318 QUARTZ ROCK. 



gular fragments. The strata are occasionally, 

 but rarely, bent, and they never present those 

 contortions which are so common in micaceous 

 schist. 



With regard to the precise place which quartz 

 rock occupies among the primary strata, nothing 

 positive can be laid down ; since it is found al- 

 ternating with all those which follow granite. 



Under the head of gneiss it was already men- 

 tioned that this rock was frequently found alter- 

 nating with it, and that the limits of the two were 

 often undefinable. The variety which occurs in 

 this case, is generally that which contains felspar 

 as an ingredient, and the change is produced by 

 the loss of the other ingredients by which gneiss 

 is characterized. In this association, the beds of 

 quartz rock are often inferior in quantity to the 

 gneiss, or subordinate ; but alternations also 

 occur among strata of dimensions so extensive as 

 not to permit the use of this term. 



In many places it will be found to alternate in 

 thin strata with micaceous schist; and it was al- 

 ready shown that many of tjie rocks ranked with 

 this substance, contain but a very sparing quail- 



