TRANSACTIOXS OF SECTIOX C. 595 



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. Tliey 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 River, west of Simla. These sprino-s rise apparently 

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

 yard or two of tlie rushing water of the Satlej. Wlien I visited the springs they 

 had a temperature of 1.30° 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 i'ts 

 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. 



Tlie 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 

 waterholding 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, some of which prepare the way for those that follow. 



In some cases the change may be brought about by the removal, in whole or 

 in part, of one or more of the essential constituents of a mineral, whereby the 

 relative proportions and mutual relations of those that remain are altered, as the 

 following examples will show. 



By loss of water limonite passes into haematite, and opal into crystalline 

 quartz. Dyscrasite, by loss of antimony, passes into native silver, and pyroxene, 

 by the removal of its lime and iron, is changed into talc. Simple oxidation or the 

 absorption of oxygen by a mineral is responsible for another class of changes, as in 

 the conversion of zinc blende into goslarite, and antimony into valentinite. 



The loss of one or more of the ingredients, concurrently with the introduction 

 of one or more new ones, causes many metamorphic changes, as in the conversion 

 of marcasite into magnetite, of witherite into barite, and of azurite into malachite. 



The well-known conversion of a peridotite into serpentine is a case in point. 

 Here, part of the iron and magnesia is removed from the olivine, and water is 

 introduced. A simple process like this, brought about by the percolation of surface 

 waters through an igneous rock, is sufficient to transform considerable areas of 

 rock masses into serpentine, as has been the case in parts of Cornwall. 



Some metamorphic processes are more complex than those alluded to above, 

 but Nature has unlimited time at her disposal, and is able to manufacture potent 

 chemical reagents as her processes proceed. For instance, the sulphides of various 

 metals of common occurrence in rocks, most of which, with the exception of those 

 of the alkaline metals, are insoluble in water, by taking up oxygen pass into 

 sulphates, most of which are soluble in that liquid at the ordinary temperature. 



These sulphates are readily carried away in solution, and become potent factors 

 of change in rocks through which water charged with these salts flows. Again, 

 carbon dioxide, so abundant in percolating water, decomposes minerals containing 

 lime pr alkali, and removes them as soluble carbonates to efiect powerful chemical 

 reactions elsewhere. 



QQ2 



