48 ANNALS NEW YORK ACADEMY OF SCIENCES 



where a large irregular mass outcrops on the west side of the railroad. 

 Its relations are rather obscure, and its size if fully exposed would prob- 

 ably be about 60 to 80 feet, but there can be little doubt as to its identity 

 as a dike. Occasional blotches of a fine grained gray substance are in- 

 cluded in it. The true pegmatitic structure is not well shown, this name 

 being applied because of the coarseness of its texture. About 30 feet to 

 the south lies one of the schist inclusions described below; between the 

 two the country rock (biotite augite norite) is altered to a diorite, some- 

 what gneissoid, but this is probably due to the schist rather than the 

 pegmatite. 



Dacite Porphyry 



The very remarkable rock, dacite porphyry, is known from only one 

 locality, viz., about 300 feet up the hill east of Montrose station. It out- 

 crops in the gutter of the road for about ten feet (though it may be traced 

 for 75 feet), and this outcrop is about three feet wide. By differential 

 weathering, it projects about three feet above the ground, and its jointing 

 and its white color, combined with its odd position, cause it to look at a 

 glance exactly like a stone wall. In thin section, the rock is seen to be 

 composed of a moderate number of large idiomorphic feldspars set in a 

 mosaic of quartz and feldspar grains. The phenocrysts are almost never 

 striated, but Williams 33 states that all of the feldspar of the rock has a 

 specific gravity of 2.63-2.67, so that it must be oligoclase and andesine. 

 The phenocrysts never show resorption, but usually exhibit beautiful 

 zonal growth. The quartz is all in the groundmass, and some of the 

 small feldspars are striated and seem to be oligoclase. Biotite, hornblende 

 and muscovite, some of the latter probably damourite, but not all, occur 

 in rather small quantity. In the groundmass, the grains often appear to 

 be interlocked, as though the rock were a kind of augen gneiss, but the 

 perfect outlines of the phenocrysts and the distribution of the muscovite, 

 as well as the field relations, preclude this. 



Dioritic and Gabbroic Dikes 



Various types of the dioritic lamprophyres constitute by far the most 

 common group of dike rocks. They are especially abundant on Verplanck 

 Point, although found quite abundantly elsewhere, especially through the 

 southern part of the district. The different types, of course, exhibit no 

 preference for any particular plutonic rock. Only in the limestone on 



33 Williams calls this rock a bed of porphyritic quartz mica diorite, (Amer. Jour. Sci., 

 (3), XXXV, 446. 1888.) and Dana a granitoid micaceous quartzite (Idem, XX, 218). 



