TRAP. 



*l'he igneous productions of the fetond kind are bafalts, 

 which iu the form of beds, tabular maffes, or peaks, cover 

 the elevated parts of the ancient furface of the coiuitry, or 

 occur on the fummits of fome mountains and ifolated emi- 

 nences. They are alfo obferved on the lldrting round the 

 iides of Mont Dor and Cantal. They are, fays jVI. Dau- 

 buiffon, 'evidently only the remains and patches of different 

 currents which have fpread over the country : they prefent 

 the fame mineralogical charafters as the bafalts of Saxony, 

 and other diftrifts ; they contain the fame fubftances, and 

 have the fame tendency to form regular prifms or columns ; 

 they cover, without diftinftion, all kinds of rocks, and are 

 never covered by them. An igneous origin cannot be 

 denied to thefe balalts ; the perfeft refemblance between 

 their parte and that of fome of the currents of lava in tlieir 

 vicinity, which can be traced to exifting craters, affords a 

 ftrong prefumption of this ; but they preient other infallible 

 marks of their origin. In following Hep by Hep certain 

 maffes of bafalt, which are near Mont Dor and Cantal, and 

 fupplying by imagination what has been taken away, where 

 their continuity is broken, you arrive at the fides of thefe 

 two enormous volcanic mountains, and we come to maffes 

 of fcoria: or of bliftered rocks, where beyond doubt we are 

 near the current : all the bafalts which have been traced 

 upwards made part of that current. A great number of 

 thefe large bafaltic platforms, which cover ifolated moun- 

 tains, difplay on' their furface bliftera, fpongy fcorias or 

 droffes, like thofe which cover the beft prcferved lavas ; nor 

 can we refufe them a fimilai- origin. Some of thefe plat- 

 forms repofe on volcanic afhes. It is true, fome ifolated 

 eminences have fummits of compaft prifmatic bafalt of a 

 black colour, and deftitnte of thofe unequivocal figns of 

 the aftion of fire which are feen elfewhere ; but thefe emi- 

 nences of compadt bafalt moft frequently ftand by the fide 

 of thofe platforms with fcoriated furfaces, before defcribed : 

 they once formed with them a continued whole, and have 

 evidently been divided from them by the excavation of the 

 ravines and vallies which now leparate them. They cannot 

 have had a diflerent origin. The convulfive aftion of time 

 and the elements mull have deftroyed the fcorified cruft : 

 only the compaft nucleus remains, deprived of the marks of 

 the aftion of fire, like the interior parts of moft currents of 

 lava, which are equally deftitute of fuch marks. Thus, 

 fays M. Daubuiffon, all the bafalts of Auvergne prefent 

 proofs either direft or indireft of an igneous origin ; though 

 the difintegration of the furface, and the derangement that 

 the currents have fuffercd, prevent us from retracing them 

 to the crater from whence they flowed, and from feeing the 

 number, form, and extent of the different currents ; but we 

 are certain that their exiftence was anterior to the excavation 

 of the vallies. 



The porphyroid.il trap is the third kind of igneous rock- 

 formation m Auvergne. It forms eight or ten diftinft moun- 

 tains : the moft confiderable are Mont Dor, Cantal, and Puy- 

 de-D6me. The fubftance of thefe mountains, from the ac- 

 count of Daubuiffon, appears to be a kind of felfpar or clink- 

 ftone porphyry : their paffage, direft or indireft, into bafalt, 

 and, above all, the volcanic fcoriae imbedded in their niafs, 

 prove that they owe their exiftence to fire. Thefe porphy- 

 ries are the moft ancient of the igneous produftions of Au- 

 vergne ; they are covered with bafalt, and contain veins of 

 that fubftance. 



In the opinion of fome geologifts, thefe mountains and 

 maffes of porphyry have never flowed as lava, but have been 

 fufed in their native beds, and fubfequently elevated. The 

 homogeneity of their pafte Ihews how complete the fufion 

 has been. The cryftals of felfpar were, in all probabihty, 



9 



formed during the igneous fluidity of the mafs. However 

 different tlie currents of lava, the bafalt, and the porphyiitic 

 trap may be, and however various the periods of their forma.- 

 tion.they appear, fays M. Daubuiffon, to be united in a certain 

 degree. Cantal, Mont Dor, Puy-de-D6me, and the other 

 porphyry mountains, are ranged in a direft line running nearly 

 from north to fouth. Almoll all the bafalts of thefe regions, 

 that can be, in fome manner, retraced to their origin, feem 

 to have taken their direftion in the fame line. It is alfo in 

 this direftion, and among the ancient produfts, that the 

 greater part of the craters have been opened whole veftiges 

 are ftiU vifible. When, at two leagues to the weftward of 

 Clermont, we fee near fixty volcanic mountains ranged in a 

 ftraight line, it can fcarcely be believed to be the effeft of 

 chance. A caufe has certainly exifted, which has produced 

 this effeft. Perhaps, fays Daubuiffon, there was in this 

 direftion a vein of matter that contained the germ of vol- 

 canic fire, which burft forth and was renewed at different 

 periods. 



Perhaps this range of mountains may be regarded as an 

 enormous bafaltic dyke, which has broken through the crult 

 of the globe along the line \vhere they are now ranged, their 

 continuity having been broken by partial fubfidence, and by 

 diluvial currents. Daubuiffon's account of the trap-rocks 

 or bafalt of Auvergne is particularly interefting, as eftablifh- 

 ing the intimate conneftion between trap-rocks and thofe of 

 undoubted igneous origin. It is further remarkable, as 

 M. Daubuiffon had previoufty pubhftied an account of the 

 bafalts of Saxony, in which he endeavoured to prove the 

 aqueous formation of bafaltic rocks. 



The moft remarkable trap-formation in Europe, and 

 perhaps in the world, extends from the county of An- 

 trim, in Ireland, through part of the Scotch Hebrides, and is 

 probably the fame which occurs in the Ferroe iflands, and 

 the ifland of Iceland. An account of the columnar bafalt 

 of this formation will be found under the articles Giant's 

 Caujeivay, and Staffa. T!ie geological relations of the 

 trap-rocks of Antrim with the regular ftrata in that country, 

 have been recently traced by the Rev. J. Connybeare and 

 Dr. Berger. The bafalt of this diftrift generally covers 

 chalk, fimilar to the chalk on the eaftern fide of England, 

 and like it refting upon beds of fand, and the grey ftratified 

 lime-ftone called lias. In England we have no inftance of 

 bafalt occurring above the chalk, or any of the ftrata over 

 the lias, or in the lias itfelf, though it occurs in the coal- 

 formation under the has. A fine feftion of the beds of 

 bafalt with the chalk, fand-ftone, lias, and coal-ftrata, may 

 be feen for many miles along the coaft, prefenting numerous 

 dykes and fractures of the ftrata, which have fometimes 

 brought the bafalt to the fame level with the chalk, and in 

 other fituations with the coal-ftrata. The bafalt in the in- 

 terior, and in fome parts of the coaft, forms ifolated caps on 

 the fummits of detached conical hills. The exiftence of thefe 

 ifolated caps has been attributed to diluvial currents, which 

 have cut through the o.nce continuous beds of bafalt. 

 Though fuch currents have doubtlefs exifted in various fitu- 

 ations, and have excavated vallies, another agent has obvi- 

 oufly been employed in the formation of the vallies of An- 

 trim. We have only to examine the coaft to convince our- 

 felves of this faft, where we may fee the ftrata fuddenly 

 thrown down or elevated feveral hundred feet, by mineral 

 veins or dykes filled with bafalt. If we trace in imagination 

 the fame difplacement into the interior, where it obvioufly 

 extends, we fhall fee a fufEcient caufe for the exiftence of 

 ifolated caps or beds of bafalt, feparated from each other by 

 vallies, or lower grounds, that have been formed by fub- 

 fidence. Through thefe veins or dykes the bafalt itfelf was 



probably 



