54 



CHAPTER V. 



ON ROCKS DENOMINATED PRIMARY, AND THE CHANGES TO WHICH 

 THEY HAVE BEEN SUBJECTED. 



The Origin of Rocks called Primary, believed by many Geologists to be igne- 

 ous. — A Classification founded on this View. — A Classification independent of 

 Theory. — Constituent Minerals of Granite. — Varieties of Granite. — Structure 

 and Appearance of Granitic Mountains. — Mont Blanc, and the Aiguilles in its 

 Vicinity. — Localities of Granite. — Granite Veins. — Passage of Granite into 

 Porphyry and Sienite. — Minerals found in Granite. — On Granite as the Foun- 

 dation Rock on which other Rocks are laid. — The relative Antiquity of differ- 

 ent Granitic Mountain Ranges. — Granite pierced through by Porphyry and 

 Currents of Lava. — Granite sometimes protruded among the upper Strata. 



In describing the different classes of rock, we may either commence 

 with the lowest or most ancient, or with the uppermost or most recent; 

 hut 1 am persuaded that the student will find it most convenient to he- 

 gin with the lowest and proceed in an ascending series to the upper- 

 most. The rocks called primary have distinctly marked mineral cha- 

 racters, and contain few, if any, organic remains. As the student 

 proceeds, he may trace the first indications of organic existence, and 

 in ascending to the upper rocks, he will observe the gradual increase 

 of genera and species that have left their remains in the different 

 heds ; in some cases indicating great changes in the condition of parts 

 of the globe, as from sea to land, or from salt water to fresh, or from 

 deep to shallow sea. If the student begin with the more recent or 

 uppermost strata, he will find them difficult to recognize by fixed 

 mineral characters, and he will be confused by the variety of organ- 

 ic species presented to his notice, but from which he can derive little 

 instruction until he is able to compare them with the fossil remains in 

 the lower strata. In the geological description of a particular coun- 

 try or district, it may, often, he more convenient to commence with the 

 heds nearest the surface, and proceed in a descending series, but 

 then the reader is supposed to be already acquainted with the sci- 

 ence. 



If any rocks can with propriety be denominated primary or prim- 

 itive, they are those which are most widely spread over the globe in 

 the lowest relative situation, and which contain no remains of organic 

 existence. Primary rocks are supposed by geologists to constitute 

 the foundation on which rocks of all the other classes are laid ; and 

 if we take an enlarged view of the structure of the globe, we may 

 admit this to be the fact, — but the admission requires certain limita- 

 dons. The same causes that have produced granite and the other 



