ON GEOLOGY. 



67 



rocks, whether simple or compound, which have been hitherto observed, 

 amount to about sixty ; of which the principal seem to be the eight following : 

 granite, gneiss, hornblend, limestone, wacke, basalt, quartz, and clay. 



Let us next pass on, then, to consider their relative situation. Of the 

 different rocks thus glanced at, and placed over each other, the whole crust 

 of the earth is composed, to the greatest depth that the industry of man has 

 been able to penetrate ; and I have already observed, that with respect to 

 each other, they occupy a determinate situation, which holds invariably in 

 every part of the globe. Thus, limestone, excepting under particular circum- 

 stances, hereafter to be explained, is nowhere found under granite, but always 

 above it. This general view of the subject may, indeed, induce a suppo- 

 sition that every separate layer which constitutes a part of the earth's sur- 

 face is extended round the entire globe, and wrapped about the central 

 nucleus, like the coats of an onion ; the kind of rock that is always lowest, or 

 nearest the centre, uniformly supporting a second kind, and this second kind 

 a third, and so on. Now, though the different kinds or layers of rocks do 

 not in reality extend round the earth in this uninterrupted manner — though, 

 partly from the inequality of the nucleus on which they rest, partly from their 

 own inequality of thickness in different places, and partly from other causes, 

 the continuity is often interrupted — yet still we trace enough of it to con- 

 vince us that the rocks which constitute the crust of the earth, when con- 

 templated upon a large scale, are every where the same, and that they inva- 

 riably occupy a like situation with respect to each other. 



The labours of Mr. Kirwan and M. de Saussure gave the earliest hints upon 

 this subject ; and the geological theories of Professor Werner of Freyburg, 

 and of M. de Cuvier of Paris, are entirely founded on the same. These theo- 

 ries, though derived in some measure from different sources of mineralogical 

 study, coincide not merely in their general outline, but in all their more pro- 

 minent parts, and only differ in their mode of accounting for the more limited 

 or local deposites. 



M. Werner, " from whom alone," to adopt the language of M. de Cuvier, " we 

 can date the commencement of real geology," so far as respects the mineral 

 natures of the strata, divided in his first view of the subject, all the various 

 rocks that enter into the solid crust of the earth, into five classes. 



Of these the first class consists of those rocks which, if we were to sup- 

 pose each layer to be extended over the whole earth, would, lie lowest, or 

 nearest the centre, and be covered by all the rest ; it comprises seven distinct 

 sets, as granite, gneiss, mica-slate, clay-slate, a peculiar kind of porphyry, 

 sienite, and a peculiar kind of serpentine. Of these granite lies the under- 

 most, and sienite the uppermost ; and in the midst of several of them we meet 

 with beds of not less than eight other kinds of rock, as though dropped into 

 them by accident — as topaz, another kind of porphyry, serpentine, limestone, 

 flint-slate, and trap, quartz, and gypsum ; which are hence called subordinate 

 rocks of this class, and which extend the whole number of sets belonging to 

 it to fifteen. 



These are supposed to have been earliest produced, and when the earth 

 first emerged from a state of chaos to a state of order; and are hence deno- 

 minated PRIMITIVE formations. They are distinguished by the following 

 character. Not a single relic of either animal or vegetable petrifaction is to 

 be found in any of them. The lowermost or older contain no carbonaceous 

 matter ; which is discoverable but very sparingly in the superior or newer. 

 They are all chemical combinations, and generally crystallized ; the crystal- 

 lized appearance being most perfect in the oldest, and gradually becoming less 

 perfect in the newer formations. I have already observed that the whole of 

 this scale of formations does not regularly coat the nucleus of the earth ; so 

 little so, indeed, that sometimes even the granite itself, the lowermost rock 

 of all, is left bare, and not pressed down or coated by a deposite of any other 

 kind of rock : arid so of the rest. Wherever this deficiency takes place, the 

 rock thus left at liberty rises uniformly higher than it is found to do where 

 pressed upon and invested with its common coatings. But every rock does 



