446 W. J. MILLER ADIRONDACK ANORTHOSITE 



"The most of the basic syenite, and all of the more gabbroic of it, is in close 

 association with the anorthosite border. . . . Now the syenite is unques- 

 tionably younger than the anorthosite, and the observed relations seem to 

 point to the conclusion that the change (in the syenite) is due to actual diges- 

 tion, by the molten syenite, of material from the (anorthosite) gabbro." " 



The Keene gneiss of the Lake Placid region differs in being coarser 

 grained, distinctly porphyritic, and not so rich in dark minerals; but 

 both Cushing's basic syenite and the Keene gneiss are intermediate in 

 position and composition between the anorthosite and the syenite-granite 

 in their respective regions, and I believe Cushing's suggested explanation 

 is the correct one. 



Another rock, earlier described by Cushing^^ from a railroad cut nearly 

 5 miles north of Tupper Lake Junction, is regarded by him as interme- 

 diate between the syenite and anorthosite. Judging by the description, 

 this rock is in most ways similar to the typical Keene gneiss, except that 

 the phenocrysts of labradorite are not so large. 



In Kemp's report on the geology of the Elizabethtovm-Port Henry 

 quadrangle,*^ he describes two peculiar types of gabbro with distinct 

 anorthosite affinities. One of these, called the Woolen Mill type, "is dark, 

 gneissoid, and of moderate coarseness of grain. It resembles a rather 

 basic member of the syenite series, but has occasional blue labradorite 

 phenocrysts which ally it with the anorthosites." The minerals contained 

 are green pyroxene, plagioclase, orthoclase, quartz, garnet, pyrrhotite, 

 apatite, and sometimes biotite and hornblende. Kemp states that this 

 sort of rock also occurs along the southern border of Blueberry Mountain. 

 Having seen this rock in the field, I quite confidently class it with the 

 Keene gneiss. He says that the rock called the Split Eock Falls type "is 

 suggestive of the anorthosite in that labradorite is the chief feldspar pres- 

 ent, but the dark silicates are more abundant, and when crushed and 

 sheared the rock yields a decidedly foliated gneiss. It then becomes a 

 hard 'dense rock, exceedingly tough. Nevertheless, large phenocrysts of 

 labradorite are not uncommon." Both of these types are demonstrably 

 younger than the anorthosite, the first showing an irruptive contact 

 against the anorthosite and the second containing inclusions of anortho- 

 site. The fact that these rocks are intrusive into the anorthosite har- 

 monizes with my own observations (see above) on the Keene gneiss— that 

 is to say, in such cases the Keene gneiss magma developed as an assimila- 

 tion product at a lower level and was then forced upward in some places 



« H. p. Gushing : N. Y. State Mus. Bull. 115, 1907, p. 479. 



« H. P. Gushing : N. Y. State Mus. Kept. 54, vol. 1, 1902, pp. r43 and r68. 



« J. F. Kemp : N. Y. State Mus. Bull. 138, 1910, pp. 37-40. 



