386 Geology. — On the Cotistituent Minerals, ^. 



We have the less regret in beheving we shall never witness 

 the demonstration of this hypothesis, because we think a line 

 exists, separating the primary rocks from those called transition, 

 which will serve all the purposes of the proposed theoretical 

 division, depending upon organization, and which every one may 

 demonstrate for himself, who has a proper knowledge of the 

 mineral character of rocks. Our desire of making what we 

 have to say on this branch of our subject, intelligible to our un- 

 initiated readers, will lead us at present into some elementary 

 details, necessary to a clear understanding of the mineral nature 

 of all rocks, and especially of the great characteristic difference 

 between those beds comprehended in the primary division, one to 

 four inclusive,* and all the other beds enumerated in the series. 

 The line of which we have spoken, we conceive to be constituted 

 where the primary rocks terminate ; for, as it will be hereafter 

 seen, although the constituents of all the rocks contained in the 

 crust of the earth, are substantially the same, yet all geological 

 writers of any reputation, now agree that the primary and se- 

 condary rocks — including in these last, all rocks lying above the 

 primary, — have been brought into their places by two different 

 agents. It appears, therefore, to be more consistent with ob- 

 servation to say, that a transitionary state of the beds of the 

 planet commenced, when the agency which brought the primary 

 rocks into their places appears to have ceased, and when that 

 which deposited the secondary rocks, appears to have com- 

 menced. In this view of the subject, we exclude, for the present, 

 the intrusive rocks, which, as we have stated at page 342, we 

 have also excluded from the tabular view. We have adopted 

 this manner of treating the subject, that our readers may pursue 

 it, uninfluenced by the prejudice which the theories and terms 

 belonging to the infancy of geology have impressed too many 

 with. We now proceed to elementary details. 



All the rocks, in whatever part of the series they may be 

 found, have, for their constituent elements, very few substances 

 or earths. The four principal ones are silica, alumina, lime, and 

 magnesia. Potash and soda also, with iron, in various states of 

 oxidation, enter, with these earths, as ingredients into many 

 rocks. We owe to Sir Humphrey Davy the brilliant discovery 



* Vide Table, page 338 



