204 MINERALOGY AND LITHOLOGY. 



Granite Veinstones. In reference to those great veins filled with gran- 

 itic minerals, such as occur at Grafton, Alstead, Springfield, Acworth, 

 etc., very little need here be said. Their distribution and mode of occur- 

 rence have been very completely described by Professors Hitchcock and 

 Huntington in Volume H ; and the texture of their material is so coarse 

 that the study of it is more mineralogical than lithological, and hence 

 much has been said in the first chapter of this volume in reference to 

 them. In general, it may be stated that these veins were supposed to be 

 fissures that were formed deep down in the earth, and into these fissures 

 water, percolating through the adjacent strata, entered. This water at 

 such a depth must have been under heavy pressure, and heated as well by 

 the inner heat of the earth, and hence would have a most powerful sol- 

 vent action, and would become heavily charged with the soluble materials 

 of the strata. From this water, after entering these veins, the granitic 

 minerals crystallized. The water from some distance all flowing into one 

 reservoir, minerals rare in the strata were concentrated in the veins; and 

 hence, mixed with quartz, orthoclase, and the mica which is so exten- 

 sively mined, such minerals as beryl, columbite, and tourmaline, which 

 contain rare elements, are common. These veins are characterized by 

 large crystals of the granitic minerals; by the occurrence in them of 

 graphic granite or pegmatite, which is a granite in which the quartz is 

 so arranged with reference to the feldspar as to produce the effect of 

 oriental writing; and by the occurrence of a large number of well crys- 

 tallized minerals. Remarks additional to those made in the previous 

 chapter on minerals, and in the preceding volume by Prof. Hitchcock, 

 will be found in the chapter on economic mineralogy. 



SIENITE.* 



Under this head are considered those massive crystalline rocks which 

 are composed of orthoclase, with either augite, hornblende, or mica as 

 a characteristic accessory. These form an interesting group of rocks, 

 which in New Hampshire are much less widely distributed than are the 

 granites, and which usually occur in plainly eruptive masses. In some 

 of them biotite exists, but they all contain either augite or hornblende, 



* The mode of spelling adopted in the preceding volumes issued by this survey is adhered to. 



