806 Dr. Mac CullocK on the Origin, Materials^ 



elation 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 be- 

 tween 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 to well-trained geologists. If, when 

 separated from their connexions, there are specimens, particularly 

 among the oldest of the shales, which no care or practice could 

 distinguish from the primary schists, the resemblance between the 

 sandstones and quartz rock is often equally accurate ; although, in 

 a general sense, the latter is distinguished by its superior com- 

 pactness and more predominant crystalline texture. Where it 

 contains mica, it may be compared to the micaceous sandstones, 

 from which it, in fact, differs only in compactness ; and, when fel- 

 spar is an ingredient, it is obvious that it bears an analogy to the 

 argillaceous ones. 



Here then, in primary limestone, quartz rock, and argillaceous 

 schist, We trace an analogy, not of a very remote nature, to the 

 secondary strata ; showing that, with certain variations, from 

 causes not difftcult to comprehend, nature has repeated herself at 

 considerable intervals of time, and has been guided by laws of 

 great general simplicity. It remains to extend this analogy one 

 step further ; but the difficulties increase, as might be expected, at 

 each remove. 



In micaceous schist, we find an analogy to micaceous sandstone 

 too obvious to be disputed ; and whatever varieties of composition 

 it may present, they depend on different proportions of the mica- 

 ceous ingredient ; the predominance of which, in particular 

 cases, may probably be attributed to the nature of the rocks from 

 which its materials were derived, possibly from tbie state of heat 

 to which it has been exposed. Its other peculiarities are ex- 

 plained in a similar way. Gneiss, if we consider its materials, 

 holds a parallel to a sandstone containing clay and mica ; and 

 here, although all analogy becomes finally very feeble, there is a 



