186 



the two rocks are found, as is not uncommon, 

 in alternation. The variety of gneiss thus pro- 

 duced, is described in its proper place. 



Independently of the association which mica- 

 ceous schist forms with gneiss, it is intermixed 

 with, and passes, by similarly easy gradations, 

 into the following rocks. 



With quartz rock it is so frequently associated, 

 and in beds so thin, that the two cannot be dis- 

 tinguished in the general descriptions of a tract of 

 country. As the ingredients of one of the lead- 

 ing varieties of quartz rock, are the same as those 

 which constitute the essential character of mica- 

 ceous schist, it requires only a change of propor- 

 tion to convert the former into the latter, or the 

 reverse ; and that change is of such frequent oc- 

 currence, as to produce endless examples where 

 neither term is more applicable than the other. 



Where talcose schist occurs in union with 

 micaceous schist, as it sometimes does, there is 

 frequently an indefinite transition between the 

 two; the mica, either superseding the talc in the 



