204 BULLETIN OF THE NEW YORK STATE MUSEUM 



example the mica, If disposed in lines, gives a foliated-like 

 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 pre 

 ferred to those 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. And 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 nat- 

 ural outcrops, where this rock is found decayed to the depth 

 of loo 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 sharp and great changes of temperature in 

 our northern cities and towns. The more rapid decomposi- 

 tion 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 kaollnlzed in part. It is an element of 

 weakness rather than of strength. The presence of the 

 easily decomposable varieties of pyrite Is not only prejudi- 

 cial to strength and durability but also to the beauty of the 

 stone as soon as it begins to decay. 



The term " granite " as used among builders and architects 

 Is not restricted to rock species of this name In geological 

 nomenclature, but includes what are known as gneisses (foli- 

 ated and bedded granites), syenite, gabbro and other crystal- 



