AND ANALOGIES, OF ROCKS. 221 



strated ; and it has already been proved, that many of 

 the primary strata hear marks, scarcely to he dis- 

 puted, of the action of long-continued heat. Thus it 

 is to he expected that their organic remains, if they 

 ever existed, should have been obliterated ; and if this 

 has not happened in the case just quoted of shells 

 under gneiss, it may be attributed to the nature of the 

 bed in which they lie. Proofs of the truth of this 

 view are found most distinctly detailed by nature in 

 Sky, and in the Isle of Mann ; as I long ago indi- 

 cated, and as has been since confirmed on the continent 

 of Europe. Where the conchiferous beds are actually 

 converted into pure crystalline limestone by the action 

 of the incumbent trap, that is undistinguishable from 

 the primary rocks of the same kind, and all the shells 

 have disappeared ; while, in some parts of the grada- 

 tion between the stratified and fused rock, their 

 gradual loss of form, and final obliteration may be 

 traced. 



Having thus disposed of one great branch of the 

 analogy between the primary and secondary rocks 

 it is necessary to see what may be inferred respecting 

 the remainder. 



The difference between shale and slate, or between 

 the primary and secondary argillaceous schists, is often 

 so small as to have been a source of error, even among 

 experienced geologists. If, when separated from their 

 connexions, there are specimens, particularly among 

 the oldest of the shales, which no care nor practice 

 could distinguish from the primary schists, the re- 

 semblance between the sandstones and quartz rock 

 is often equally accurate ; though, in a general sense, 

 the latter is distinguished by its superior compactness 

 and more predominant crystalline texture. Where 

 quartz rock contains mica, it may be compared to the 



