BaiLDlX(i AND OIJNAMF.NTAL STONKfi. 400 



iiiid llio i)i'ese]if'0 of a lar/^c iiinoniit, therefore i(MHlers the roe,l< ilifllcult 

 to polisli, and wlieii polished it docs Dot retain its Insler so lon<;- as <h) 

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

 presence in large luuouuts is therefore deleterious to stones winch are 

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

 occurs is also an iniportaut factor. A large auiount of it scattered in 

 very tine flakes throughout the mass of the rock inliuences its value as 

 a polislied 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 mi^a 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 laminrs are arranged in 

 one definite plane, it imparts a stratitied 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 thero<',k 

 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 

 <!()nsideral)le extent for the same; jiurposes, they will be treat<'d of to- 

 gether in the following ])ages. 



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 comjiosition 

 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° ^vith each other, and in this respect is distinguislied 

 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 charac.terizing 

 accessory in granite is more common than has ordinarily been supi)Osed. 

 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-[)oint, as hornblende 

 [)ossesses a much better cleavage than pyroxene, while the pyroxene is 

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

 ease while working. The cracking out of little i)i(^(;es from the black 

 iMgredieTitof the Quincy granites has been freciuently noticed, and is 

 due to the circumstance that this granite is not the Iiornblendegranite 

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

 Quincy granite contains a i)eculiar variety of pyroxene which is so brit- 

 tU; that it is dilficult 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 ])yroxene and hornblende may be iden- 

 tical in composition, they are frequen'Jy associated together in the same 

 i-ock ; a fact which is very evident when thin sections are examined 



