GEOLOGY. 



ftrata between the chalk near London, and the granite of 

 Devonftiire, is much greater than what it really is, taking 

 it for granted that the dip of the llrata is always regularly 

 to the eaft ; but this is not the cafe. A ftratum of any 

 confiderable extent has waves and irregularities, by which it 

 is generally fpread over a large fpace compared with its true 

 thicknefs and fuppofed angle of inclination. A ftratum not 

 more than feventy yards in thicknefs may extend in the line 

 of its dip ten miles or more, and may appear to have a con- 

 fiderable dip \\hen obferved in certain fituations ; and were 

 we to calculate its thicknefs from the extent of ground 

 which it covers, and from the dip, we might infer that it 

 exceeded two thoufand yards or more. More accurate 

 obfervations will convince us, that the numerous fractures or 

 bendings of the ftrata in the line of their dip generally 

 fpread them over a much wider fpace than the angle of 

 jnchnation and thicknefs would lead us to believe ; and the 

 comparifon which has been made of Aiding a number of 

 books under each other to reprefent the fuppofed thicknefs 

 of the « hole ftrata of England is utterly inapplicable to the 

 cafe. In feftions of particular diftrifts, to reprefent the 

 arrangement and dip of the ftrata, it is impoffible to deli- 

 neate the irregularities and wavings of the ftrata on a fmall 

 fcale. In the fection of England by Mr. Bakewell, (fee 

 Plate 111. Jig. I. Geology,) the various ftrata from the Ger- 

 man ocean to Crofs-fell are reprefented rifing regularly 

 from under each other, like a number of books in a llanting 

 pofition, it being imprafticable to reprefent on fuch a fmall 

 fcale all the irregularities of each ftratum. In Plate III. 

 Jig. 2. the feftion is on a larger fcale, and the lias ftratum, 

 d,'d,d,d, is reprefented riiing from under the green fand^, ^, i, 

 near Bridport in Dorfetftiire, and continued to the valley, 

 M, at Axmouth ; whereas had not the ftrata been diflocated 

 by a great number of fraftures, as reprefented in the fettion, 

 the lias ftratum d would have terminated or cropped out 

 eaft of the letter L, which reprefents the fituation of the 

 town of Lyme. The aggregate thicknefs of all the beds of lias 

 cannot be more than two hundred yards, and the regular 

 inclination is at leaft one yard in twenty, which would make 

 the loweft bed of has crop out about four thoufand yai'ds, 

 or two miles and a quarter weft of its lirft appearance near 

 Bridport. 



Plate III. Jig. 6. Geology, reprefents the waving ilruc- 

 ture of the beds of flate, provincially called (hillet in 

 Devonfliire. In fome fituations, as near Moreton, beds of 

 black Hme-ftone are interpofed, and take the twilled form 

 of the flate. Now in pafling from Exeter to Dartmoor 

 over the fraftured edges of the ftrata, as reprefented^/f^. 6, 

 the traveller may crofs portions of the fame ftratum a, a, a, 

 repeatedly at a confiderable diftance from each other, and 

 were he to fuppofe each of thefe portions to be a feparate 

 ftratum, and to calculate accordingly, the thicknefs of the 

 whole bed of flate, from the red ground on the eaft, to 

 Dartmoor on the weft, he would make it not lefs than ten 

 miles ; whereas in all probability it may not exceed three 

 or four hundred yards. On a fmaller fcale, the ftrata are 

 often extended over a large fpace by fraAures in many of 

 the coal diftriils ; by which a bed of coal is brought near 

 the furface feveral times in the line of its dip, as reprefented 

 Plate 1. Jig. I. Geology, and defcribed in the article Coal. 

 See Coal, and Veins, Mineral. 



It too frequently happens, that geological obfervers 

 meafure nature by the ftandard of their own limited expe- 

 rience in pafling through a country, and defcribe certain 

 rock formations as deftitute of organic remains, becaufe 

 they have not found them in travelhng through a diftrift. 

 The alpine part of Weftmoreland and Cumberland near 



the lakes confifts of grey wacke, clay-flate, compad felfpar, 

 porphyry, fienite, trap, chnkftone, and granite ; a thin bed of 

 ftratified lime-ilone is interpofed, and runs through a fpace of 

 fifteen miles, containing organic remains of coralloids, though 

 the rocks which cover this lime-ftone, to a great depth, and 

 the rocks on which it refts, contain no obfervable veftiges of 

 organic hfe. Their relative pofition in the valley of Long 

 Sleddale in Weftmoreland, is reprefented Plate 111. fg. 5- 

 Geology. The flate which covers the hme-ftone appears to dip 

 at an angle of feventy degrees ; but on more attentive exa- 

 mination it will be found, that what might be mif- 

 taken for regular ftrata are merely the fchiftofe lamina of 

 the flate arranged in the direftion of the cleavage, the dip 

 of the flate being in reality the fame as that of the lime- 

 ftone on which it refts. Under the lime-ftone occurs a bed 

 of horn-ftone, refembhng compatt felfpar, butinfufible ; this 

 is eighty yai-ds thick, and refts on other beds of fchift, as 

 reprefented Plate III. Jg. 5. Geology. This fchift and 

 horn-ftone contain no organic remains, and appear to be 

 connedled with the granite, which makes its appearance in 

 the adjacent valley at no great diftance. The difcovery of 

 organic remains under rock formations of great extent 

 which are deftitute of them is a circumftance of great in- 

 tereft to the geologift wherever it occurs, and proves the 

 neceflity of caution in deciding whether certain rocks were 

 formed prior to the exiftence of organic beings. Ba- 

 faltic or trap rocks, whofe fituation is not conformable with 

 the general dip or pofition of the ftrata, and which bear a 

 near fimilarity to volcanic rocks in appearance and compo- 

 fition, are defcribed under the articles Trap, Rowlev- 

 Ragg, Whinstone, and Veins, Mineral ; and alfo the 

 article Basalt, Addenda; which fee. Plate IV . Jig. 2. 

 Gfo/ofji, reprefents the arrangement of a feries of columnar and 

 amorphous beds of bafalt placed over regular ftrata in an 

 unconformable pofition, and interfered by veins or dykes 

 of bafalt b I, in which the ftrufture is coliunnar ; but the 

 columns or bafaltic prifms in thefe dykes are arranged 

 horizontally. 



Fig. 4. reprefents a bafaltic rock on the coaft of the 

 county of Antrim, which inclofes a bed of chalk that it 

 appears to have broken and enveloped ; an effect which 

 feems to require that the bafalt ftiould once have been in 

 a melted ftate like lava. The bafalt is reprefented in the 

 Jig. immediately beneath the above, as it occurs in veins in 

 another fituation on the fame coaft, cutting through the 

 chalk, and changing it to a certain diifance into cryftalline 

 lime-ftone or marble. See Trap and Whinstone. 



Plate IV .Jig. I. Geology, reprefents the fedtion of a regular 

 metalHc vein.which divides into two, and meets again, lea\-ing an 

 intervening fpace, filled with earthy minerals called rider. The 

 vein is reprefented as feparated from the rock by a thin lining 

 of clay c c, which generally accompanies veins. The 

 interfedlion of metallic veins in the fame plate reprefents 

 two veins containing the fame kind of ore, and having the 

 fame dip as a, a, a, a, interfered by a vein of a different 

 kind b b, which has cut through and difplaced the former ; 

 in thefe inftances, the vein bb \i fuppofed to beofpofterior 

 formation to the veins a, a, a, a. In the fame_^. is (hewn 

 the difplacement of a vein without any interfection of other 

 veins. ( See Veins, Metallic. ) For an account of volcanic 

 rocks, fee the articles Volcano and Volcanic Products, 

 and alfo Systems of Geology. For an account of the or- 

 ganic remains in rocks, fee Petrifactions, Rocks, 

 Strata, and Fletz Rocks. For an account of the or- 

 ganic remains of extincl fpecies of large quadrupeds in 

 alluvial foil, fee Mammoth, Mastodon, Mecu-onix, and 

 Megatherium, Addenda. 



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