ROGERS, GEOLOGY OF THE CORTLANDT SERIES 29 



tite is not very abundant. Several small grains of what seemed to be 

 quartz were seen. 



A small area of a very simple gabbro lies in the extreme southeastern 

 corner of the district, where it is intimately associated with diorite and 

 biotite augite norite. The gabbro phase is composed entirely of augite 

 and feldspar. This rock, or more probably a mixture of the three, was 

 used for the basement work of the Cornell Dam across the Croton River, 

 but while the crushing strength was all that could be desired, the abun- 

 dance of chloritized joints was found to affect seriously the value of the 

 large blocks required, so that the engineers were finally forced to employ 

 the granite described above. A large quarry, however, was opened, and 

 most of it still remains above the level of the lake. 



Another type of gabbro is found, however, which is very different. It 

 occurs in two localities adjoining schist inclusions; one at the north end 

 of Lake Meahach and the other just west of Montrose. In the field, it 

 closely resembles limestone, being gray and homogeneous in appearance 

 and weathering in layers. Under the microscope, it appears that the 

 rock is made up chiefly of two minerals, feldspar and augite. The latter 

 is brownish but is non-pleochroic and otherwise typical. Biotite, apatite, 

 ilmenite and titanite are present in small amount. 



The most extraordinary feature of the rock is the amount of shearing 

 which it has undergone, which far exceeds anything noted elsewhere in 

 the series. The augite is distributed in bands, in some of which the min- 

 eral is actually granulated, and these surround "augen" of broken augite 

 and feldspar. This lenticular effect is visible in the specimen, especially 

 when polished. The whole rock resembles a badly crushed gneiss, rather 

 than any of the other members of the Cortlandt Series, and it may pos- 

 sibly represent one of the inclusions of Highland gneiss which are found 

 elsewhere in the district. Since, however, it seems to be associated with 

 the schist inclusion, it is probably a true igneous rock, from the strained 

 and crushed zone which might be expected to border such a foreign mass, 

 and movement of the latter just before the consolidation of the igneous 

 rock may have contributed to this extraordinary crushing. 



Norite 



The norites are perhaps a trifle more abundant than any other group, 

 although they do not, as Williams and Dana supposed, constitute the 

 great bulk of the series. The general norite magma exhibits several 

 facies, which are distinct rock types and which yet often pass into one 

 another by insensible gradations. That the classification made upon a 



