58 NEW YORK STATE MUSEUM 



Section 3 



THE CRYSTALLINE SILICATE ROCKS 

 PRELIMINARY DISCUSSION AND DEFINITION OF TERMS 



Before entering upon the description of the different quarry- 

 materials, it may be well to explain that the classification of rocks 

 into three principal groups • — igneous, sedimentary and meta- 

 morphic — which has been followed hitherto scarcely serves the 

 purpose of an economic classification that is based on general 

 quarry features and uses. From a practical standpoint, there is 

 no line of division to be drawn between many metamorphic gneisses 

 and schists and the igneous rocks, since they may have the same 

 applications and present the same problems in quarrying and dress- 

 ing. It is customary, therefore, to include the metamorphosed 

 silicate rocks which are useful for structural stones with the mas- 

 sive igneous types, and that practice will be followed here. 



The other metamorphic rocks include slates which ^are placed in 

 a separate division, marbles which with some nonmetamorphic lime- 

 stones are also separately described, and quartzites which from an 

 economic point of view belong in the class of sandstones. 



The crystalline silicate rocks of the Adirondacks and southeastern 

 New York embrace a variety of individual types such as granite 

 in the strict sense, syenite, diorite, anorthosite, gabbro of different 

 kinds, diabase, and an assemblage of gneisses and schists that in- 

 cludes both igneous and sedimentary derivatives of varied mineral 

 composition. 



GRANITE 



As an architectural stone, granite outranks the other igneous 

 rocks of the State, which is true also wherever the crystalline 

 silicate rocks are exploited. Its prominence is due in part to its 

 relatively widespread occurrence, but largely to the combination 

 of qualities in regard to color, uniformity and ease of extraction 

 and dressing which is less often found in the other stones. The 

 prevalent taste for light-colored stone in buildings has much to 

 do with its general favor. 



Although quarrymen and builders use the term granite rather 

 indiscriminately to designate almost any of the silicate rocks that 

 have been named, it probably belongs to a single rock series of 

 igneous origin which is characterized in composition by the pres- 

 ence of potash, feldspar and quartz. These two minerals always 

 predominate, but are often accompanied b}^ others in greater 



