GEOLOGICAL OCCURKENCE. 103 



at A. The intruded rock (number 164) is medium to coarse grained, but 

 shows a finer texture in the narrow passage between the inclosed masses 

 of pyroxenite. 



At number 11 opening the ellipsoidal or leopard rock occurs in abun- 

 dance, and constitutes a large part of the wall in places. At one point 

 in the face of the wall a crystal of apatite, six inches or more in diameter, 

 was seen inclosed in the leopard rock, with the ellipsoidal masses dis- 

 posed in a concentric manner about it. Examples showing the same re- 

 lations between more or less crushed apatite and the leopard rock were 

 frequent in the refuse of the dumps, but the presence of crystallographic 

 form is exceptional. 



At various places on the hill dome-like masses of the coarse grained 

 syenite rock appear as local enlargements of the dike. One of these near 

 the office shows fragments of pyroxenite scattered through the intrusive 

 mass. 



These observations, which may be duplicated man}^ times in the 

 vicinity, are sufficient to demonstrate — 



1. That the coarse grained syenitic rock, the leopard rock and the 

 streaked gneiss belong to the same rock-body ; and, 



2. That this body represents an intrusion of syenite later than that of 

 the pyroxenite. 



An interesting feature of these syenitic rocks is their remarkably fresh 

 condition. This appearance, which is prominently characteristic of the 

 rock in the hand specimen, is also shown in the thin section, where ver}'' 

 little evidence of decomposition is to be observed. Epidote and chlorite, 

 generally common as decomposition products, arj rare in these rocks. 



Megascopical Characters. 

 coarse syenite-gneiss. 



The first of the syenite-gneisses consists of a very coarse grained mix- 

 ture of microcline and monoclinic pyroxene chiefly, with a variable 

 amount of quartz. The rock is divided into irregular angular blocks, the 

 largest being one or two inches across, separated by thin anastomosing 

 sheets of granular feldspar, augite and quartz. These interstitial areas are 

 sometimes thick enough to be readily traceable in the hand specimen. 

 Many, however, are scarcely or not at all recognizable megascopically, 

 but are brought out with distinctness under the microscope. 



The microcline is of a dark gra}^, often purplish color, crystallized in 

 large individuals, frequently from one to two inches in diameter. The 

 larger cleavage faces show a slight undulatory surface, with variable re- 

 flection and bright pearly sheen. 



XV— Bull. Geol. Soc. Am., Vol. 7, 1895. 



