Pirsson and Washington — Geology of Bed Hill, N. H. 437 



Its especial interest lies in the fact that it is the same magma 

 as that which forms the main mass of the mountain, produc- 

 ing the same minerals but with an entirely different texture, 

 which can only be the result of its crystallization under quite 

 different physical conditions, such as would obtain, for instance, 

 at the contact with colder rocks or in a dike. It represents 

 therefore, as stated above, an excellent example of the aschistic 

 dikes of Brogger, or the granitic porphyritic ones of Rosen- 

 busch's classification. 



Grano-alaskose (Aplile). 



The presence on the eastern side of the mountain of aplitic 

 dikes associated with trap-like ones in the contact zone of 

 gneiss has been already mentioned. They are all of them 

 very clearly persalic rocks ; some indeed are quite devoid of 

 any femic or alferric minerals. They are fine-grained, and 

 composed essentially of feldspars and quartz. The relative 

 amounts of these two and the character of the feldspar are 

 the only features of especial interest concerning them, and a 

 specimen from a six-inch dike in the gneiss outcrops above the 

 highroad at the east foot of the north peak was selected as a 

 typical one for study and measurement to determine these 

 relations. 



Megascopic. — Holocrystalline ; grayish white ; speckled with 

 minute dark points of a black mineral. Fine-grained ; average 

 diameter about 0*25 mm . Of a sugar granular habit. Weath- 

 ers light brown. 



Microscopic. — Mostly alkalic feldspar and quartz granules. 

 A small amount of oligoclase is present and the mass is dotted 

 with shreds of biotite, largely altered. A few minute zircons 

 were seen. The feldspar is quite fresh ; the alkalic variety 

 shows considerable albite in microperthite intergrowths. The 

 average character of it is estimated to be Or 3 Ab 2 , but this, of 

 course, is only a rough approximation. 



Mode. — The rock is well adapted for measurement by the 

 Rosiwal method, which gives the following mineral composi- 

 tion : 



Vol. per cent. Wt. per cent. 



Quartz 37*5 37'9 



Alkalic feldspar 55-0 54-3 



Oligoclase 4*7 4-7 



Biotite 2-8 3*1 



Zircon trace trace 



Total 100-0 100-0 



