The Monteregian Hills. 219 



following minerals : hornblende, pyroxene, biotite, olivine, 

 plagioclase, nepheline, sodalite, apatite, magnetite, sphene, 

 and in some cases a very small amount of orthoclase. 



There is a marked tendency on the part of all the 

 constituents to assume an idiomorphic development. The 

 long lath-shaped plagioclases and large hornblende indi- 

 viduals have an approximately parallel arrangement, and 

 between these lie the other iron-magnesia constituents 

 with the smaller plagioclase individuals, the nepheline 

 and the other components of the rock. These interstitial 

 constituents do not differ greatly in size from the others, 

 and show the same tendency to a parallel arrangement. 



Hornblende. — Although almost every thin section of 

 the rock contains not only hornblende, but pyroxene and 

 biotite also, their relative proportion varies considerably. 

 The hornblende is distinctly the most abundant, except in 

 the finer-grained variety forming the summit of the 

 mountain in which it is distinctly subordinate in amount 

 to both pyroxene and mica. It is deep brown in color and is 

 sometimes hypidiomorphic in its development, but often 

 occurs with perfect crystalline form, showing the prismatic 

 and the orthopinacoidal faces. Its extinction is larger 

 than is usual in brown horneblendes, judging from the 

 recorded instances, reaching 20°. It possesses a strong 

 pleochroisni as follows : 



a = pale yellowish-brown. 

 b = deep-brown. 

 C = very deep-brown. 

 Absorption = C > b > 3. 



It is often twinned parallel to coPco or to a steep 

 orthodome, and sometimes presents a faint zonal structure, 

 marked by a slight difference in extinction [of the several 

 zones indicating a slight change in composition as growth 

 proceeded, and occasionally a greenish tint is noticeable 

 about the margin of the individual. It sometimes holds 

 inclusions of magnetite and is often intergrown with the 

 pyroxene. In the essexite from one place on the south 



