TRAP. 



• ' From fomc unknown caufe, a fudden riling of the water 

 took place in a turbid and agitated ftate, and covered the 

 whole globe : it then became calm, and during the period of 

 ks fetthng, dcpolited the different rocks of overlying trap, 

 I and again retired to its former level with conliderable 

 rapidity. The broken ftratification, fo charaifteriftic of this 

 formation, was caufed partly by the rapid retiring of the 

 water. The heaps of trees, the beds of clay, faiid, and 

 gravel, and their conftant occurrence in the lower pai't of 

 this formation, are evident proofs (fays Mr. Jamefon ) of the 

 rapid and tumultuous rifing of the water. The fubiequent 

 calmnefs of the water is proved by the finenefs of tlie me- 

 chanical and the increafing finenefs of the cliemical folutions, 

 as we approach the upper part of the formation. Jamefon's 

 Mineralogy, vol. iii. p. 85. 



According to this theory, the regular ranges of columns 

 fo frequent in trap-rocks, were produced by the (hrinking 

 in or drying of the mafs. As the inundation was general, 

 the rocks of this family muft be fpread univerfally over the 

 globe. Indeed the fame geologift fuppofes there were two 

 inundations, the firft, which depofited the overlying porphyry 

 and fienite, the fecond, which depofited the baialtic or flcetz 

 trap-rocks. On this theory, Mr. Bakewell remarks in his 

 Introdudlion to Geology, " it is fcarcely poffible for the 

 human mind to invent a fyftem more repugnant to exifting 

 facts. Were bafaltic rocks, as Werner fuppofes, depofited 

 from an aqueous fluid, that covered the whole globe after 

 the formation of the fecondary ftrata, every part of the 

 dry land and every valley mull have been incrufted or filled 

 with bafalt ; it would be the prevailing rock of every dif- 

 tria : on the contrary, overlying trap or bafalt exifts only 

 in detached maffes, in particular lituations, nor do fragments 

 of bafalt occur in any quantity fufhcient to warrant the 

 behef that it was ever formed univerfally over tlie globe." 



Though the mode of aqueous formation, fuggelled by 

 Werner, is utterly inadmiifable, we do not think it impro- 

 bable that many bafaltic rocks may have been formed in the 

 humid way, by eruptions of mud of limited extent, like 

 thofe which ilfue at prefent from the volcanoes in South 

 America. .The opinion that all bafalt or trap-rocks have 

 originally flowed in the form of lavas, we do not feem 

 authorized, from exilling fads, to maintain ; and the great 

 latitude given to this mode of formation, has induced other 

 geologifts to rejeft it altogether. But it has been well ob- 

 lerved, that there is a wide difference between afcertaining 

 the agency by which a rock has been formed, and the man- 

 ner in which that agent has been applied. Numerous ob- 

 fervations, which have been recently made on trap-rocks in 

 volcanic countries, have led to the opinion that many of 

 fhefe rocks have been foftened, and even fufed, in their 

 original pofition, without ever having flowed as currents of 

 lava. 



The principal objedlions againft the agency of fire in the 

 formation of thefe rocks are enumerated by Brochant. 

 (Min. tom. ii.) Many of thefe objeftions, we think, can 

 be fatisfadiorily anfwered. 



1. " Bafalts are extremely rare among the produ6ls of 

 burning volcanoes, and modern eruptions have not produced 

 any." — This objection is founded on an afliimption of fafts, 

 which are not admitted by other geologills. The obferva- 

 tions of Daubuiflbn in Auvergne, of fir George Mackenzie 

 in Iceland and the Ferroe iflands, and of Humboldt in South 

 America, prove that trap-rocks are extenfively fpread 

 among volcanic produfts ; though, in many inftances, they 

 appear to have been formed by the agency of fubterranean 

 heat afting on beds of fchift, which have been melted or 

 foftened in their original pofition, and been fubfequently 



elevated, without ever having flowed as lava. If modern 

 eruptions have produced no bafalt, we may recoUeft that it 

 is only the external parts or fcoria: tliat are expofed to 

 our view ; and we have no opportunity of examining the 

 currents of fubmarine lavas, formed at great depths under 

 the ocean, iiVcircumftances fimilir to thofe which may have 

 attended the formation of bafalts. That many bafaltic rocks 

 were formed under the fea, is evident from tlieir alternating 

 .with beds of hme-ftone containing marine fliells. 



2. " Whatever origin may be attributed to the columnar 

 or tabular ilrudure of bafalt, it is not pecuhar to trap- 

 rocks : there are gypfums, marles, and fand-ftooes, which 

 frequently prefent this ftrufture." — The columnar and glo- 

 bular ilrufture may be formed by the flow refrigeration of 

 a melted mafs of bafalt, as was proved in die experiments of 

 Mr. Gregory Watt. (See Rowley Rag.) The co- 

 lumnar ftru<Eiure has been obferved by colonel Imrie in a 

 current of lava which had flowed from a volcano in Feli- 

 cuda, one of the Lipari iflands. This current could be 

 traced from the mouth of the volcano to the fea. That the 

 columnar llrufture may alfo exift in fome rocks of aqueous 

 origin, cannot invalidate the above fafts. The particular 

 forms which the columns of bafalt fometimes prefent, wliiclk 

 will be fubfequently noticed, appear to be the effefts of 

 partial fufion. 



3. " Bafalts often repofe immediately on coal, as at 

 Meifner, near CalTel. Now, if this bafalt were volcanic, 

 it mull neceflarily have produced the combufl;ion of thefe 

 beds of coal." — To this objeftion it will be fufficient to 

 reply, that numerous infliances may be cited of baialtic 

 dykes and beds of bafalt having charred the coal in their 

 vicinity, and reduced it to a ftate of coke or cinder. ( See 

 Veins, Mineral.) And from the experiments of Dr. 

 Maccullock on powdered jet and wood-coal, inclofed in gun- 

 barrels under preffure, fo as to confine a confiderable part 

 of the volatile produfts, it appears that they were converted 

 into true mineral coal by a red heat ; and had the confift- 

 ence, fradlure, and every property of pit-coal. Where 

 bafalt in contad with coal has reduced it to a coke, it ap- 

 pears highly probable that the volatile parts have been driven 

 ofi" by heat ; and where the coal preferves its true charader, 

 the volatile parts have been retained by compreffion. 



4. " The remains of animals and vegetables which are 

 found in fome trap-rocks, could not have refilled the adion 

 of volcanic heat without being deftroyed. It is the fame 

 with many very fufible minerals. Some fuch fufible minerals 

 are alfo found in volcanic rocks ; but thefe inftances are 

 rare, and cannot ferve as a bafis for a general rule." — In 

 the experiments of fir James Hall on lime-ftone expofed to 

 heat under compreffion, it was found that chalk, which con- 

 tained Ihells, might be melted and reduced to a cryftalliue 

 itate by coohng, without deftroying the organic ftrudure 

 of the fliells. Moil of the organic remains in bafaltic rocks 

 are in a fofter amygdaloid, or in wacke, which may pro- 

 bably have been formed from eruptions of volcanic mud, like 

 that called moya, from the South American volcanoes. The 

 fufible minerals found in bafalt and fome volcanic produds, 

 khere is every reafon to believe, aflumed a dillind cryilalline 

 form when the mafs was confolidating. 



5. " Cavities filled with water in fecondary trap-moun- 

 tains, oppofe the fuppofition of their igneous origin." — That 

 thefe cavities have been filled by infiltration is rendered al- 

 moll certain, from tlie circumllance that the water may be 

 expelled by a gentle heat ; and if the water can find a paf- 

 fage through the Hone from the cavity to the furface_, it 

 may alfo find a paflage from the furface to the centre of 

 the mafs. 



U 2 6. "There 



