GKOLOGIC SECTION AND ITS INTERPRETATION 179 



rock cuts, which aflford very interesting exposures.* In the first cut a 

 fairly coarse augite-syenite is sliown which has not been severely gran- 

 ulated, is practically non-foliated, and has a very evident cataclastic 

 structure (plate 19).t The second cut shows quite similar material, 

 though more crushed and with a better foliation. The third cut is 

 quite extensive, and a generalized section of the exposures is given in 

 the accompanying figure. 



The augite-syenite constitutes the center and south end of the section. 

 It is more thoroughly granulated and more gneissoid than in the pre- 

 ceding exposures. Separating the two syenite areas. is a thickness of 

 12 feet of Avell banded gneisses. Above is a layer 2 feet thick of a white, 

 granular rock composed of quartz and wliite pyroxene in the proportion 

 of 1 to 2. This is followed by layers of granular, black pyroxene gran- 

 ulite and light colored quartzose rocks, the latter consisting essentially 





Fi(;ui!K 1. — Sectinii in Railrond Cut iirnr Loon Lake, New York. 



A, augite-syenite. jS, well banded quartzose gneisses. C, quartzose gneisses. D, biotitie 

 sheared strip— strike north 10 degrees west. Dip ol' bedding and foliation (15 degrees to the west. 



of quartz and potash feldspars, the quartz forming from 60 to 70 per 

 cent of the rock. The structure and composition indicate a sedimentary 

 origin for these included bands, and they are precisely like rocks which 

 invariabl}' accom^iany the crystalline limestones of the region, the white 

 pyroxene being especially characteristic. At the lower contact with the 

 sj^enite is a probable shear-plane, along Avhich biotite is abundantly de- 

 veloped. This syenite is succeeded to the north by finely granular, red, 

 granitic gneisses of doubtful origin, but also very similar to rocks which 

 are of common occurrence associated with the limestones. The foliation 

 planes of the syenite have the same dip and strike as the included 

 gneisses. 



There can be no question of the sedimentary origin of the gneisses in- 

 cluded under " B " in the section (see plate 20). With that as a start- 

 ing point, the uniform dip and strike in the exposure, together with the 

 finely granular character of the syenite, give the impression that the 

 whole forms a regularly bedded series ; but when the syenite is com- 

 pared with the rock in the other cuts it is seen to be unquestionably the 



* These exposures were visited iu company with Professor A.<'. (iill in July, 1897, and with 

 Professor J. F. Kemp in August, 189S, neither of whom cibjci'tcd i(] the interpretation here given. 



t A photograph of this exposure is reprodneed in plate I'.i to indicate tlie color change due to 

 weathering. 



