66 



ON GEOLOGY. 



to the uninhabitable, and the civilized an almost infinitely smaller proportion 

 still. Hence our experience must be extremely limited ; a thousand facts 

 may be readily conceived to be unfolded that we are incapable of account- 

 ing for ; and, at the same time, a variety of contradictory hypotheses to be 

 formed with a view of accounting for them. 



So far as the superficies of the earth has been laid open to us by ravines, 

 rivers, mines, earthquakes, and other causes, we find it composed of a multi- 

 tude of stony masses, sometimes simple, or consisting- of a single mineral 

 substance, as limestone, serpentine, or quartz; but more frequently compound, 

 or constructed of two or more simple materials variously intermixed and 

 united; as granite, which is a composition of quartz, felspar, and mica; and 

 sienite, which is a composition of felspar and hornblend. These stony masses 

 or rocks are numerous, and they appear to be laid one over the other, so that 

 a rock of one kind of stone is covered by a rock of another kind, and this 

 second by a third kind, and so on, in many instances, for a very considerable 

 number of times in succession. In this superposition of rocks it is easily 

 observable that their situation is nol arbitrary. Every stratum occupies a 

 determinate place ; so that they follow each other in regular order from the 

 deepest part of the earth's crust, which has been examined, to the very sur- 

 face. Thus there are two things respecting rocks which claim our peculiar 

 attention — their composition and their relative situation. And independently 

 of the rocks thus considered as constituting almost the whole of the earth's 

 crust, there are other masses of fossil materials that must be likewise 

 minutely studied ; which traverse rocks in a different direction, and are 

 known by the name of veins ; as if the rocks had been split asunder in dif- 

 ferent places from top to bottom, and the chasms had been afterward filled up 

 with the matter which constitutes the vein. And hence the veins which 

 intersect rocks are as much entitled to our attention as the structure and 

 SITUATION of the rocks themselves. 



Rocks, as to their structure, may be contemplated under two divisions, 

 simple and compound. 



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

 templation. 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 present sur- 

 vey of the subject; and in this form we meet with them of two kinds: 

 CEMENTED, or composcd 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 chiefl}'^ of felspar, mica, and quartz, with gar- 

 nets, shorl, or hornblend occasionally intermixed with the mass. The gra- 

 nite that fovms the flag-stones 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 inde- 

 terminate, 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 peculiarities. 

 This is a branch into which I cannot plunge, for it would lead us from 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 Sys- 

 tem of Mineralogy, or M. Brogniart's Traite Elementaire, or M. Cuvier's 

 Essay on the Theory of the Earth, prefixed to his Fossil Remains. I can only 

 observe, aX present, that the total number of rooky masses, or different kinds of 



