Page:The Journal of geology (IA journalofgeology11893univers).pdf/74

archipelago, but in the Arabian Sea and the Bay of Bengal they extend to distances of 800 miles from the mouth of the Indus and Ganges, and cover areas of more than 700,000 and 900,000 square miles, respectively. By reference to a map of the ocean currents it may be seen that their courses affect the distribution of these deposits. Sweeping at all seasons past the west coast of Australia and directly toward the east coast of Africa, parallel to which it then diverges, the principal current prevents any extended distribution of sediments in a direction normal to these coasts. But the currents of the Arabian Sea and the Bay of Bengal, flowing alternately east and west around these great embayments, past the mouths of the two great silt-bearing rivers, distribute fine material in suspension throughout the area of their circulation.

Favorable conditions:

(a) Evaporation of an enclosed sea.—When a limited body of water, such as a lake, is subjected to a change of climate, so that evaporation exceeds precipitation of rain, the volume will shrink, outflow will cease, and the solution of salt will be concentrated. If the process is sufficiently continued the solution will become saturated, first for one salt, then another, and they will be deposited in the order of their insolubility. This process is important as an indication of climatic variation in the past; it has been fully described by Gilbert, Russell and Chatard for Pleistocene lakes and the chemical relations, and these studies suggest the conditions to which appeal must be made to explain the less exact facts known in ancient formations of the kind.

(b) Precipitation from brackish waters.—The chemical precipitation of lime and magnesia from sea-water is a much mooted question. There are two lines of evidence relating to it which