Varieties of Granite. 



467 



are superadded. In Europe the basis of this rock is said to be fine 

 grained : but in Massachusetts it is more commonly rather coarse 

 grained. 



Perhaps the most remarkable porphyritic granite in the state oc- 

 curs in place in the west part of Harvard. (No. 1465.) It is a 

 gray rather coarse granite with white feldspar, and the imbedded 

 crystals are often two inches across ; and being white, they give to 

 the rock a striking appearance ; and it has actually been mistaken by 

 some writers for a conglomerate ! 



In Chester a large protruding mass of granite in the west part of 

 the town is porphyritic. The imbedded crystals are much less than 

 those just described, and of a gray color. The rock resembles por- 

 phyritic gneiss ; but lacks both the laminar and stratified structure. 

 (No. 1463.) 



Probably a part of the rock which I have described as porphyritic 

 sienite in the south east part of the state, as in Abington, N. Bridg- 

 water, Fair Haven, &c, may more properly be regarded as granite. 

 Indeed, as this rock is usually destitute of hornblende, perhaps the 

 whole should be regarded as granite. 



A very peculiar porphyritic granite occurs in the argillaceous slate 

 of Guilford Vt. just without the limits of Massachusetts. I should 

 have described this rock as a porphyry, were it not obviously a gran- 

 ite that has been partially fused. We can easily trace the gradations 

 from the perfect granite to a rock composed of compact feldspar and 

 imbedded masses of quartz. (Nos. 1467 to 1470.) At first we per- 

 ceive nothing peculiar, except that the granite exhibits occasional 

 spots of feldspar of nearly a milk white color, and a little indistinct- 

 ness in the foliated structure of the feldspar. At length the feldspar 

 becomes nearly all compact, and the mica, reduced in quantity, 

 is disseminated in the mass as well as the quartz. Finally the feld- 

 spar is perfectly compact and only grains of quartz appear in it. The 

 rock now begins to assume a slaty structure, and seems in fact to be ar- 

 gillaceous slate that has undergone a kind of fusion. The granite in 

 this case is fine grained and would form a beautiful stone for architec- 

 tural purposes. 



% It may be seen by the specimen No. 1 455. that the granite from 

 the quarries in Pelham N. Hampshire, exhibits a porphyroid aspect, 

 similar to that just described. But I have not visited the locality and 

 cannot say whether all the circumstances above described exist in 

 that place. 



4. Graphic Granite. (Nos. 1471 to 1480.) In this variety, which 



I 



