And it is indeed the fact that large portions of the vast region comprised between the lower Volga, the AralIrtysh water-divide, the Dzungarian Ala-tau, and the outliers of the Tian-shan and Hindu-kush systems are actually covered with Aralo-Caspian deposits, nearly always a yellowish-grey clay, though occasionally they assume the character of a more or less compact sandstone of the same colour.
It has been found sporadically near the Aleutian Islands, between the Philippines and Marianne Islands and to the south of the Galapagos group. It is made up to a large extent of the siliceous frustules of diatoms. It is usually yellowish-grey and often straw-coloured when wet, though when dried it becomes white and mealy.
In circumference, covered by a thick bed of a reddish-brown clay, and apparently resting on a yellowish-grey sandstone.