GEO 



than with usj f the se- 



happens one' re inter- 



the south / fc ta O f t en 



tweenjlg- 2, fthe val- 



vertu-w g, w between 



ains have 

 lad been 

 Ind their 

 led away, 

 [he plains 

 ns. The 

 ten inter- 

 inginall 



possible directions, and occupying 1 vacui- 

 ties, which appearto be of somewhat later 

 date than the original formation of the 

 mountains. The volcanic mountains in- 

 terrupt those of every other description, 

 without any regularity, as if their origin 

 were totally independent of all the rest. 

 The internal constitution of the earth 

 is little known from actual observation, 

 for the depths to which we have penetrat- 

 ed are comparatively very inconsiderable, 

 the deepest mine scarcely descending 

 half a mile perpendicularly. It appears, 

 that the strata are more commonly in a 

 direction nearly horizontal than in any 

 other ; and their thickness is usually al- 

 most equable for some little distance; but 

 they are not disposed in the order of their 

 specific gravity, and the opinion of their 

 following each other in a similar series, 

 throughout the greater part of the globe, 

 appears to rest on very slight foundations. 

 From observations on the attraction of 

 the mountain Schehallien, Dr. Maskelyne 

 inferred the actual mean density of the 

 earth to be to that of water as four and a 

 half to one, judging from the probable 

 density of the internal substance of the 

 mountain, which he suppose clto*beasolid 

 rock. Mr. Cavendish has concluded- more 

 directly, from experiments on a mass of 

 lead, that the mean density of the earth is 

 to that of water as five and a half to one. 

 Mr. Cavendish's experiments, which were 

 pei-formed with the apparatus invented 

 and procured by the late Mr. Michell, ap- 

 pear to have been conducted with all pos- 

 sible accuracy, and must undoubtedly be 

 preferred to conclusions drawn from the 

 attraction of a mountain, of which the in- 

 ternal parts are perfectly unknown to us, 

 except by conjectures founded on its ex- 

 ternal appearance. Supposing both series 

 of experiments and calculations free from 

 error, ;t will only follow, that the internal 

 pai is of Schehallien are denser, and per- 

 haps more metallic, than was before ima- 

 gined. The density assigned by Mr. 

 Cavendish is not at all greater than might 

 be conjectured from observations on the 



vibration of pendulums : Newton had long 

 ago advanced it as a probable supposition, 

 that the mean density of the earth might 

 be about five or six times as great as that 

 of water, and the perfect agreement of the 

 result of nv.ny m *'ern experiments with 

 this conjecture affords us a new proof, in 

 addition to many others of the accuracy 

 and penetration of that illustrious philoso- 

 pher. See GLOBKS. 



GEOLOGY has for its object the struc- 

 ture and formation of this globe : it, of 

 course, embraces the consideration of the 

 materials of which it is composed, and the 

 circumstances peculiar to its original for- 

 mation, as well as the different states un- 

 der which it has existed, and the various 

 changes which it has undergone. 



It necessarily follows, from the very li- 

 mited depth within which our actual exa- 

 minations have been made, that our facts 

 and real observations are confined to what 

 may be considered, comparatively, as 

 merely the crust of the globe With re- 

 spect to its more internal part, we have 

 hitherto only been aided by conjecture, 

 which, it must be admitted, has too fre- 

 quently led to theories the most extrava- 

 gant and absurd. From the experiments 

 of several learned men, it, however, ap- 

 pears, that the density of the globe is 

 greatest towards its centre. Boscovich is 

 of opinion, from his very ingenious calcu- 

 lations, that the centre is a spherical nu- 

 cleus, possessing an equal degree of den- 

 sity to within some leagues of the earth's 

 surface; but although it is thus conclud- 

 ed, that the interior of the earth is solid, 

 contrary to the conjectures of several an- 

 cient philosophers, yet it is by no means 

 pretended, that even in this, its more solid 

 parts, there may not exist cavities of u 

 greater or less size, connected, perhaps, 

 with each other, and extending consider- 

 ably, in all probability, towards the sur- 

 face. 



The solid masses of the globe, which 

 have come within our examination, have 

 been distinguished into primitive and se- 

 condary ; among the former were placed 

 the rocks of granite, gneiss, porphyry, 

 serpentine, and limestone, of a peculiar 

 character ; and, among the latter, were 

 considered the rocks of secondary lime- 

 stone, of phosphate of lime, of gypsum, 

 and of some of the sand-stones ; of chalk, 

 and of silex. This division is not, how- 

 ever, at present universally adopted ; 

 other divisions having been assumed, 

 which have appeared to agree better with 

 the different systems which have been 

 proposed : these divisions we shall there- 

 fore more fully notice, after pointing 1 out 



