BUILDING AND ORNAMENTAL STONES. 409 



and the presence of a large amount therefore renders the rock difficult 

 to polish, and when polished it does not retain its luster so long as do 

 the other minerals, its surface soon becoming dull by exposure. Its 

 presence in large amounts is therefore deleterious to stones which are 

 intended for exterior polished work. The condition in which the mica 

 occurs is also an important factor. A large amount of it scattered in 

 very fine flakes throughout the mass of the rock influences its value as 

 a polished stone less than does the presence of large and thick crystals 

 scattered through the rock in smaller number. The method of the ar- 

 rangement of the mica is an important item; if scattered at haphazard, 

 and lying in all directions among the quartz and feldspar crystals, the 

 rock will work nearly as well in one direction as another. If it is scat- 

 tered through the rock in such a way that its laminae are arranged in 

 one definite plane, it imparts a stratified appearance to the rock, causing 

 it to split more readily in the direction of this lamination than across 

 it. When this stratified appearance becomes strongly marked the rock 

 is called a gneiss. Since, then, the distinction between granite and 

 gneiss is simply one of structure, and as the two rocks are used to a 

 considerable extent for the same purposes, they will be treated of to- 

 gether in the following pages. 



If hornblende is the characterizing accessory, the rocks are usually 

 without distinct lamination, as this mineral commonly exists in a gran- 

 ular form. Hornblende is subject to as wide variations of composition 

 as is mica, but its white and very light colored varieties do not usually 

 occur in our granites. Hornblende cleaves parallel to two planes, which 

 make angles of 124° with each other, and in this respect is distinguished 

 from black mica, which has but one cleavage. Its folia are also ine- 

 lastic. 



Hornblende takes an easier and more durable polish than mica and its 

 presence is preferable on this account. Pyroxene as a characterizing 

 accessory in granite is more common than has ordinarily been supposed. 

 Indeed all rocks which contain pyroxene abundantly have usually been 

 confounded with hornblende granites. The distinction between these 

 two minerals is important from an economic stand-point, as hornblende 

 possesses a much better cleavage than pyroxene, while the pyroxene is 

 much more brittle than the hornblende, and cracks out with greater 

 ease while working. The cracking out of little pieces from the black 

 ingredient of the Quincy granites has been frequently noticed, and is 

 due to the circumstance that this granite is not the hornblende-granite 

 it lias usually been supposed to be. Hornblende is very tough, but the 

 Quincy granite contains a peculiar variety of pyroxene which is so brit- 

 tle that it is difficult to produce a large surface which does not show 

 some little pits, due to the breaking out of a portion of the black 

 grains of pyroxene. Although pyroxene and hornblende may be iden- 

 tical incomposition, they are frequently associated together in the same 

 rock ; a fact which is very evident when thin sections are examined 



