I58 NEW YORK STATE MUSEUM 



mens, the microscope reveals the presence of augite and hyper- 

 sthene in small amounts and usually some magnetite or ilmenite. 

 In portions of the mass that show effects of crushing in the breaking 

 down or mashing of the feldspar, there is found a development 

 of secondary minerals such as garnet, biotite and calcite. The 

 garnet has a tendency to form aggregates about the magnetite, 

 which owing to their red color stand out plainly from their sur- 

 roundings. The mineral has drawn upon the magnetite for the 

 iron and the feldspar for the lime and silica necessary to its growth. 

 Biotite is not so common as the garnet with which it is closely 

 associated and no doubt genetically related. The following analysis, 

 quoted from a paper by Prof. Albert R. Leeds, will serve to 

 show the chemical character of the anorthosite. The sample from 

 which it was made was taken from the summit of Mt Marcy. 



Si0 2 54 



A1 2 3 26 



Fe 2 0< ' * 



FeO 



CaO 10 



MgO 



K 2 



Na 2 4 



H 2 



47 



45 



3 



66 



80 



69 



92 



37 



53 



100. 19 



In places the typical anorthosite, as above described, gives way 

 to a much finer grained rock in which the pyroxene minerals are 

 more prominent, showing a transition to gabbro. With this 

 mineralogic change the feldspar individuals decrease in size and 

 number and the color becomes greenish. Such phases are apt to 

 have a gneissoid texture as they are less resistant to metamorphic 

 influences than the anorthosite, a feature that is illustrated as well 

 by the development of hornblende in the place of the pyroxene 

 minerals and of abundant garnet. " They are undoubtedly a differen- 

 tial product of the anorthosite, but it is not always clear whether 

 the gabbro has separated in place or has come up through the 

 anorthosite in the form of dikes. 



Ore bodies. Ore is found in both the anorthosite and the gabbro. 

 The anorthosite is the commoner wall rock and the ore bodies 

 which it incloses have perhaps the greater possibilities for commer- 

 cial utilization, due to their uniform character and higher average of 



