GEOLOGICAL POSITION AND GEOGRAPHICAL DISTRIBUTION 203 



be present. The more commonly occurring are: hornblende, 

 pyroxene, epidote, garnet, tourmaline, magnetite, pyrite and 

 graphite. And the character of the rock is often determined 

 by the presence of these accessory constituents in quantity. 



The chemical composition also varies from that of the 

 average or typical kind. The mineralogical differences 

 mark the varieties, thus there are : hornblende granite, 

 biotite granite, tourmaline granite, etc. 



The texture of the granites is determined by the aggre- 

 gated minerals entering into their composition. And they 

 vary from coarse-crystalline, 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. 

 When the feldspar and quartz are In a letter-like arrange- 

 ment, the rock is known as pegmatite or graphic granite. 

 A porphyritic structure is produced by the occurrence of the 

 feldspar, in well-developed crystals in the mass. In conse- 

 quence of the wide variation due to the mode of arrange- 

 ment 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 are so because of 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 amount of dark-colored mica mixed 

 with the feldspar and quartz ; and the darker-colored varie- 

 ties owe their color in most cases to hornblende or tourma- 

 line which may be present. 



The beauty, ease of working, durability and value of the 

 granites for use in construction is related closely to their 

 mineralogical composition. Their arrangement In the mass 

 iand their relative proportion determine the color and give 

 beauty. The presence or absence of certain species influence 

 the hardness and homogeneous nature and the consequent 

 ease with which the stone can be dressed and polished. For 



