CusiIING 



— Geology of 



Clinton County. 



507 



plagioclase feldspar, microcline, biotite, hornblende ;ui<l titanite are present in 

 varying amounts, and small apatites and zircons are almost universal. Locally 

 a green augite also appears, but this is not a common occurrence in this 

 county. With the appearance of the more basic minerals the foliation of the 

 rock becomes apparent and increases in prominence as they increase in abund- 

 ance. Sometimes the quartz retreats and locally may l»e entirely lacking. An 

 increase in plagioclase frequently accompanies this change, so that the rock 

 grades from a quartz-orthoclase into a plagioclase gneiss, often without 

 making any noticeable difference in the external appearance of the rock. 



2. Widely exposed in Black Brook township are gneisses of quite 

 different appearance, and, judging from published descriptions of the Adiron- 

 dack gneisses, of an unusual type. For the most part they are very finely 

 granular, but alternate with numerous, often broad, coarse, pegmatite-like 

 bands. The fine-grained rock is of a prevailing lavender-grey or lilac-grey 

 tint, showing, when freshly broken, small, brilliant cleavage faces of feldspar. 

 It is quite free from basic minerals, hence foliation is not especially apparent. 

 The coarse rock is not unlike similar coarse bands in the ordinary gneisses, 

 but is more abundant here, alternating regularly with the other. There are also 

 present occasional thin bands of a basic gneiss weathering with a rust)' hue. 



In thin sections, both the coarse and fine varieties are seen to be made up 

 mainly, and often wholly, of microcline and quartz, the microcline having 

 almost universally a markedly microperthitic habit. Apatite is seldom absent, 

 and often abundant. Plagioclase and microperthitic orthoclase are frequently 

 present, sometimes in considerable quantity, and some slides show small 

 amounts of magnetite, biotite and hornblende, these being more plentiful in 

 the coarse bands. 



Closely associated with these gneisses are others which, outwardly, more 

 nearly resemble the ordinary gneiss, and which have plagioclase for their 

 most prominent constituent, together with microcline, orthoclase, quartz, 

 magnetite and ferro-magnesian silicates in varying amounts. By their distri- 

 bution the impression is created that through them the microcline gneisses 

 grade into the ordinary orthoclase gneisses. 



These microcline gneisses are in rather close juxtaposition to an area 

 occupied by the limestone series, and perhaps should be classed with that. 

 The thin bands of rusty basic gneiss interfoliated w ith them resemble gneisses 

 of that series rather more closely than they do the basic bands in the ordinary 

 gneisses. Aside from that indication the field evidence does not lean strongly 

 in either direction. 



