GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



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are practically absent over considerable areas, and, where they 

 do occur, are mostly few and small. Yet such rocks are re- 

 peatedly found shading locally into others with much less ap- 

 parent gneissoid structure, with feldspar augen quite frequent 

 and with definite cataclastic structure; rocks whose original 

 igneous textures are sufficiently well preserved to show their 

 origin beyond a doubt. 



In some cases, notably at Little Falls and Middleville in Herki- 

 mer county, where outliers of these rocks occur and where the 

 augen are bigger and more numerous than at any other known 

 localities, the rock seems to have originally been rather coarsely 

 porphyritic. But for most of the rock in the region this does not 

 seem to have been true. 



Mineral composition. At the type locality, Loon lake, the rock 

 is a quartzose augite syenite, and, since this is the prevailing 

 character over much of the region, the description of the type 

 will serve well for a general description of the rock. 



In the Loon lake rock microperthite and oligoclase feldspars, 

 augite and hypersthene (or bronzite), hornblende, magnetite, 

 quartz, garnet, apatite and zircon are always present, and 

 locally biotite. titanite, pyrite and allanite appear in addition. 

 The rock is essentially composed of microperthite, augite and 

 hypersthene. with quartz, oligoclase and garnet always present 

 in varying and usually slight amount. 



The feldspar is mostly microperthite. A little plagioclase 

 always appears and seems universally to be oligoclase. Most of 

 the plagioclase present is intergrown with orthoclase in the 

 microperthite. and the chemical analysis indicates that this must 

 be albite. The feldspar is usually perfectly fresh and contains 

 to some extent minute, dustlike inclusions, as well as including 

 small zircons, apatites and titanites and occasionally small 

 augites and quartzes also. Orthoclase is only present as a con- 

 stituent of the microperthite. 



Both augite and hypersthene are usually present, the former 

 mostly predominating. Parallel growths of the two frequently 

 occur, often of repeated fine lamellae, the contact faces being as 

 usual. The augite is deep green in thin section, quite like the 

 green of the hypersthene. 



