OH GEOLOGY. 



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invention at times takes the place of induction, and the man of wisdom 

 has his dream as well as the man of fancy. 



Let us descend from such magnificent flights : let us quit the possible for 

 the actual ; and equally incapable of following up the fugitive material of 

 which the visible universe consists, into its elementary principles and col- 

 lective mass, let us examine it as far as we are able, in the general laws, 

 structure, and phasnomena it exhibits in the solid substance of the globe 

 on which we tread. 



It is this inquiry that constitutes the science of geology, a brief outline 

 of which is intended as a study for the present lecture ; — a science than 

 which few are of more importance, but which is only at present in its in- 

 fancy, and of coiirse almost entirely indebted for its existence to the un- 

 wearied assiduity and discoveries of modern times. 



The direct object of geology is, to unfold the solid substance of the 

 earth — to discover by what causes its several parts have been either arranged 

 or disorganized — and from what operations have originated the general 

 stratification of its materials, the inequalities of its surface, and the vast 

 variety of bodies that enter into its make. 



In pursuing this investigation, many difficulties occur to us. The bare 

 surface, or mere crust of the earth's structure, is the whole we are capable 

 of boring into, or of acquiring a knowledge of, even by the deepest clefts 

 of volcanoes, or the deepest bottoms of different seas. It is not often, 

 however, that we have the power of examining either seas or volcanoes so 

 low as to their bottom. The inhabitable part of the globe bears but a small 

 proportion 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 inca- 

 pable of accounting 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 mul- 

 titude of stony masses, sometimes simple, or consisting of a single mineral 

 substance, as limestone, serpentine, or quartz ; but more frequently com- 

 pound, or constructed of two or more simple materials, variously inter- 

 mixed 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 an- 

 other 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 superposi- 

 tion of rocks it is easily observable that their situation is not arbitrary. 

 Every stratum occupies a determinate place ; so that they follow eacli 

 other in regular order from the deepest part of the earth's crust, which has 

 been examined, to the very surface. Thus there are two things respecting 

 rocks which claim our peculiar attention—their composition and their rela- 

 tive situation. And independently of the rocks thus considered as con- 

 stituting 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 rocka had been split asunder in different places from top to bottom, 

 and the chasms had been afterwards filled up with the matter which con- 

 sttitutes the vein. And hence the vbins which intersect rocks are as much 



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