QUARTZ ROCK. 323 



points of contact, appear to indicate an origin 

 chiefly mechanical, and resulting from the agglu- 

 tination of sand. These are its varieties when in 

 the purest state ; and I may add, that cavities are 

 sometimes found in the specimens, containing 

 regular, although minute, crystals. 



The next, and perhaps the prevailing variety 

 of this rock, consists of a mixture of quartz and 

 felspar ; and, from the latter, it generally derives 

 a reddish hue. Its texture is various, the grains 

 of felspar being at times imbedded in a crystalline 

 quartz, while, more frequently, the whole rock 

 is a mass of agglutinated grains of the two sub- 

 stances, in various proportions and of various 

 sizes. In every instance it has a foliated dispo- 

 sition ; and, sometimes, even the texture is the 

 same, since it splits into laminae of extreme thin- 

 ness. 



The next remarkable variety is that in which 

 quartz is intermixed with mica; but I have al- 

 ready said all that is necessary on this subject 

 when treating of its alternation and affinity with 

 micaceous schist I may only add, that, in these 

 cases, it often admits of being split in the direc- 



Y 2 



