JURA. QUARTZ ROCK. 221 



ddubt that the stratified quartz of that coal district is a 

 secondary sandstone, which has been reduced to that state 

 in consequence of the influence of masses or veins of the 

 trap by which it is accompanied. 



The varieties in the mineral character of quartz rock 

 are numerous, but I shall limit this notice of them to the 

 most important and the best marked. 



It is occasionally, but rarely, found in a compact state, 

 and crystalline throughout ; little differing from quartz as 

 it occurs in veins; but even in these cases showing a 

 constant tendency to divide in parallel beds. More gene- 

 rally, when pure, it has an aspect obscurely granular, 

 which by degrees becomes somewhat lax and arenaceous ; 

 the grains varying in size and in the intimacy of their 

 union. In some of these examples it appears to be a gra- 

 nular crystallized mass ; in others it possesses a mixed 

 mechanical and chemical texture ; while in a third, the 

 rounded aspect of the grains, and the small number of the 

 points of contact, appear to indicate an origin chiefly 

 mechanical, and resulting from the agglutination 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 

 substances, in various proportions and of various sizes. In 

 every instance it has a foliated disposition; and, some- 

 times, even the texture is the same, since it splits into 

 laminae of extreme thinness. 



The next most remarkable variety is that in which the 

 quartz is intermixed with mica, but I have already said all 

 that is necessary on this subject when treating of its alter- 

 nation and affinity with micaceous schist. I may only add, 



