Varieties of Quartz Rock. 



173 



map. The order which I now follow, and which agrees with that 

 given on the Tabular View of the Rocks in the Atlas, accords, as 

 nearly as I can determine it, with the order of nature : whereas on 

 the map, I was obliged to have some reference to convenience of ex- 

 hibition ; one of the groups being miscellaneous, 



Mineralogical Characters. 



Quartz rock was first described by Dr. Macculloch ; and its chief 

 ingredient, as its name implies, is quartz. But it takes into its com- 

 position, mica, feldspar, and sometimes blue schistose clay. The fol- 

 lowing varieties are found in Massachusetts. 



1. Pure Quartz. This exists in several states. First, hyaline, 

 white : generally in beds in mica slate. Secondly, compact, white, or 

 reddish, or dark blue ; in beds in argillaceous slate. It is quite ob- 

 vious that this blue variety has, in some way or other, been coloured 

 by the slate ; either when first deposited from aqueous solution, or 

 when subsequently melted, if it ever has been, by heat. Thirdly, 

 course, granular, color gray, or reddish. These are the most com- 

 mon varieties. Fourthly, fine, granular, or arenaceous ; sometimes 

 disintegrated so as to form a beautiful white sand. (Nos. 545 to 565.) 



2. Granular Porous Quartz, with occasional fragments of foliated 

 feldspar and black hornblende. This is the rock descrcibed in the 

 first part of my Report, under the name of buhrstone ; for which it is 

 used. It is certainly a remarkable rock. Without close inspection 

 it would not be distingushed from the Paris buhrstone. But it is in 

 fact finely granular ; and what is most remarkable, is the occurrence 

 in it of occasional fragments of foliated feldspar and hornblende, rare- 

 ly larger than a pea, and often not larger than the head of a pin. 

 These have evidently been subject to attrition ; and they give to the 

 rock somewhat of a mechanical texture. I have observed also in this 

 rock, traces of mica slate: and the longest direction of the pores cor- 

 responds with that of the strata seams, and gives the rock a slight 

 claim to a slaty structure. The pores or cavities are rarely large, 

 but they are disseminated through the whole rock ; and most frequent- 

 ly coated with a yellowish substance, which may be hydrate of iron. 

 The quartz in these cavities is slightly mamillary sometimes, show- 

 ing a tendency to crystallization. (No. 567.) 



3. Quartz and Feldspar; the former in much the largest quantity. 

 This variety usually occurs in connection with gneiss, and not in 

 large quantity. (Nos. 571, 593, 600.) 



