182 NEW YORK STATE MUSEUM 



amount of the dark colored mica mixed with the feldspar and 

 quartz; and the dark 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 gran- 

 ites for use in construction is related closely to their mineralo- 

 gical composition. Their arrangement in the mass and their 

 relative proportions 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 they are 

 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. The more rapid 

 decomposition of the micas makes those varieties in which they 

 occur in large flakes or aggregations more liable to decay. The 

 condition of the feldspar also is often such as to influence the 

 durability. When kaolinized in part, it is an element of weak- 

 ness rather than of strength. The presence of the easily decom- 



