42 Granite and 



sandstones, and sandstones, as they approach granites, 

 lose their (sometimes fossiliferous) characters, and become 

 changed into crystalline limestones, serpentine, &c., and 

 quartz rock. In other cases gradual changes of a differ- 

 ent kind are observed in slaty and schistose rocks as they 

 approach granites. Clay-slates are simply clays con- 

 solidated by pressure, often affected by cleavage, and 

 sometimes chemically altered. Approaching granites 

 ordinary slates often assume a foliated structure by the 

 development of distinct mineral layers of quartz, felspar, 

 and mica. This is gneiss. Analyse some kinds of mica- 

 slate, gneiss, and common sandy clay, and their average 

 composition will not differ more than three clays, three 

 pieces of gneiss, and three bits of granite often do from 

 each other. 



Granite, is sometimes merely gneiss still further 

 metamorphosed by heat in the presence of moisture ; 

 and, though this is not the popular notion, I have long 

 held it, and some other geologists share this opinion. 

 When slate is changed to gneiss, there is no develop- 

 ment of materials which were previously absent, but 

 simply a re-arrangement of its constituents, according 

 to their chemical affinities, in rudely crystalline layers, 

 which seem in gneiss to have found facilities for their 

 development in pre-existing planes, whether of bedding 

 or of cleavage ; or, in other words, if the rocks be 

 uncleaved when metamorphism occurs, the foliated 

 planes show a tendency to coincide with those of bed- 

 ding ; but if intense cleavage has preceded, the foliation 

 will generally tend to follow the planes of cleavage. 

 Furthermore, in gneissic rocks, garnets, schorl, stauro- 

 lite and staurotide, hornblende, and other minerals are 

 frequent in some localities, especially near and in con- 

 tact with granite. All the chief materials of these are 



