Feather stonhaiigli's Geological Report. 



27 



are easily recognised for the greater part. They have all 

 been evidently deposited from water, and are classed amongst 

 the sedimentary rocks, in contradistinction to the others, 

 which are deemed to be ignigenous. There are a few rocks 

 which have an ambiguous character : those limestones which 

 have been deposited from solutions take a crystalline aspect, 

 and many strata proximate to ignigenous rocks have been sub- 

 jected to some change in their external appearance, but the 

 exceptions will not interfere with, this classification. 



The common granite is easily recognised by its granular 

 crystalline structure, composed of felspar, quartz, and mica, 

 nearly equally diffused and united into a mass without any 

 apparent cement. The varieties are numerous, occasioned 

 by the varying proportions of its constituent parts, and the 

 substitution of other minerals for some of them. Amongst 

 the most remarkable are those known as graphic granite, a 

 beautiful mineral composed almost entirely of lamellar felspar 

 and quartz, so disposed as to present an appearance of literal 

 characters. This variety contains beryls, garnets, and other 

 minerals, and, where it is found in extensive deposites, as in 

 the State of Delaware, about ten miles from Wilmington, is 

 valuable for the manufacture of porcelain. Another variety 

 is the porphyritic granite before mentioned, where large 

 rhomboidal crystals of red and white felspar are imbedded in 

 a paste of small-grained granite. 



The gneiss is generally a stratified rock, often abounding so 

 much in mica as to constitute its base. The plates of this last 

 mineral are distributed parallel to its strata, which occasion it 

 to split easily in that direction. Granite olten passes into this 

 rock, the quartz being absent. There is a singular rock on 

 the prairie at the grand portage east of Lac qui parle^ on the 

 St. Peter's. Immense masses, some of them twenty feet high, 

 abound there, with a laminated structure so perfect as to form 

 a true stratification. The lamina are in many instances only 

 an inch in breadth, and dip to the southeast with an almost 



