and Metamorphism. 5 1 



ently capriciously eats its way towards the surface by 

 the hydrOthermal fusion or alteration of parts of the 

 earth's crust, in a manner not immediately connected 

 with the more superficial phenomena of volcanic action 



FIG. 12. 



and for this, among other reasons, it may happen 

 that strata which are contorted, have in places been 

 brought within the direct and powerful influence of 

 great internal heat. Under some such circumstances, 

 we can easily understand how stratified rocks may have 

 been so highly heated that they were actually softened ; 

 and most rocks being moist (because water that falls 

 upon the surface often percolates to unknown depths), 

 chemical actions were set going, resulting in a re- 

 arrangement of the substances which composed the 

 sedimentary rock. Thus certain strata, essentially 

 composed of silica and silicates of alumina, and alkalies 

 such as soda and potash, may have become changed into 

 crystalline gneiss. 



This theory of re-arrangement leads me to another 

 question connected with, but not quite essential to 

 my argument, as far as relates to physical geography 

 viz., What is the origin of granite, which in most 

 manuals is only classed as an igneous rock ? For my 

 part, with some other geologists, I believe that in one 

 sense it is an igneous rock that is to say, much of it 

 has often been completely fused. But in another sense 



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