452 



Scientific Geology. 



Sinai in Arabia is composed of genuine sienite, he proposed to change 

 sienite into Sinaite : But geologists have unfortunately neglected the 

 hint. 



Miner cblogical Characters. 



1. Feldspar and Hornblende. This differs from greenstone by the 

 predominance of the feldspar : and yet it is obvious that in respect to 

 many specimens, and even large deposits, it is difficult to decide 

 whether they should be referred to the former or the latter rock. In 

 almost all cases both the ingredients are more or less crystalline; 

 though sometimes, as at Nahant, the feldspar seems to be passing to 

 a compact state. The hornblende is almost universally black ; the 

 feldspar white, greenish, and yellowish. (Nos. 1271 to 1285.) 



I apprehend that by a careful examination of the specimens of this 

 variety, nearly all of them would be found to contain more or less 

 quartz : but quartz and feldspar, when seen through a microscope, re- 

 semble each other so much, that it is not easy to decide. At any 

 rate, this variety insensibly admits quartz and mica into its composi- 

 tion, and thus approximates to granite. It is hardly necessary to say, 

 that on the other hand it passes into greenstone. This variety, al- 

 though elegant, is rarely wrought for ornamental purposes, on ac- 

 count of the proverbial toughness of the hornblende. 



2. Feldspar, Quartz, and Hornblende. This variety embraces 

 nearly all the sienite in the State that is employed for architectural 

 purposes, including the quarries at Gluincy and those at Cape Ann. 

 Feldspar is the most abundant ingredient. This is foliated, and com- 

 monly of a grayish, bluish, or yellowish color. A hyaline quartz, va- 

 rying in color from quite light to quite dark gray, is very uniformly 

 mixed Avith the feldspar, so as to exhibit homogeneousness in the midst 

 of variety. In general, the hornblende, which is black, is very sparing 

 ly disseminated, and hand specimens often contain not a particle. In- 

 deed, over extensive tracts I have sometimes not met with any. Hence 

 I regard this hornblende as an unessential ingredient; and conse- 

 quently have arranged under this variety a rock very common in the 

 vicinity of Boston, differing from that just described only by the ab- 

 sence of hornblende. It is most common a considerable distance 

 south of Boston in the counties of Norfolk and Plymouth. In some in- 

 stances, as may be seen by the specimens, Nos 1286 to 1308, the feld- 

 spar is flesh red, or a lilac red, and in others of a blood red color. 

 Its great resemblance in structure and composition to the Quincy 



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