PART 3. 



ECONOMIC GEOLOGY 

 Building Stone a 



GRANITIC ROCKS 

 Granite, Gneiss, Syenite, Trap and Norite 



Granite. Typical granite is a crystalline, granular mixture of 

 a feldspar, quartz and hornblende. In addition to these essential 

 constituents, one or more accessory minerals may be present. 

 The more common are the micas, museovite and biotite, garnet, 

 tourmaline, magnetite and pyrite. The character of the rock is 

 often determined by the presence of these accessory constituents 

 in quantity, as in some cases the hornblende is entirely replaced 

 by mica. \ 



The chemical composition also varies from that of the average 

 or typical kind. The mineralogical differences mark the varie- 

 ties, thus there are: hornblende granite, biotite granite, tourma- 

 line granite, etc. 



The texture of granites is determined by the aggregated miner- 

 als entering into their composition. It varies from coarse-crystal- 

 line, in which the individual crystals may be an inch or more in 

 length, to fine-crystalline and aphanitic, wherein the minerals 

 are hardly visible to the eye. In consequence of the wide varia- 

 tion due to the mode of arrangement of the mineral constituents, 

 there is an equally great variety noticeable in the texture. 



The color also is dependent upon the minerals. As feldspar 

 is the predominant constituent it gives character to the mass, 

 and the red varieties owe their color to the red or pink feldspars 

 in them, as in the case of the granite of Grindstone Island in 

 the St Lawrence. The shades of gray are due to the varying 



a This chapter on building stone is abridged with alterations and additions from 

 Bulletin No. 10 of the New York State Museum, by John C. Smock. 



