ROGERS, GEOLOGY OF THE CORTLANDT SERIES 25 



The sodalite syenite is a very fine grained black rock, in which biotite 

 is the only mineral visible. It is somewhat brecciated and is considerably 

 altered. Under the microscope, the biotite is seen to be subordinate in 

 amount to a green hornblende, the crystals of which are often dislocated 

 across the cleavage, leaving cracks filled with an isotropic substance. 

 There is 'little orthoclase, moderately altered, and no plagioclase. The 

 sodalite is fairly abundant, and is typically developed, in rounded iso- 

 tropic grains, n< 1.54. It has none of the structures of analcite, and 

 the latter moreover could hardly form in so acid a rock and one in which 

 the orthoclase is so slightly altered. Somewhat more abundant than the 

 sodalite is a mineral which seems difficult of identification. It occurs in 

 grains which are often hexagonal, having a relief of about 1.51 and a 

 medium (second order) birefringence. It is biaxial positive. Its hex- 

 agonal outline would suggest some derivative of nephelite, which would 

 be expected in this association, but it is not cancrinite or any of the other 

 common alteration products. Thomsonite usually occurs in a more 

 fibrous form, although except for this, the characters correspond rather 

 closely. The exact determination of this mineral, however, would hardly 

 affect the name given the rock, which stands as another suggestive occur- 

 rence of a zone on the border of the mica schist. 



Diorite 



The diorites are more important in the western part of the district, 

 where they cover an area of about two miles. Since Dana and Williams 

 worked chiefly in this section, the diorites appeared more important to 

 them than they really are, for, except for this area, they appear only as 

 small isolated patches. There is one at Pleasantside and another about 

 half a mile southeast of that place; three along the southern border of 

 the series, and a confused mass on Montrose Point. On Stony Point, 

 across the river, they are found. There is also a garnetiferous phase a 

 mile and a half east of Pleasantside associated with the emery ; and dio- 

 rites, often very micaceous, constitute the immediate wall rock in several 

 of the emery cuttings on the hill just east-northeast of that village. 



As stated above, Williams subdivides the diorites entirely on the basis 

 of their dark minerals, arriving thereby at brown and green hornblende 

 diorites, mica-hornblende diorite and mica diorite. He states in this 

 connection that the brown hornblende diorites tend to pass into norites 

 and pyroxenites, while those with green hornblende show aspecial affin- 

 ity for the mica-bearing rocks. These observations are quite correct; 

 there is every gradation from a diorite whose sole ferromagncsian mineral 



