6 GEOLOGICAL MEMOIRS.
surround it ; it is especially much less than that of the small lakes
which are embosomed in mountains, such as the Alpine lakes. The
sand forms a margin along the shore : this margin becomes especially
wide on the eastern coast, which is low, and receives the principal
watercourses. But two- thirds of the bed of the Sea of Aral is formed
of mud, which fills those deepest parts in which the movement of the
waters is necessarily less felt. It is only on the eastern shore that the
Mollusca have been developed in any abundance, and on bottoms of
sand having less than 25 metres of water. In the Sea of Aral one
can well see how irregular their distribution may be.
The Caspian presents the example of an inland and brackish sea ; like the Sea of Aral, it has been thoroughly explored by the Russian navy. Its depth is in relation to the height of its shores ; thus in the northern part it becomes remarkably shallow on account of the steppes which surround it, and of the powerful rivers, such as the Volga, which tend incessantly to silt it up. These rivers flow over strata which are eminently sandy, such as the Permian and the Trias, so that they deposit sand over this northern portion ; it may be estimated that half of the bed of the Caspian is covered by sand. Mud is deposited in the south, which is the deepest part of this sea. The Mollusca of the Caspian are developed in zones, which recede from the mouth of the rivers, or may even be interrupted ; they abound especially on the bottoms of sand, and are rarely found below a depth of 50 metres.
But little is yet known regarding the Black Sea. With respect to its orography, it may be observed that it is funnel-shaped, and at the same time that its southern portion is the deepest and most abrupt. Sand occupies only a small surface ; at the same time, on the north-west, where the Black Sea receives the Danube and other great rivers, sand has accumulated along the shore, forming a zone which attains a width of 60 kilometres. Shell-beds occur in it but rarely, and this must be attributed to the fact that the waters are only slightly salt, and the shore generally steep. These deposits, besides, are at a distance from the mouths of the rivers, and occur especially on bottoms of sand.
The Mediterranean presents two great regions, which are separated by Italy, Sicily, and the shallows which connect the latter with the coast of Tunis. Its eastern region is the most extensive and the deepest. As in the preceding seas, it is towards the south that its depth is greatest, while, on the contrary, it is very shallow in the Adriatic.
Mud occupies the greater part of the bed of the Mediterranean, a circumstance which is easily explained, this sea not being subject to tides, and its basin being of great depth.
Sand generally forms a margin along the shores ; but it disappears, or becomes rudimentary at the foot of the mountainous coasts. At the mouth of the Ebro, the Rhone, the Po, and the Nile these deposits, on the contrary, cover extensive surfaces. Sand surrounds the islands, especially Corsica, Sardinia, Cyprus, and the Baleares. It attains an exceptional development on the coast which borders the states of Tunis and Tripoli : in fact this shore inclines
