QUARTZ ROCK. 175 



ver, all be reduced under three divisions, consisting 

 of quartz alone, of quartz and mica, and of quartz and 

 felspar. A compound of quartz and schistose elay is 

 far less common ; hut all this is detailed in the Clas- 

 sification of Rocks. Its texture is interesting, as pre- 

 senting examples of the purely mechanical, and of the 

 chemical, together with instances of that mixed or in- 

 termediate nature so common in the primary rocks. 

 In the purest examples of the chemical or crystalline 

 texture, it consists of quartz alone, sometimes undis- 

 tinguishable from that of veins, except by a laminar 

 structure, or a tendency to divide into thin beds, pa- 

 rallel to the plane of stratification. In the second va- 

 riety, grains of felspar, rounded, not crystallized, are 

 sometimes also imbedded in continuous crystalline 

 quartz. The crystalline texture gradually gives way 

 to an obscurely granular one; in which the quartz is 

 cither confusedly crystallized, or is an aggregate of grains 

 united by a cement of the same substance. This 

 granular texture varies much, in the size of the parts 

 and the compactness of the whole; till, becoming lax 

 and arenaceous, the rock cannot be distinguished from 

 many secondary sandstones. In these cases, the tex- 

 ture is chiefly mechanical; the grains being rounded, 

 and the points of mutual contact few; while this hap- 

 pens also in that variety which contains felspar. But 

 it becomes unquestionable when the rock consists of 

 agglutinated gravel : while rounded pebbles dispersed 

 throughout the beds, with angular and rounded frag- 

 ments of dissimilar rocks, such as clay-slate, jasper, 

 and micaceous schist, leave no doubt of its origin. 



It is worthy of remark, that Quartz rock sometimes 

 contains cylindrical, generally flattened concretions, 

 similar to those of the secondary sandstones; adding 

 a singular point of analogy to those already mentioned. 



