218 PREVAILING THEORY CONCERNING [Ch. XI. 



neath by fusion or sublimation. But we must not anticipate 

 the discussion, as it is only proposed at present to develope 

 the various and most approved principles of the igneous 

 theory. 



Such an exposition, however consistent with the brevity 

 here required, is not so easy a task, as at first sight it appears 

 to be ; but as it forms the basis of the proposed discussion, it 

 is requisite that it should not only be free from mis- 

 statements, but also be made clear and intelligible to the 

 reader. By way of recapitulation, therefore, we will call in 

 the aid of Lyell's eloquent and perspicuous pen, by quoting 

 such passages from his writings as will elucidate the nature of 

 the leading doctrines of this theory.* 



The class of rocks usually termed " primary," (a name 

 which is not always applicable, since the formations so de- 

 signated sometimes belong to different epochs), may, in 

 general, be justly regarded as of higher antiquity than the 

 oldest secondary groups, which we identify by organic re- 

 mains, because there are rounded pebbles of granite, as well 

 as gneiss, in the conglomerates of the oldest fossiliferous 

 groups. 



In order to explain to the reader the relation which we 

 conceive the primary rocks to bear to the tertiary and 

 secondary formations, it may be stated, that sedimentary 

 rocks, containing organic remains, occupy a large part of the 

 surface of our continents ; but that here and there volcanic 

 rocks occur, breaking through, alternating with, or covering 

 the sedimentary deposits : the primary rocks, however, cannot 

 be precisely assimilated to either of the preceding ; they are 

 often seen underlying the sedimentary, or breaking up to the 

 surface in the central parts of mountain-chains, constituting 

 some of the highest lands, and, at the same time, passing 

 down and forming the inferior parts of the crust of the earth. 

 These primary rocks are divisible into two groups : the one 



* Principles of Geology, vol. iii. p. 352. et seq. 



