PRIMARY ROCKS. 



71 



is very ambiguous. Primitive rocks occur either as 

 protruded masses, as overlapping masses, resulting 

 from the spread of matter after ejection, or as vein- 

 stones, filling fissures, apparently consequent on 

 some violence to which the strata have been sub- 

 jected." 



Primary rocks are chiefly composed of the hard 

 minerals, quartz, feldspar, and hornblende ; mica and 

 talc are disseminated in smaller proportions, and 

 limestone and serpentine occur in beds or masses, 

 but less frequently than the above-named minerals 



CLASS t 

 Primary Rocks. 

 1. Granite. 2. Gneiss. 3. Mica Slate. 



Subordinate Rocks which occur among Primary. 



1. Hornblende Rock. I 3. Crystalline Limestone. 



2. Serpentine. | 4. Quartz Rock. 



Some of these subordinate rocks occur also among 

 transition rocks. Bakewell justly remarks, that gran- 

 ite, gneiss, and mica slate might with propriety be 

 regarded as belonging to one formation, as they 

 are essentially composed of the same minerals, vary- 

 ing in different proportions, and, accordingly, are 

 rather modes of the same rock than different spe- 

 cies. We often, indeed, see them passing into each 

 other, as one of their constituent minerals becomes 

 more or less abundant. 



Granite. 



Granite is a compound rock, composed of quartz, 

 feldspar, and mica, arranged in various proportions, 

 and varying accordingly the aspect of the mineral. 

 The crystals of each may be large or small ; in one 

 portion of the rock equally mixed ; in another, un- 

 equally blended ; so that we often find the quartz or 



