Shetland, and they are well known also in Cornwall and the West of Ireland. In time the whole roof of such caves may give way, and the latter then appear as narrow ravine-like or gorge-like inlets. This can happen only when the land-surface does not rise to any great height above the sea. When the rocks above a sea-cave are too strongly built or too thick to permit of a downfall of the roof, the cave may attain very considerable dimensions. But as all rocks are traversed by lines of weakness, a limit must be reached beyond which caves cannot be widened. By and by the rocks will cease to be self-supporting, and collapse must take place.

Caves of marine origin are seldom met with far removed from existing coast-lines. They are naturally confined to the latter, and to those lines of old sea-level known generally as " raised beaches." Their position at the base of old sea-cliffs renders them liable to early obliteration, for they tend to be obscured by, and eventually to be concealed underneath, a talus of ddbris. They are not singular, however, in that respect, for many of the most interesting and important of the limestone caverns of Western Europe have been hidden in the same way, their discovery having been the result either of mere accident or of patient scientific research.






ALL the varied topographical features of the land owe their origin either to subterranean or to superficial agents, or to both. This is true of elevations and depressions alike. It would seem possible, therefore, to classify hollows according to the mode of their formation. Not a few, however, are of complex origin, having resulted partly from hypogene and partly from epigene action. Indeed, we might group all basins roughly in two divisions, according as they owe their origin more or less directly to crustal deformation and fracture, or to the action of surface-agents. Epigene action, however, is so manifold and diverse—the agents of erosion, of transport, and accumulation act in so many different ways—that a more detailed grouping is desirable. Any classification adopted must be more or less arbitrary and incomplete, but it will serve our purpose to group basins as follows :—

1. Tectonic basins.

2. Volcanic"

3. Dissolution"

4. Alluvial

5. /Eolian

6. Rock-fall

7. Glacial

1. Tectonic Basins. These owe their origin directly to deformation of the earth■s crust, whether the result of warping or of fracture, or both. In this class are included many inland seas, and most of the larger lakes of the globe. The Aralo-Caspian depression, with its numerous sheets of water and desiccated basins, the Dead Sea, Issyk-Kul, the lakes of Equatorial Africa, the Great Salt Lake of Utah, and very many others are true tectonic basins. A large number of such basins occur in relatively dry and rainless regions. On the other hand, many are met with in temperate regions. The great fresh-water lakes of North America and Europe (Superior, Huron, Michigan, Ladoga, Onega, etc.) occupy tectonic basins. These lakes, it will be noted, are confined to the glaciated areas of the two continents, and their character as tectonic basins has been modified and obscured by glacial erosion and accumulation. There seems no reason to doubt, however, that the depressions are the result of crustal deformation. Tectonic basins are usually somewhat flat-bottomed or gently undulating. Occasionally they are traversed by narrow winding hollows, which have been traced for longer or shorter distances. These have frequently the character of river-ravines and valleys, and are suggestive, therefore, of a former land-surface which has become depressed. Similar indications of depression are afforded by the highly indented coastlines of some of the larger lakes of this class, the long inlets and projecting headlands recalling the appearances presented by the fiord-coasts of Norway and Scotland.

The crustal deformation may consist of simple subsidence—a wide area of relatively flat or gently undulating land sinking below the level of adjacent tracts; or the subsidence may be the effect of dislocation and displacement. Again, basins have come into existence between contiguous high grounds undergoing elevation. Once more, the formation of an anticline across the drainage-area of a lowland region might bring extensive lakes into existence. Similarly it is conceivable that lakes might be formed in mountain-valleys by the swelling up of the crust at the base of the mountains, or by the formation of new flexures in the mountains themselves, having a direction transverse to the valleys. We cannot, however, point to any particular valley-basin formed in this way. Earth-movements of this kind would seem to take place very slowly, so slowly, as a rule, that rivers are able to saw across the obstructions as fast as they rise.

« PreviousContinue »