THE MONTEREGIAN HILLS 26 1 



augites which occur in rocks of this class. The extinction is 

 high, reaching 45°. 



Biotite. — This is deep-brown and almost identical in color 

 with the hornblende and is strongly pleochroic, C yellowish- 

 brown, and ^ deep-brown. It occurs intimately associated with 

 the hornblende and augite, and also frequently as a border 

 around the iron ore. While usually present in comparatively small 

 amount, in the finer-grained essexite forming the summit of the 

 mountain it is much more abundant than the hornblende. In 

 this variety of the essexite both the mica and the hornblende 

 often possess a poikilitic structure owing to the presence of 

 numerous inclusions of plagioclase, which mineral also often 

 penetrates the individuals of biotite and hornblende in the form 

 of well-developed crystals. 



Olivine. — This species is found in the finer-grained variety of 

 the essexite at the summit of the mountain, and was also 

 observed in the thin sections from the essexite at one point on 

 the east side of the mountain not far from the summit. It is 

 very pale-green in color and occurs as little grains inclosed in 

 the biotite and pyroxene. 



Plagioclase. — The plagioclase in the rock has well-developed, 

 lath-like forms and is, almost without exception, excellently 

 twinned according to the albite law. Twinning according to 

 the Carlsbad and pericline laws is also very common, occurring in 

 the same individuals which show the albite twinning. The laths 

 of plagioclase can in a few cases be seen to be distinctly twisted, 

 evidently owing to pressure exerted upon them bv other crys- 

 tals during the consolidation of the rock, since the rock was 

 submitted to no dynamic action subsequent to its crystallization. 



As before mentioned, all the plagioclase individuals are not 

 of the same dimensions. There are larger laths associated with 

 the large hornblende crystals, and between these are smaller 

 laths. The two sets are not, however, sufificiently well marked 

 to cause the resulting structure to be classed as porphyritic. 

 The plagioclase in the rock is not all of the same composition, 

 but varies somewhat, even in the same hand specimen, ranging 

 from an extremely acid labradorite to an oligoclase. It, how- 



