THE MONTEREGIAN HILLS 263 



SiOo ------ 62.05 



AI2O3 - - - - . 22.60 



Fe^Oa ----- .75 



CaO .-.--- 3.g6 



NaaO ----- 7.g5 



K2O ------ 1.80 



Volatile ----- .80 



99.91 

 Sp. G.= 2.659. 



This feldspar has the specific gravity and general composi- 

 tion of an acid andesine, although the high content of K3O may 

 possibly indicate the presence of some potash feldspar as an 

 intergrowth. 



Nepheline. — This is quite subordinate to the feldspar in 

 amount. It possesses the usual low index of refraction, with 

 extinction parallel to the cleavages, which latter can usually be 

 seen. It is sometimes quite fresh, but at other times is found 

 more or less completely altered to a mineral which occurs as 

 little fibrous bundles, showing strong double refraction and 

 parallel extinction. The fibers usually have a more or less dis- 

 tinctly parallel arrangement. This mineral remains practically 

 unaltered when treated with concentrated hydrochloric acid for 

 twenty minutes, although the nepheline in which it is imbedded 

 is destroyed. It is either muscovite or kaolin. The nepheline 

 is allotriomorphic and occurs chiefly in the corners between the 

 larger crystals of feldspar and other minerals, and is penetrated 

 by them. It is especially abundant in those portions of the rock 

 which are rich in the dark-colored constituents. When occur- 

 ring in this manner it appears, with the sodalite, to have been 

 the last constituent of the rock to crystallize out. It is usually 

 much more abundant than the sodalite. The nepheline also 

 occurs in places as irregular-shaped lath-like inclusions in the 

 feldspar. 



Sodalite is usually, although not invariably, present. It 

 strongly resembles the nepheline in appearance and shows the 

 same alteration product. It is, however, quite isotropic. Like 

 the nepheline, it occurs either in the spaces between the other 



