326 



Scientific Geology. 



tains some mica, might have been originally a micaceous sandstone. 

 But more of this hypothesis as we proceed. 



It is gratifying to find that the quartz rock of Massachusetts cor- 

 responds so closely with that of Europe. But we shall find this to 

 be the case generally with our primary rocks ; a proof of uniformity 

 in the mode of their production. 



10. MICA SLATE. 



It is usual to place this rock next to gneiss, or as the second in re- 

 spect to age among the stratified rocks. And in Massachusetts it is 

 not unfrequently associated with gneiss. But it is also associated 

 with every other rock, as high in the series at least as argillaceous 

 slate : I mean in a conformable position. Hence I have thought it 

 best to introduce it before talcose and hornblende slate and serpentine ; 

 because these latter rocks, in the district under consideration, are con- 

 nected, scarcely without an exception, only with gneiss and the oldest 

 varieties of mica slate. 



Mineralogical Characters. 



It is hardly necessary to remark, in respect to a rock so common 

 and well known, that its essential ingredients are quartz and mica: 

 and the anomalies of composition are fewer in this rock than in most 

 others; although the varieties of aspect are numerous. As I under- 

 stand the subject, however, it is necessary that the mica should be the 

 predominant ingredient, in order to constitute a rock mica slate. But 

 in this case we must look to the whole mass of the rock, rather than 

 to hand specimens : for single specimens may often exhibit the quartz 

 in excess, and yet be regarded as mica slate. The following varie- 

 ties of this rock I have found in Massachusetts. 



1. Quartz and Mica: the former granular and laminar ; the lat- 

 ter in distinct scales and highly glistening. This variety is associ- 

 ated with the oldest rocks, as granite and gneiss ; and is obviously 

 more highly crystalline than the other varieties. The longitudinal 

 arrangement of the mica gives this variety sometimes a fibrous ap- 

 pearance. (Nos. 614 to 626.) 



2. The same, containing a small proportion of Feldspar, and thus 

 passing into gneiss. (Nos. 627 to 636.) It is only when the mica 

 greatly predominates that this rock can with any propriety be denom- 

 inated mica slate. 



