72 GEOLOGY. 



of different kinds in New England, Massachusetts being 

 truly the granite state of the Union, though New Hamp- 

 shire is generally called so. 



147. Uses of Granite. Granite, on account of its dura- 

 bility, is one of the most valuable materials for buildings 

 and monuments. It was much used by the ancients, and 

 there are obelisks of granite in Egypt that have been ex- 

 posed to the weather for three thousand years, and yet 

 are in good condition. Commonly, the finer is the tex- 

 ture of granite, the more durable is it. Syenite, which is 

 so durable, is of a fine texture, but it owes its durability 

 partly to the toughness which hornblende imparts ( 

 125). If there be pyrites or any other ore of iron dis- 

 seminated through the rock, it impairs its value essen- 

 tially, because by decomposition rust is produced, defac- 

 ing and often crumbling the stone. Some granites, which 

 present a good appearance on being taken out of the 

 quarry, very soon crumble on continued exposure to the 

 air without any obvious cause. This shows the neces- 

 sity of proper examination in selecting granite for build- 

 ing. As feldspar contains a large proportion of alumina, 

 feldspathic granite is much used in obtaining the kaolin 

 for making porcelain. 



148. Gneiss. This is a rock which has precisely the 

 same mixture of minerals in it that granite has, but the 

 arrangement is different. It is a stratified rock. It is 

 foliated ; that is, it is in leaves, and is split easily in dif- 

 ferent thicknesses, varying from a few inches to a foot 

 or more. This cleavage occurs where the mica is most 

 abundant. When it cleaves in thin slabs it is used for 

 flagging, but when it can be got out in thick blocks it 

 is used for building. Some of the buildings of Amherst 

 College are constructed of this material, and present a 

 fine appearance. There are many quarries of it in Mas- 

 sachusetts, and some in Connecticut. 



149. Slates, Shales, and Schists. These terms are oft- 

 en used as being synonymous ; but when they are used 



