J21S JURA, -^-QUARTZ ROCK. 



from its stratified arrangement, is capable of being readily 

 quarried. From the same cause it can also frequently be 

 procured in parallel beds of various thickness ; while, often 

 possessing natural joints like many other schistose rocks, 

 it breaks readily into fragments of a rhomboidal, if not of 

 a rectangular shape, which admit of being easily trimmed 

 by the hammer. The difficulty of producing a smooth 

 surface by the pick, might possibly limit its uses to certain 

 classes of masonry ; but in those, the facility of giving it a 

 form fit for building, would render it an economical sub- 

 stitute for granite, which it also far excels in durability. 

 As a lining for furnaces it must also be of great use ; 

 undergoing no alteration except at the very surfaces of 

 contact with the other earths which it may meet in the 

 fire. 



It has appeared from the preceding remarks, that quartz 

 rock is a member of the primary strata. With regard to 

 the precise place which it occupies among these, nothing 

 positive can be laid down ; since it is found alternating 

 with all those which follow granite. But as the doctrine 

 of an invariable order among these rocks is unfounded, 

 it is no more subject to uncertainty than limestone, gneiss, 

 micaceous schist, clay slate, or graywacke ; among which 

 it holds a place, and with all of which it alternates. In 

 Scotland however it appears to occur most frequently 

 in company with micaceous schist, with gneiss, and with 

 red primary sandstone. 



In many places it will be found to alternate regularly 

 with micaceous schist ; and it i$ well known that many 

 rocks, generally ranked with this substance, contain but a 

 very sparing quantity of mica, while their predominant 

 ingredient is quartz in various states of mixture. Such 

 varieties, holding an intermediate state between two rocks, 

 have an equal claim to be ranked under either ; but as it 

 is a fruitless attempt to define that to which Nature has 

 not set bounds, and as no advantages are gained by multi- 

 plying tcims to express such gradations, it is most proper 



