234 GEOLOGY OF THE COMSTOCK LODE. 



The feldspars of the massive and metamorphic rocks are ordinarily 

 fresh,^ and they appear to decompose only under peculiar conditions, the 

 details of which are not fully understood, but the circumstances point to 

 the intervention of external energy. Such behavior is characteristic of 

 compounds the formation of which is accompanied by a liberation of heat. 

 The silicates containing a single base appear to liberate but a very small 

 amount of heat, for the thermal effect even of the formation of sodium 

 silicate is very small indeed, and that of calcium and aluminium silicates 

 is, by inference, smaller still. The separation of the feldspars into silicates 

 of the earths will probably, therefore, be accompanied by the absorption 

 of heat, and so will the solution of sodium silicate. I know of no experi- 

 ments to show precisely what is the thermal effect of the conversion of 

 calcium silicate into calcium carbonate, but the behavior of the carbonate 

 and silicate of sodium, and of calcic carbonate, leaves little doubt that it 

 must be the evolution of a small amount of heat, less than that evolved 

 by the formation of calcium carbonate. 



Formation of kaolin. — If kaoHu results from the decomposition of these feld- 

 spars, there must be a still further separation of silica, and an introduction 

 of hydrogen. The structural formula adopted suggests interesting possi- 

 bilities. It is, namely, by no means impossible that the silicon represented 

 in the last formula as basic should be replaced by hydrogen by the reaction 



. 2Si-f 4H20 = 2Si02-f 8H. 

 Were this the case, the result would be silicic anhydride and 



=41 > Si*0^«, 



w) 



or twice 



A120^2SiO^+2H''0 



(the ordinary formula for kaolin), if the water is regarded as combined. 

 The heat liberated by the reaction 



2Si + 4H='0 = 2SiO=-f 8H 



' It has been alreatly remarked that the decomposition of an insignificant percentage of a feldspar 

 crystal robs it of its transparency. Many dull, chalky-looking feldspars, when seen under the micro- 

 scope, prove to bo very slightly altered. 



