I 



20 NEW YORK STATE MUSEUM 



miles northeast of Upper Jay where is to be found one type of 

 anorthosite made up of crystals of labradorite from one to several 

 inches across with evidence of only moderate crushing, along with 

 another type in which only occasional large rounded cores are left, 

 and still other types in which practically all the feldspar has been 

 granulated into a fine to medium-grained rock. These types are 

 in zones which show perfect gradations from one extreme to the 

 other. 



Most of the typical Marcy anorthosite is practically devoid of 

 foliation, hence the general absence of dip and strike signs from 

 these areas on the geologic map. In some places, where the rock _ ^ 

 is only moderately coarse grained, there is a noticeable tendency ■ 1 

 for the feldspars to show a crude parallelism. It is usually impos- 

 sible to determine satisfactorily the dip and strike of such foliation _ 

 in ordinary outcrops in the woods. The more gabbroic phases of ■ I 

 the rock do, however, often exhibit a fair to well-defined foliation 

 due to the parallel arrangement of the dark-colored minerals. An _ 

 important consideration is the frequent gradation from well- 1 1 

 foliated to slightly or nonfoliated anorthosites or anorthosite- 

 gabbros within short distances, often not more than a few rods. 

 The causes of the foliation and granulation of the anorthosite are 

 evidently closely related and this matter is considered below in 

 the chapter on Structural Geology. 



Microscopic features. In accompanying table i, the six thin 

 sections were selected to illustrate the usual mineralogical varia- 

 tions of the Marcy anorthosite. About a dozen mineral species in 

 all were noted. By far most of the feldspar is seen, under the 

 microscope, to be striated labradorite, to possibly bytownite in 

 some cases. Where more acidic plagioclase is present, it is always 

 in subordinate amount. In thin section, with a low power of the 

 microscope, the larger labradorites are usually seen to be more or 

 less filled with very dark dustlike particles. With a higher power 

 these are seen to be practically opaque, slender prismatic, or some- 

 times tabular, forms with parallel arrangement often strung out 

 parallel to the twinning bands of the feldspar. Professors Gushing 

 and Kemp, who have noted such inclusions, think they are most 

 likely ilmenite. They no doubt give the dark color to the labra- 

 dorite. Monoclinic greenish gray pyroxene with good cleavage — 

 usually augite but sometimes diallage — appears in all the slides. 

 The chlorite in slide 5 was quite certainly derived from pyroxene. 

 The hornblende exhibits good cleavages and pleochroism from 



