92 NEW YORK STATE MUSEUM 



tone to most specimens. Both the quartz and augite fluctuate, the 

 proportion of the former mineral being usually about that found 

 in a moderately silicious granite, but may shrink to very small 

 amounts. The augite gives way at times to hornblende or biotite, 

 a result that may be traced in part to secondary alteration. 



The arrangement of the constituents may be described as gneiss- 

 oid, yet it often lacks the parallelism of typical gneisses. The 

 texture is mostly granular, such as would be produced by shearing 

 and granulation of a massive rock with perhaps a certain amount 

 of flowage under compression. Coarse phases in which little crush- 

 ing effects are observable and grading into a pegmatite rock are not 

 unusual in the area. They may be explained as massive aggregates 

 which have escaped the general dynamism that has effected the 

 granulation of most of the gneisses, or possibly they represent a 

 recrystallization of the latter under certain favorable conditions 

 which have obtained only in portions of the mass. That they are 

 all intrusions from a distinct magma hardly seems possible under 

 the circumstances owing to the frequent similarity of composition 

 to the granular varieties as well as their textural gradation into the 

 latter. On the whole the characters of the acid gneiss indicate its 

 relationship to the granites. 



There are few exposures of sedimentary types among the 

 gneisses. On the south side of the Ausable, just below the con- 

 fluence of ,the two branches at Ausable Forks, a micaceous lami- 

 nated rock outcrops in a small area where the overburden of sand 

 and soil has been washed off. It has the peculiar rusty weathered 

 appearance common to these gneisses, due to the oxidation of 

 contained pyrite. Some layers are extremely quartzose. The 

 exposure has special interest from the fact that the strata are cut 

 off on one side by syenite which breaks across in an irregular manner 

 like an intrusive. It is the only place in the district where such 

 evidence of the nature and relative age of the syenite has been 

 found. The micaceous gneiss can not be traced for any distance, 

 as the river and its deposits conceal the outcrop. The elevations on 

 the opposite side of the river just north of Ausable Forks are mostly 

 syenite, but there are involved masses of amphibolite and of a light 

 colored plagioclase gneiss that probably belong to the sedimentary 

 series. Crystalline limestone has been noted by Kemp as occurring 

 at Trout pond, 3 miles south of Clint on ville ; it does not appear to 

 be present, however, anywhere in the vicinity of the mines. 



The strike of the gneisses varies considerably, but is mainly in a 

 northerly direction. The common readings are east of north, up to 



