56 NEW YORK STATE MUSEUM 



blance between the Mahopac and the Storm King granites is chiefly 

 mineralogic and is most apparent in the microscope. (See plate 17 

 for microscopic appearance.) 



Although it would be possible to map this granite separately, it 

 was finally decided on the grounds of its evident close relation to 

 the Reservoir type, to consider it simply a facies of that invasion and 

 include these together as one map unit. 



Storm King granite. The Storm King granite is a medium to 

 coarse-grained rock, rather dark colored, slightly greenish and some- 

 times greasy looking, with a marked but crude gneissoid structure. 

 The feldspars are gray or red, the quartz is gray, and there is strong 

 black streaking of hornblende or augite. Biotite is not abundant. 

 Garnet occurs more in the marginal portions of the mass and is 

 probably produced by absorption of the invaded Grenyille. 



The quartz content varies from that proper to a normal granite 

 to very low, so that a considerable portion of the rock approaches 

 the composition of syenite. High quartz content and red feldspar 

 commonly occur together. The pegmatitic phase is a distinctly red 

 granite, very coarse-grained and not at all gneissoid, with lower 

 ferromagnesian content than the main mass of the rock. 



In thin section it is a coarse-grained rock with good granitoid 

 structure. The characteristic features are (i) intense pleochroism 

 of the biotite and hornblende which turn from green and light 

 yellow-brown to almost black, (2) abundance of microperthitic 

 intergrowths. 



The essential minerals are quartz (in some specimens), perthite, 

 microcline, oligoclase, biotite, hornblende, and light-brown augite 

 in the darker varieties. The accessories are a little garnet occasion- 

 ally associated with the hornblende, rather rounded zircon, apatite 

 and magnetite. Allanite is rare. The magnetite appears in grains 

 and also along the cleavages of augite (plates 18 and 19). 



The rock is fresh with no evidence of alteration except slight 

 sericitization of some of the feldspar, and development of chlorite 

 small, usually not over 5 or 6 feet wide. They are the latest of the 

 sort caused by regional deformation, but strain effects are 

 well shown in the quartz and feldspar. Crush zones, although rare, 

 are known in this formation. Some of them are so completely 

 healed as to form perfectly solid rock. 



Basalt, Diabase, Diorite. The basic dikes of the Highlands are 

 small, usually not over 5 or 6 feet wide. They are the latest of the 

 Precambrian rocks, cutting all the others, and are themselves quite 



