100 [Assembly 



formation, running to the eastward of the Highlands generally, and 

 occupying portions of Westchester and Dutchess counties in New 

 York, and thence extending into Connecticut and Massachusetts. 

 The ordinary gray or dark-colored bluish limestones and the various 

 colored sandstones have a much wider distribution, but are still 

 limited to certain belts of country. 



Treating these in their order, we may arrange and discuss them 

 as follows : 



I. 



Granites, including Sienites, Gneiss, or Gneissoid and Sienitic 

 Rocks ; their Geological Position and Geographical Dis- 

 tribution. 



The term granite, in its strict signification, means a crystalline 

 rock composed of quartz, felspar and mica in intimate mixture, the 

 separate minerals being composed of crystalline grains. It is a very 

 common condition of the granitic rocks, that the mica may be absent, 

 and in its stead we have hornblende, and in this form the rock is 

 termed a sienite.* On the other hand, the presence of mica in thin 

 scales, forming lamination, or rendering the lines of bedding visible 

 by coloration or otherwise, produces what we term gneiss ; though 

 some geologists would apply the term gneiss to all stratified granitic 

 rocks. 



The proportion of mica in gneiss is not necessarily larger than in 

 some of the granites ; but the faces of the thin laminae being 

 arranged parallel to the lines of bedding and the freest line of 

 cleavage, causes it often to appear in larger proportion, f 



Quartz, felspar and hornblende without mica or with a very small 

 proportion of this mineral, constitute some of the best granites ;^ 

 while in the lighter gray or whitish gray granites, the quartz, or 

 quartz and felspar, are the chief component parts, and there is little 

 either of hornblende or mica. The grains or aggregations of these 

 minerals may sometimes be so large that each one presents its dis- 

 tinctive mineralogical or individual character, becoming so coarsely 

 crystalline as to be unfit for building purposes. 



Granites of New York. 



In the lower portion of the Adirondack region, or the Laurentian 

 System bordering Lake Champlain and extending from Saratoga to 

 Clinton county, the rocks consist mainly of a gray gneissoid granite, 

 which is sometimes traversed by coarser crystalline veins, and some- 

 times nearly or entirely losing its gneissoid character from the small 

 proportion of mica, but always i-egularly stratified. The latter 

 character is presented in the exposures at Little Falls and other places; 



* The Eg3'ptian sienite or syenite, according to Delesse, contains mica. 



+ A distinction has sometimes been made Detween gneiss and granite, that the one is 

 stratilied and the other not. This does not hold true ; for nearly all, if not all, the gran- 

 ites that are extensively quarried are stratified, and I believe all of them cleave in one 

 direction more freely than in another, while the other free line of cleavage or breaking is 

 rectangular to the first. 



