26 Feather stonhaugK' s Geological Report, 



system which have been wrought out of the grauwackean 

 chaos during the last four years, by the perseverance and 

 sound judgment of Mr. Murchison. 



Before entering upon any description of these inferior rocks, 

 some observations may be introduced here, upon the two 

 general classes into which all deposites'may be arranged — 

 those of chemical and those of mechanical origin. Although 

 the object of the geologist is rather to arrive*at just conclu- 

 sions respecting the causes to which mineral beds owe their 

 position, than to the elementary nature of the materials of 

 which they are composed, yet some mineralogical acquaint- 

 ance with their mineral constituents is requisite to discrimi- 

 nate between them, and every student is supposed to have 

 prepared himself, in some degree, to comprehend the chemi- 

 cal laws under the influence of which mineralogical bodies 

 take their peculiar forms. Crystalline bodies, however imper- 

 fect, are the result of chemical agencies; and where a rock is 

 composed of regular crystals of one mineral, imbedded in a 

 massive paste of imperfectly crystallized mineral matter of 

 another kind, we may come safely to the conclusion that the 

 whole mass has had a crystalline origin. There is a porphy- 

 ritic granite of this kind, called Shapfell granite, in England, 

 which is met with in various parts of the world. It is the 

 rock upon which the coal measures of Chesterfield, Virginia, 

 rest, and I have observed it on the eastern slope of the Blue 

 ridge, in Madison county, in that State, and in various parts 

 of Georgia. This rock has evidently never been disturbed 

 as to its aggregation since its first chemical production. The 

 other class of rocks is of a different character. When ob- 

 served by the microscope, the aggregate parts appear to con- 

 sist of small fragments of crystalline minerals, having been 

 subjected to much attrition by water, in consequence of 

 which they have lost their sharp edges, and have become 

 rounded. Deposites of this kind, the constituents of which 

 have been mechanically separated from crystalline bodies, 



