70 



PRIMITIVE ROCKS. 



fore, we see a mountain bounded by a plain, we are 

 not to suppose that the strata of which it is formed 

 terminates at its base, but rather that they dip 

 under the other rocks at angles more or less incli- 

 ned, penetrating far into the interior of the earth, 

 and often rising again in remote districts.* It is 

 proper, perhaps, here to remark, that Mr. Lyell, the 

 distinguished President of the London Geological 

 Society, m his Elements of Geology, objects to the 

 word primitive^ on the ground that it is not true that 

 all granites and rocks called primitive were first 

 formed, and the aqueous and volcanic rocks super- 

 imposed. He thinks that granite has originated at 

 different epochs, some antecedeiU, others subse- 

 quent to the origin of many fossiliferous strata, and, 

 by the agency of internal heat, ejected in the form 

 of veins or dikes through the superincumbent rocks, 

 perhaps changing the very rock containing fossils 

 into crystalline granite or gneiss. He therefore pro- 

 poses the term h^pogene, from the Greek words sig- 

 nifying ''under" and ''to be born," signifying that 

 such rocks are formed beneath the others, and have 

 not assumed their present form and structure at the 

 surface. He also calls such rocks Plutonic, because 

 he believes they could not have acquired theircrys- 

 talline structure unless they had been modified by 

 the agency of heat under great pressure in the 

 depths of the earth. 



De la Beche also remarks, " that in the earlier days 

 of geology, granite was considered the fundamental 

 rock on which all others were accumulated ; but 

 this opinion, like many others, has now given way 

 before facts: for we have examples of granite rest- 

 ing upon stratified and fossiliferous rocks of no very 

 great comparative antiquity. It must, however, be 

 confessed, that granite appears sometimes to alter- 

 nate in considerable thickness with the inferior strat- 

 ified rocks, and that the separation of it from gneiss 



* BakewelPs Geology. 



