222 Pirsson and Schuehert — OHskcmy Formation. 



and had suffered little transport. This is shown by the abund- 

 ance of blocks, their large size, angular shapes, unworn condi- 

 tion and characteristic resemblance to the nearby rocks in 

 place. It is presumed that they represent the debris of broken 

 down beds of the immediate vicinity. The supposition that 

 the blocks are local in origin is further supported by the rather 

 noticeable absence in this neighborhood of the coarser material 

 of glacial transport. Glacial sands and gravels containing peb- 

 bles or very small bowlders are common, and in morainal forms, 

 as in drumlins, etc., are rather frequent topographic features, 

 but the larger glacial erratics, which are so striking a feature 

 of the region farther south, were not observed in the stream 

 beds, on the lake shores, or in other places where erosion 

 might have brought them to the surface. 



Toward the lower end of Lake Parlin there is a large intru- 

 sive mass of trap rock. It is exposed in outcrops on the road ; 

 large bowlders of it are seen along the lake shore, and massive 

 outcrops at several places. It rises into Parlin Mountain, 

 where, as it was found at the north end and on the trail to 

 Long Pond as it winds about the mountain, it is inferred that 

 it forms the main upper portion of this eminence. No con- 

 tacts w r ere seen and its intrusive nature is inferred from the 

 size and position of the mass, the nature of its jointing, and 

 the petrographic characters of the rock. On a fresh surface of 

 fracture it is a dark gray to green and of uniform and tine tex- 

 ture. Examination in thin section shows that it is a greatly 

 altered diabase. Originally it consisted of plagioclase and a 

 brownish augite with accessory iron ore and apatite, with occa- 

 sional very fine, delicate micrographic intergrowths of quartz 

 and orthoclase filling interstices between the plagioclases. The 

 structure was the characteristic ophitic one, but the proportion 

 of plagioclase to augite was large, making the rock one of 

 feldspathic type. 



From this condition the rock has been greatly altered ; the 

 augite has been changed into chloritic substance, which pre- 

 serves the outlines of the ophitic fillings between the feldspars 

 and contains in a few places still unaltered pieces of the 

 unchanged augite. While this chloritic substance, for the 

 most part, appears of the nature of ordinary green, pleochroic 

 chlorite with low double refraction, it contains frequent minute 

 bundles of radiant fibers of a more yellowish color and with 

 rather high double refraction, which is suspected to be an iron- 

 bearing serpentine. Some separated silica also is seen, bat no 

 formation of any carbonate was observed. 



The feldspars are also altered and the original albite twin- 

 ning so much obscured or obliterated that their character can- 

 not be accurately determined, but they are thought to be 



