62 NEW YORK STATE MUSEUM 



Special descriptions. No. 65 of taible 6 is very typical of a 

 medium-grained diabase without olivine from a dike 30 feet wide 

 with fine-grained margins. The diabasic texture is plainly visible 

 to the naked eye. Under the microscope the labradorite is seen to 

 be in distinct laths, the biotite pale yellow to deep brown, and the 

 augite pale brown with good cleavages. This rock is lighter gray 

 than the usual diabase of the quadrangle. 



In the southern part of the gorge of the river one-half of a mile 

 south of Keene, a dike (no. 9) fully 5 feet wide cuts the mixture 

 of granite and anorthosite. It stands in a vertical position, is trace- 

 able for fully 100 yards, and shows a fairly good columnar struc- 

 ture. The rock is really an olivine basalt porphyry. It is repre- 

 sented by no. 66 of table 6. Two generations of crystals are evi- 

 dent, all of the olivine and some of the augite belonging to an 

 earlier period of crystallization, these two minerals clearly standing 

 out as phenocrysts (black and pale yellowish green respectively) 

 up to nearly a centimeter across. The groundmass is very fine 

 grained, dull black, and without a diabasic texture. Under the 

 microscope the olivine is colorless with crystal boundaries fairly 

 well defined, and it shows some alteration to serpentine and hema- 

 tite along the fractures. In thin section the augite is pale brown, 

 mostly as rather idiomorphic crystals with good cleavages, and 

 often with excellent zonal structures. The biotite is in the form of 

 slender prisms with light to dark-brown pleochroism. The biotite 

 evidently crystallized before the feldspar because the latter fills 

 spaces between the biotite crystals. Much of the labradorite is in 

 the form of small lath-shaped crystals. 



In the gorge of West Branch Ausable river at High fall, there 

 are several dikes (probably branches of a single large one) sharply 

 cutting the granite parallel to its jointing. They are traceable for 

 fully 100 yards in the walls of the gorge, the largest dike being 

 about 5 feet wide. Just above High fall one of the dikes is brec- 

 ciated. No. 61 of table 6 gives its mineral content. The rock under 

 the microscope, is seen to be badly decomposed, which accounts for 

 the large amount of secondary calcite. 



On top of the ridge i mile east of Upper Jay there are seven or 

 eight short dikes from i inch to i foot wide and roughly parallel, 

 as indicated on the map. The largest is exposed for 75 feet. The 

 middle ones are slightly faulted in a number oif places. 



A dike 4 or 5 feet wide is beautifully exposed at the lower end 

 of The Flume where it cuts the Wliiteface anorthosite vertically. 

 It contains some inclusions of the anorthosite. 



