8 DESOKIPTIVE GEOLOGY. 



the central granites are made np of coarsely crystalline material: tlieir 

 color is white or reddish-gray. Quartz and feldspars are the predominant 

 minerals, while the amount of mica present is very variable, and in those 

 localities where found abundantly gives, of course, a darker color to the 

 rock. The quartz is granular, usually of a gray color, frequently colorless 

 and translucent. The feldspars give the prevailing color to the granites. 

 Both orthoclase and plagioclase are present, but the former is much more 

 abundant. The orthoclase occurs in broad, tabular crystals, from micro- 

 scopical forms up to those measuring one atid a half inches in length ; the 

 larger crystals having usually a decidedly reddish tinge. The mica is most 

 frequently found as a brilliant, dark biotite in small plates, and in the whiter 

 granites quite well preserved. 



In addition to the well-defined, essential minerals mentioned, the nar- 

 row crevices and fissures in the granite and the spaces between the crystal- 

 line particles seemed filled with a reddish, decomposed, hydrated oxide of 

 iron. Owing to the coarse and friable nature of the granite, with the inter- 

 stices occupied by a soii; of ferruginous material, decomposition takes place 

 easily ; and, where this latter ingi'edient is at all common, the disintegration 

 proceeds with striking rapidity until the surface of the rock is protected 

 from atmospheric agencies by a heavy layer of detrital matter, frequently 

 to a depth of two or three feet. The ease with which this granite crumbles 

 could be well observed, a few years ago, by studying the fresh railroad- 

 cuts and the surfaces of the ground that had been recently exposed by the 

 workmen removing the coarse gravel for the purposes of the railroad-bed. 

 This disintegration gives a marked character to the hills, and has left 

 masses of rock that have withstood erosion, but carved in the most curious 

 forms, which present roimded edges and a conchoidal weathering, with no 

 angular fragments or blocks. Many of these masses have received names, 

 such as Skull Rock, The Tower, Lighthouse Eock, from their supposed resem- 

 blance to the objects named. 



There are many localities in the central granite mass which differ 

 somewhat from the above description ; localities where the body of the rock 

 is more compact and finer-grained, with no large feldspar crystals. In 

 places, there appears to be a gradual transition from the coarser- to the 



