200 MINERALOGY AND LITHOLOGY. 



been compared to pepper and salt. These little black specks are found, 

 on microscopic examination, to be in part biotite and in part hornblende, 

 though, as in the last class, these ingredients are variable, both in their 

 amount as relates to one another, and to the mass of the rock. At some 

 points, as, for example, at Jackson falls, these rocks are characterized by 

 their frequent inclusion of fragments of other rocks. This circumstance, 

 among others, has caused them to be all considered as eruptive. Though 

 this may not be improbable, this circumstance of itself is not sufficient to 

 prove the eruptive origin of a granite, for it has been more than once 

 pointed out that some granites are partially fragmental in their nature. 

 The little grains of hornblende in thin sections often appear quite well 

 crystallized. Plagioclase, and little crystals of apatite, are conspicuous. 



A very peculiar granite has been cut by the railroad at Bemis station. 

 It is fine in its texture, and light gray in color. In thin sections its 

 accessories are found to be white hornblende and muscovite. Besides 

 these, sphene, in irregular grains, is abundantly scattered through it. 

 Plagioclase, pyrite, and pyrrhotite are also ingredients. This rock is, 

 so far as I am acquainted, an isolated occurrence. 



A granitic rock from Portsmouth contains green dichroic hornblende 

 and muscovite, but so much plagioclase that it brings to mind the quartz 

 diorites that are found in the region. 



Hornblende Granites. The granites in which hornblende alone ap- 

 pears as the characteristic accessory are fewer in number than the 

 last. Prominent among these is the Chocorua granite. In thin sec- 

 tions, the hornblende of this granite is very dark in color, and strongly 

 dichroic. A little scale of mica is occasionally found. As accessory 

 constituents, titanic iron, hematite in blood red, translucent grains, fluor 

 spar in grains with an attempt at crystallization, and epidote in grains 

 with a most delicate feathery fringe, are present. The orthoclase is 

 quite pure, but is made up of those irregular laminae which are repre- 

 sented in Fig. I on PI. 8, and which are only seen in polarized light. 

 They result from the circumstance that in the two interlaminated parts 

 of the crystal the axes of elasticity are somewhat different in their ar- 

 rangement, and hence the interference colors obtained in any given posi- 

 tion between the Nicol prisms are different. This granite has a greenish 

 tint, which is imparted to it by epidote. 



