500 THE JOURNAL OF GEOLOGY. 



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, paral- 

 lel 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 Ben- 

 gal, flowing alternately east and west around these great embay- 

 ments, past the mouths of the two great silt -bearing rivers, 

 distribute fine material in suspension throughout the area of their 

 circulation. 



CHEMICAL DEPOSITION. 



Favorable conditions : 



[a) Evaporation from an enclosed sea. 



(3) Precipitation of lime and magnesia from ocean waters, 

 charged by solution from the land, through evaporation, 

 through reaction of salt water on fresh, and through varying 

 atmospheric conditions at the surface of the sea. 



(#) 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 satur- 

 ated, 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 precipi- 

 tation of lime and magnesia from sea-water is a much mooted 

 question. There are two lines of evidence relating to it which 



