466 Notices of Memoirs — 



The genesis of individual species of minerals is a fascinating 

 study, but the subject is too large to enter upon here. 



Water gains access to rocks in several ways. It falls as rain ; it 

 rises from hidden depths ; it leaks from the sea into horizontal beds 

 or into strata dipping away from it ; and it penetrates through faults 

 and fissures. Eain in its descent takes up from the air oxygen, 

 nitrogen, carbonic acid, and in some cases small amounts of nitric 

 acid. It is thus in itself a powerful solvent and potent agent in 

 producing chemical change. In its passage through the surface soil 

 it dissolves humic and other organic acids, the products of vegetable 

 decay, which add greatly to its solvent power and enable it to break 

 up many silicates and to dissolve even silica. By the time the rain- 

 water reaches the solid rocks below the surface soil, it has become 

 a very active agent in producing chemical change in them. It is by 

 such agents, persistently applied during long periods of time, that 

 large areas of ultra - basic igneous rocks have been altered into 

 serpentine. 



Hot springs are a well - known instance of water rising in 

 considerable quantity from plutonic depths. They are known to 

 occur in the plains of India, and are especially abundant in the 

 Himalayas. I visited two very interesting ones at Suni, in the bed 

 of the Satlej Eiver, west of Simla. The springs rise apparently 

 under the very bed of the river, and come to the surface on both 

 banks within a yard or two of the rushing water of the Satlej. 

 When I visited the springs they had a temperature of 130° F., and 

 contrasted strongly with the cold water of the river flowing past 

 them, which had descended from high Himalayan glaciers and had 

 a temperature of 49° F. 



The native inhabitants of neighbouring villages told me that the 

 hot springs always appear at the very edge of the river, whatever 

 may be the height of its waters during drought or flood. This 

 statement is probably true, for I think the springs well up from 

 below through the walls of a fault that traverses the bed of the 

 Satlej at a high angle to its course, and the springs thus come to 

 the surface on both its banks. 



The metamorphic influence of these springs on the rocks in this 

 locality has been very powerful. The ancient volcanic rocks there 

 exposed have, for some distance up the river, been altered by aqueous 

 agents almost out of recognition. The original structural characters 

 of these lavas have been almost completely broken down, and an 

 amorphous substance substituted for the crystals and minerals of 

 which they were originally composed. 



This result shows that the crystals and minerals of these old lavas 

 must, for all practical purposes, have been completely porous to the 

 aqueous agents brought to bear on them. 



The general transmutation of one mineral into another by the 

 action of heated water holding mineral agents in solution, aided by 

 heat and pressure, may take place in a variety of ways. Some 

 of these processes are simple, but others are highly complex. Many 

 are the results of a single operation, others of a series of changes, 



