4 2'22 ON THE ORIGIN, MATERIALS, COMPOSITION, 



micaceous sandstones, from which -it, in fact, differs 

 only in compactness ; and, where felspar is an ingre- 

 dient, 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 difficult to 

 comprehend, nature has repeated herself at consider- 

 able 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 mica- 

 ceous sandstone too obvious to be disputed ; and 

 whatever varieties of composition it may present, they 

 depend on different proportions of the micaceous 

 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 the state of heat to which, as before remarked, it 

 has been exposed. Its other peculiarities have already 

 been explained in a similar way. Gneiss, if we con_ 

 sider its materials, holds a parallel with a sandstone 

 containing clay and mica ; and here, although the 

 analogy becomes finally very feeble, there is a chain 

 through the varieties of this rock which connects it 

 with the secondary sandstones as perfectly as quartz 

 rock is thus traced. The causes for the evanescence 

 of this analogy, have been already shown to consist in 

 the posterior influence of heat, causing it to approxi- 

 mate to granite, and, finally, to graduate into it. It was 

 then also shown, that the action of heat converted 

 shale into hornblende; and thus, in the frequent 

 alternations of gneiss and hornblende schist, we 



