





CLEAVAGE AND FOLIATION. 



99 



has been in this manner modified. It would be easy to find uppeb 

 unaltered shale, slate, a piece of gneiss, and a piece of Gaileby - 

 granite, of which the ultimate chemical constituents would Wa U-c»se45 

 agree as nearly as they would in two distinct pieces of 

 gneiss. Gneiss is composed of free silica or quartz ; felspar 

 (which is essentially a silicate of alumina), soda, potash, or 

 lime ; and of common mica, the chief ingredients of which 

 are silica, alumina, potash, and peroxide of iron. In all 

 shales and slaty rocks these chemical ingredients will be 

 found. Serpentines are also metamorphic rocks, p. 214. 



For long the laminar structure of gneissic rocks was a 

 mystery. It frequently lies in wavy layers (No. 130), or 

 else in small and rapid contortions (Nos. 126, 129, and 135.) 

 Many of the elder geologists contented themselves with the 

 easy assumption that all the earth was originally formed of 

 granite, the result of its first cooling from a state of igneous 

 fusion ; and that gneiss, being formed of the same consti- 

 tuents as granite, was made from the sedimentary waste 

 of that rock deposited in a primeval boiling sea, which ac- 

 counted for its wavy and wrinkled structure. The meta- 

 morphic theory destroyed this ready hypothesis, and it is 

 now universally known that gneissic rocks and granites 

 are of every geological age. 



I must now refer to the phenomena of cleavage. (See 

 Case 46, Nos. 26 to 47, and p. 123.) 

 win, in his celebrated work on the "Geology of South 



Mr 



Ame 



rica," showed that in part of the And 



]f the cleavage and foliation 



he, therefore, conceives that 



; /< 



coincide, and 

 V he the ex- 



effect 



of the process of ivhich cleavage is the f 



ment of particles in the rocks began with cleavage and 

 ended in their entire crystalline re-arrangement in the same 



foliation. Mr 



Sharpe observed 



tnat the foliated layers of the rocks of Scotland lie in large 

 sweeping synclinal or anticlinal lines, which, he said, bear 

 no relation to the original lines of stratification. This 



d 2 



