154 GEOLOGICAL AND 



oped to some extent, and numerous inclusions of acicular 

 microliths, which sink to the finest dust-like forms, fill 

 this whole surface. Some of the largest of them I found 

 to be hornblende and others are pyroxene. There are also 

 some fluid and quartz inclusions. The bluish color and 

 iridescence of this feldspar is ascribed to the orderly ar- 

 rangement of these microliths and interpositions. There 

 are some orthoclase and biotite and the hornblende is filled 

 with minute grains of magnetite and rutile. Sections cut 

 from the gabbro at Davis neck, Bay View, Gloucester, 

 are identical in character with this last. Other sections 

 from House island have olivine in place of hypersthene 

 and in one section I find the biotite to be completely 

 bleached. There are in this region numerous noncrys- 

 talline diabase dykes, some of which are cut by the elreo- 

 lite-syenite, and others that as distinctly cut the syenite. 

 At Woodbury's point on the Beverly shore this syenite is 

 cut by a coarse porphyritic diabase which contains feld- 

 spar crystals that are from three to six inches long ; and 

 cutting through this dyke, and also cutting the syenite, is 

 a dyke of ryolitic granite (granophyre, of Prof. Rosen- 

 busch) that is probably the last formed rock in the region. 

 Thin sections studied show it to be composed of quartz, 

 orthoclase and biotite with perfect micro-crystals of horn- 

 blende which sink to dust-like proportions, very abundant 

 as inclusions both in the quartz and orthoclase. There are 

 also some zircons and magnetite inclusions in the biotite. 

 Some of the hornblende microliths are of the blue glauco- 

 phane variety. 



Several thin sections of the micro-granite veins that cut 

 the elajolite-syenite, when studied with the polarizing 

 microscope, are seen to be composed of orthoclase, some 

 glassy plagioclase crystals, quartz veins due to segregation 

 in part, epidote, numerous plates of polysynthetic twinned 



