§ 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 inrelation 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 
coastof Tunis. Its eastern region isthe 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 _sur- 
rounds 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 
