CONSTITUENTS OF GRANITE. 



In other specimens, the proportion of feldspar being large, and 

 that of mica and quartz small, the rock is called POEPHYEOUS 



GEANITE. 



25. In general, the little laminae of mica are distributed irre- 

 gularly through the granite, their faces being turned in all 

 conceivable directions. In certain specimens, however, they are 

 observed to be placed parallel to each other, so as to give the rock 

 a lamellated, slaty, or schistous texture. The granite, in such 

 cases, takes the name of GNEISS, from the Danish " ynister." 



26. Having thus briefly described the composition and condition 

 of the fundamental layer of matter, upon which the solid shell of 

 the earth is based, and indicated the circumstances and characters 

 which are evidence of its igneous origin, we shall now proceed to 

 explain the condition and character of the superincumbent strata, 

 which, as will presently appear, have had an origin of a very 

 different kind, and dates of incomparably more recent formation. 



27. The strata which rest immediately upon the igneous rocks 

 have been denominated TRANSITION or METAMOEPHIC, inasmuch 

 as they partake partly of the character of the igneous rocks, and 

 partly of that of the rocks incumbent upon them. They partake, 

 in certain instances, of so much of the former and so little of the 

 latter, that in the earlier epochs of geological research, they were 

 classed with the igneous rocks, from which, nevertheless, they are 

 distinguished by sufficiently evident marks of incipient strati- 

 fication. 



28. It has been explained that the materials composing the 

 igneous rocks are confusedly and irregularly agglomerated with- 

 out the least appearance of even an approach to any regularity 

 of structure, and it has been shown, that this is at once the con- 

 sequence and evidence of their igneous origin. Such, however, 

 is not the character of the superincumbent rocks. The materials 

 of which these are severally composed are found to be distributed, 

 one over another, in regular layers, bounded by parallel and 

 horizontal surfaces, resembling the courses of masonry. Now 

 such a distribution never could have resulted if these, like the 

 primary rocks, had, previously to their formation, been in a state 

 of fusion. 



29. Such an arrangement, on the other hand, is precisely that 

 which would ensue, if the materials, composing the strata, having 

 been mixed with, and suspended in water, had, after the fluid 

 became tranquil, gradually subsided and settled at the bottom. 

 In such a case, the matter thus subsiding would be deposited in 

 a regular series of layers, one above the other, with level and 

 parallel surfaces. The lowest layer would be composed of the 

 heaviest part of the matter held in suspension by the water, that 



41 



