28 I NEW YORK STATE MUSEUM 



Gabbro 



Five gabbro areas are indicated on the geologic map. These 

 rocks are all considered to belong to the later Precambric (Post- 

 syenite) series of gabbros now well known throughout the Adiron- 

 dacks. Within the quadrangle, as usual elsewhere, these rocks show 

 decided variations, especially textural and structural, and this, to- 

 gether with the lack of definite relationships with the surrounding 

 rocks in certain of the areas, causes some uncertainty in regarding 

 them all as of the same age. All these rocks, however, are true 

 gabbros with certain general characteristics (for example, com- 

 position) in common, and they are even less variable than the finely 

 exhibited gabbros of the North Creek quadrangle recently discussed 

 by the writer.^ 



In appearance some, and in composition all, of these gabbros 

 most nearly resemble the basic phases of the syenite. The greater 

 proportion (about 40 per cent) of dark-colored minerals with 

 hypersthene usually prominent; nearly constant presence of am- 

 phibolite facies; and the occasional diabasic texture are rather 

 characteristic of the gabbros as against the basic phases of the 

 syenite. 



The most typical gabbro occurs in the dike a mile east of the 

 south end of Wells village. This dike shows a width of about 

 40 feet and a length of about 300 feet, with nearly east-west 

 strike parallel to the foliation of the country rock. The main body 

 (interior) of the dike is medium grained, devoid of foliation, and 

 has a good diabasic texture. On the borders there is some very 

 gneissoid amphibolite. Several small pegmatite dikes cut the gabbro. 

 The country rock is quartz-hornblende syenite with which the 

 gabbro is in sharp contact at one place. Toward the east end an 

 inclusion of very gneissoid syenite appears to be without sharp 

 contacts against the gabbro, its borders having been fused and 

 mixed with the gabbro. No. 52 of the table on page 31 shows the 

 composition of this rock, the presence of so much hornblende to 

 the exclusion of hypersthene being exceptional for the quadrangle. 

 Possibly this is because original pyroxene has all been altered to 

 hornblende. The feldspar is mostly labradorite. 



Near Alvord P. O., there occurs a small mass of gabbro as a 

 single large exposure not in actual contact with the country rock 

 which here is granite porphyry, but there can be little doubt as to 

 its later intrusive character. It is moderately gneissoid and, though 



ijour. Geol., vol. 21, no. 2, 1913, p. 160-^. Also N. Y. State Mus. Bui. 

 170, p. 26-35. 



