GRANITIC ROCKS 373 



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 

 amount of the dark-colored mica mixed with the feldspar and 

 quartz ; and the darker-colored varieties owe their color, in most 

 cases, to hornblende or tourmaline 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 

 and 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 

 example the mica, if disposed in parallel surfaces, gives a foliated 

 structure and tends to produce what is known as rift, and the 

 granite is more readily split in the planes of the mica than across 

 them. Again the mica flakes may be so large and irregularly 

 massed that the surface is not susceptible of a uniform degree of 

 polish. Hornblende, on account of its superior toughness, is less 

 brittle than pyroxene under the polishing, and the hornblende 

 granites are said to be preferred to those rocks which contain 

 pyroxene in quantity. 



The more nearly alike in hardness and the more intimately 

 interwoven the texture of the minerals, the more capable it is of 

 receiving a good polish. Hence it follows that the very coarse 

 crystalline granites are not so well suited for ornamental work. 



The enduring properties of granites vary with the nature of 

 the minerals in their composition. Although popularly they are 

 regarded as our most durable building stone, there are some 

 notable exceptions, which are evident in the natural outcrops,' 

 where this rock is found decayed to the depth of 100 to 200 

 feet, and in the active disintegration which is in progress in 

 structures of the present century. Foliated varieties placed on 

 edge in buildings, tend necessarily to scale under the great 

 changes of temperature in our northern cities and towns. 



