Wadsworth.] 312 [February 20, 



crystals of considerable size, and sometimes in plates several inches 

 in diameter. The vein, from which most of the best specimens have 

 been taken, is an offshoot of one of the great trap dikes which cross 

 the Cape from north to south nearly parallel to each other, and con- 

 sists chiefly of massive quartz and feldspar almost completely segre- 

 gated, the quartz lining the lower wall, while the feldspar lines the 

 hanging wall of the vein." 



Dr. T. Sterry Hunt also says 1 : — "The hornblendic granites of 

 Gloucester, Salem, and Quincy, Massachusetts, seem also, from their 

 lithological characters, to belong to the class of exotic or true erup- 

 tive granites." Again 2 : — " The fine green feldspar of Cape Ann, 

 Massachusetts, and the micas, cryophyllire and lepidomelane. with 

 zircon, described by Professor Cooke, from the same region, occur in 

 veins in the hornblendic granites of that locality." 



The last writer upon the geology of this region, Mr. W. O. Crosby, 

 makes the following statement 3 : — " The typical hornblendic granite 

 of this region, as shown at the quarries in Quincy, Rockport, and 

 other places, is a coarsely crystalline aggregate of orthoclase, quartz, 

 and hornblende. The hornblende is usually small in amount, and the 

 rock frequently passes, through the disappearance of hornblende, 

 into binary granite. The feldspar is usually grayish or bluish, though 

 red and green tints are frequently met with. It is worthy of note 

 that this rock is destitute of mica, or at least its presence is a very 

 rare occurrence. This typical granite frequently passes into finer 

 grained varieties, which, when hornblende is absent, pass through 

 eurite into felsite. The more hornblendic varieties are usually fine 

 grained; and the increase of hornblende is attended by a diminu- 

 tion of quartz, so that the rock exhibits, through the entire absence 

 of quartz, frequent passages into diorite." 



It seems strange that experienced observers, who have all person- 

 ally examined this region, should make so diverse statements, espe- 

 cially, as at least ninety-five per cent, of the stone brought from 

 Rockport to this vicinity, for architectural purposes, is micaceous 

 and destitute of hornblende. Examples can be seen in the basement 

 walls of the building of this Society, and in those of the Massachu- 

 setts Institute of Technology, or upon almost any street in Boston. 

 In Cambridge, Boylston Hall will serve as a- typical example of 



1 Chemical and Geological Essays, page 188. 



2 Chemical and Geological Essays, page 200. 



3 Report on the Geological Map of Massachusetts, page 13. 1876. 



