58* 



ON GEOLOGY. 



entitled to our attention as the stkucture and situation of the rocks) 

 themselves. 



Rocks, as to their structure, may be contemplated under two divi- 

 sions, simple and compound. 



The simple division is, however, rather a speculative than a practical 

 contemplation. It is possible that rocks, and of immense magnitude, may 

 exist in parts of the globe we are not acquainted with, that are perfectly 

 simple and unmixed in their structure ; but it is seldom, perhaps never, that 

 they have been actually found in such a state, at least to any considerable 

 extent. 



It is only under a compound form, therefore, or as composed of more 

 than one mineral substance, that rocks are to be contemplated in our pre- 

 sent survey of the subject ; and in this form we meet with them of two 

 kinds : cemented, or composed of grains, or nodules, agglutinated by a 

 cement, as sandstone and breccia or pudding-stone ; and aggregated, or 

 composed of parts connected without a cement, as granite and gneiss. 

 The component parts of the cemented rocks are often very multifarious ; 

 those of granite and gneiss much less so, consisting chiefly of felspar, mica, 

 and quartz, with garnets, shorl, or hornblend occasionally intermixed with 

 ihe mass. The granite that forms the flagstones of Westminster Bridge 

 are supposed to have been brought from Dartmoor ; and, like the rest of 

 the Dartmoor granite, is remarkable for the length of its crystals of felspar, 

 which in some instances are not less than four inches. 



The aggregate rocks, like the cemented, are sometimes found of an in- 

 determinate, but more generally of a determinate or regular form ; and it is 

 the office of that branch of mineralogy to which M. Werner has given the 

 name of oryctognosy^ to distinguish and describe them by these peculiari- 

 ties. This is a branch into which I cannot plunge, for it would lead us 

 irom that general view of the science to which our present course of study 

 is directed, into a detailed analysis. Those who are desirous of pursuing 

 it in this line of developement may consult with great advantage Professor 

 Jameson's System of Mineralogy, or M. Brogniart's " Traite Elementaire," 

 or M. Cuvier's Essay on the Theory of the Earth, prefixed to his Fossil 

 Remains. 1 can only observe, at present, that the total number of rocky 

 masses, or different kinds of rocks, whether simple or compound, which 

 have been hitherto observed, amount to dbout 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 ot^ jr, 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 in- 

 variably in every part of the globe. Thus, hmestone, excepting under parti- 

 cular circumstances hereafter to be explained, is no where found under gra- 

 nite, but always above it. This general view of the subject may indeed induce 

 a supposition that every separate layer which constitutes a partof the earth's 

 surface 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 reahty extend round the earth in this uninterrupted manner— though 

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



