LITH0LOGY. 201 



Another of these hornblendic rocks from the top of Mt. Carrigain is 

 very pecuhar in its microscopic structure. Macroscopically, this rock is 

 fine in texture and dark in color, and its individual constituents cannot 

 be determined. Under the microscope its thin sections show that the 

 dark constituent is greenish hornblende, though it is partially altered 

 into epidote and chlorite; but the circumstance of note is, that many of 

 the minute grains of feldspar are inlaid with quartz, in the manner pecu- 

 liar to graphic granite. The rock is hence a kind of microscopic peg- 

 matite. The figures that are formed are often of rare perfection. A 

 grain of the orthoclase, much magnified, is represented in Fig. 3 on 

 PI. xi. Mr. Michel Levy* found this character in quite a number of 

 French granites (granitoid porphyries, etc.) and considered this charac- 

 ter, among others, in attempting to find constant relationships between 

 microscopic characters and geological age. The pegmatitic character of 

 a rock he regards as evidence of the simultaneous crystallization of 

 quartz and feldspar, and this character separates such rocks from those 

 in which one clearly solidified before the other, and indicates in a degree 

 an approach toward those characters of crystallization that are peculiar 

 to newly erupted rocks. Our rocks have not been sufficiently studied to 

 enable us to say anything about such a matter; but though such investi- 

 gations are very interesting, the results are, as yet, at least hypothetical, 



A very fine-grained hornblende granite is found in a well defined dyke, 

 which cuts vertically through the coarse-grained gneissoid rocks on the 

 Swift river in Albany. 



A light-colored hornblendic granite comes from Stark ; a red variety 

 comes from Jackson; — but an enumeration of any peculiarities that I 

 have observed in these or other rocks of this class would be a mere 

 repetition of what has gone before. 



Protogene. This is a name given to those granites which, as an acces- 

 sory, contain chlorite, talc, rotten mica, or other decomposition products, 

 and which are made characteristically green by the presence of one of 

 these ingredients. In the neighborhood of Littleton such rocks reach a 

 considerable development. They are mainly composed of quartz, ortho- 

 clase, and chlorite, but in thin sections some hornblende is also seen. 

 The little plagioclase that the rock contains is much altered. This 



* Bulletin de la Soc. Geol. tie France, 1875, p. 199. 

 VOL. IV. 26 



