10 Report on Building Stone of New York. 



and gives character to the mass. Often it happens that the mica is 

 almost entirely wanting, and is replaced by one of these accessory 

 constituents. Thus there may be a hornblendic granite or a graphitic 

 or epidotic variety, etc., according to the nature of the mineral. 



Syenite differs from granite in having little quartz, no mica and 

 hornblende. 



Parallel in composition with granites, but differing in texture, are 

 the gneisses or gneissic rocks. They are stratified or lie in strata or 

 beds. In them the minerals are in thin, lenticular layers and laminae 

 which are parallel to the bedding-planes of the rock. The mass is 

 said to be laminated, or schistose in structure. Sometimes the com- 

 ponent minerals are so large that in hand specimens it is not possible 

 to decide if they be gneiss or granite. And they may be fine-gran- 

 ular and coarse-granular in texture. Graphitic, epidotic, hornblendic, 

 garnetiferous and other varieties occur, determined by the accessory 

 constituents. 



Mica schist is an aggregate of quartz and mica mainly and having 

 a more marked schistose structure than the gneisses. The lenticular 

 form of the quartz is especially noticeable. And on account of its 

 structure it is more readily split in the plane of its bedding than the 

 granites or the gneisses. Generally the plates or scales of mica lie 

 rudely parallel to one another and they help in giving the mass a 

 more laminated appearance and cause it to have a more fissile char- 

 acter. 



It must be understood that there is a wide range in the relative 

 proportions in which these essential, rock-forming minerals occur in 

 these several kinds of crystalline rocks. One or another may pre- 

 dominate and give character to the mass. As in the deposits which 

 are to-day in process of formation, these old rock masses differ within 

 narrow limits. And not only do we find variation from one locality 

 to another, but also in the same ledge and quarry, and in some places 

 in the same bed. These mineralogical differences generally indicate 

 a different chemical composition also, although not in all cases. But 

 to the quarryman and builder the former are of much more import- 

 ance, since they give strength and make it durable, or determine its 

 clearage and the style of working. The texture is wholly controlled 

 by the mode of aggregation and the nature of the minerals. Where 

 uniformly distributed and not in lines or layers the mass is split with 

 equal ease in any direction, and it is capable of being dressed with 

 like degree of fineness on any side. But such crystalline aggregates 



