490 Eahle — Biotite-ting unite Dike from Massachusetts. 



istic moire appearance of anorthoclase is lacking, the sections 

 showing multiple extinctions and appearing more as perthitic 

 intergrowths than as homogeneous mixtures of the two feld- 

 spars. Carlsbad twinning is quite common. The visible 

 phenocrysts are brachypinacoidal plates of albite which show 

 basal and prismatic cleavage cracks. 



Nepheline is next to the feldspars in .amount and plays the 

 role of quartz, filling the interspaces formed by the network of 

 feldspar laths. Having been the last mineral to form, most of 

 it is consequently in xenomorphic, angular sections, but here 

 and there a well-defined hexagonal plate is seen. The sections 

 are mostly altered to a grayish, muddy, granulated material 

 which is apparently a mixture of nepheline with kaolin and 

 very fine granular quartz, the alteration proceeding to a 

 hydrous aluminium silicate, with a separation of some free 

 silica, rather than to a zeolite. They still retain their index 

 above that of the neighboring feldspar and gelatinize to some 

 extent with HC1, as shown by fuchsin staining. 



^Egirine is disseminated in the rock, in fragments and small 

 crystals, in sufficient amount to give the rock its slightly 

 greenish cast. Its crystallization preceded that of the feld- 

 spars and it is now present as rounded crystals or broken, 

 irregular fragments. The sections vary from deep grass-green 

 to almost colorless, and the more deeply colored show a marked 

 pleochroism, a — bluish green, 6 = grass green, c = greenish 

 yellow ; the axis of greatest elasticity lies nearest c, and the 

 extinction is practically parallel in most of the sections. 



Magnetite is common and marks the remains of plates of a 

 former dark silicate. Most of this original silicate has entirely 

 disappeared, leaving only the black patches of secondary mag- 

 netite, but an occasional section shows a greenish-brown min- 

 eral between the heavy. black borders, which from its absorption, 

 parallel extinction, and characteristic shimmer, is evidently 

 biotite or perhaps lepidomelane. From the similarity of all 

 the black sections it is reasonable to infer that they were origi- 

 nally this biotite, and since the analysis shows so little mag- 

 nesia in the rock, the biotite must have been very poor in this 

 oxide and high in iron. Washington notes the poverty of 

 magnesia in all of the rocks of Essex County, so far analyzed 

 by him. 



Sodalite is seen in small, colorless isotropic sections of low 

 refraction and showing dodecahedral cleavage. The amount 

 however is much too small to account for all of the chlorine in 

 the analysis, and it is quite probable that most of the chlorine 

 is due to impregnation from the sea water. None of the small 

 amount of isotropic mineral in the slide is believed to be anal- 

 cite, and the dike can hardly be included in the same class with 

 the one at Pickards Point. 



