TRAP. 



6. " There are not obferved in trap-rocks eitlier that 

 bhck colour, or thofe indications of vitrification, that are 

 apparent at leaft in certain portions of the produds of burn- 

 ing volcanoes : real craters have never been obferved. AH 

 thofe ivhich have been cited were hollows, chafms filled with 

 water, fo common in fome mountains."' — This is an affump- 

 tion of fads which recent obfervations difprove. The ba- 

 falts in the Ferroe iflands, and in Iceland, have, in fome 

 inftances, the black appearance whish is here denied : the 

 columns and beds of bafalt contain alfo in their lower, and 

 fometimes in their upper parts, fcorise, and thofe indications 

 of vitrification that are apparent in burning volca:;oes ; aiid 

 the fides of the bafaltic dykes are generally vitrified. That 

 many of the hollows, fuppofed to have been craters, were 

 hollows formed by depreffion, is probable ; but what otlier 

 appe.irance could the crater of an ancient volcano be expefted 

 to'prefent, than a hollow or chafm which might be filled 

 with water ? 



7. " Mandelftein (or porous amygdaloid) has certainly 

 fome refemblance to porous lava ; but there are mandelfteins 

 evidently not volcanic. Befides, the cavities in the man- 

 delfteins of trap-mountains contain ver)- different minerals, 

 and fuch as could not have undergone the aftion of fire, 

 without being changed." — The remarks made previoufly on 

 very fufible minerals inclofed in bafalts, and fome lava, 

 apply equally to tliis objeftion. It is alfo probable that 

 foft and amygdaloidal bafalts may have been formed from 

 eruptions of mud, as before ftated. 



8. " Although from experiments made on bafalts and 

 lava, and from obfervations made on burning volcanoes, it 

 is known that ftony fubftances may, after perfeft fufion, 

 rsfume their ftony charafter ; but when this -takes place in 

 b'-imino- volcanoes, there are always found in the vicinity 

 fubftances which are fcorified or vitrified, and denote the 

 aAion of fire." — It has before been remarked, that fcoriz 

 and vitrified fubftances occiu- in the bafalts of Iceland and 

 Ferroe : they accompany the bafalts in Auvergne, and the 

 vicinitv of Etna. In almoft all bafaltic countries, fome 

 of the rocks prefent fo ftriking a fimilarity to volcanic pro- 

 ducts, that the refemblance has often ftruck the moft com- 

 mon obferver. If other bafaltic rocks do not prefent the 

 fame appearances, it may be owing to their having been 

 formed by fubmarine eruptions, under circumftances very 

 different from thofe which take place on land ; and it is not 

 contended that all bafalts have originally flowed as lava. 



g. " In various countries, beds of balalt are obferved to 

 alternate with fand-ftone and ftratified lime-ftone." — It is 

 obferved in many parts of England and Scotland, and is 

 probably general, that where beds of trap are interfperfed 

 between fand-ftone or lime-ftone, they are generally very 

 irregular both with refpeft to thicknefs and extent, and 

 have feldom the fame uniform arrangement as the ilratified 

 rocks with which they alternate. That the lime-ftone was 

 formed under the fea, is proved by its containing marine 

 fhells ; and there is no improbability in the fuppofition, that 

 currents of fubmarir.e lava may have flowed at different 

 periods over beds of {hells or fand, and converted the one 

 into lime-'.lor.e and the other into fand-ftone. 



IC. " In manv bafaltic aiftrieU, bafalt is only found on 

 the fummiti of hiUs ; and it is evidently perceptible, by the 

 correfpondence of the beds, that all thefe fummits were 

 parts of one and the fame bed, wliich fpread over all the 

 diftrift. This is not the form of volcanic depofits : they 

 take a certain direftion, and no finilar inftances of fuch vail 

 •volcanic depofits au-e known." — The frequent occurrence 

 of detached caps of bafalt forming the fummits of hills, in 

 an extenlive diftrict at nearly the fame level, conftitutes one 



of the moft remarkable pecaharities of the trap or bafaltic 

 formation, and deferves particular attention, as marking 

 fome of the lateft revolutions that have changed the furface 

 of the globe. To form a more diftinft idea of the fituation 

 of thefe caps, we refer to Plate IV. Geology, Jig. 3, where 

 a mafs of unconformable trap, a, lies upon the ftratified 

 rocks b, and may be feen on the fummits of other ftratified 

 hills at f, d, and r. In fome inftances, as in the Ifle of Skye, 

 as defcribed by Dr. MaccuUock ( Geological Tranfa6lions, 

 vol. iii.), the mafs at a appears to be connefted with the 

 lower rock e, and to form a part of it, as if it had broken 

 thi-ough tlie ftrata, and been thrown over the furface in a 

 ftate of fufion. The ftrata confift of grey lime-ftone con- 

 taining {hells ; but where the lime-ftone is in the vicinity of 

 the trap, it is converted into white cryftalline ftatuary 

 marble. A fimilar change is produced on chalk, or earthy 

 hme-ftone, when fubjefted to heat under preffure, as was 

 proved in a fei^es of experiments made by fir James Hall ; 

 and as we are unacquainted with any other caufe which 

 could produce tliis change, it appears probable that the 

 hme-ftone has been converted into marble by fubterranean 

 heat, and that the trap has flowed over the furface like lava, 

 but under the prefTure of the ocean ; the exiftence of marine 

 {hells in the hme-ftone proving that fuch muft have been u^ 

 original fituation. The caufes which have changed the re- 

 lative level of the lime-ftone, with refpedt to the fea, mxy 

 have broken the continuity of the ftrata, and of the bafalt 

 which lies upon them. The bafaltic mountains of Au- 

 vergne, in the departments of Puy-de-D6me and Cantal, in 

 France, have excited much attention from the exiftence of 

 ancient volcanic craters in their vicinity. According to 

 Daubuiffon, the fundamental rock of that diftrift is granite, 

 covered in fome places with a marley lime-ftone. Nearly all 

 the foil is covered with volcanic or igneous produftions : 

 they are of three kinds, and appear to have been formed ?: 

 three diftinft epochs. The moil recent are currents of lava, 

 which lead to craters ftill exifting, though dormant. The 

 fecond formation confifts of maffes or tables of bafalt, fepa- 

 rated by openings or valhes. The third confifts of moun- 

 tains compofed of a kind of volcanic porphyry. The 

 currents of lava are obferved to have run from the bottom 

 of a number of conical hills, from two hundred to four 

 hundred yards in height. Thefe hills, of which there are* 

 about one hundred, are formed of heaps of fcorise, frag- 

 ments of lava, and rapillo or tufa. Their fummit often pre- 

 fents a hollow in the form of a crater : they reft immediately 

 upon granite. The lava at the bottom is of a bafaltic na- 

 ture ; it is of a greyiih-black colour, with a fine compact 

 grain ; it contains grains of augite, olivine, and felipar. 

 The fuperficies is bhftered and ftudded with afperities, 

 which fometimes exceed a yard in height. The interior is 

 more compaft. The currents are fpread in the adjacent 

 plain ; they have fometimes reached the bottom of certain 

 valhes, and have followed their courfe for three or four 

 leagues. In advancing progreflively, they always defcend 

 to lower elevations ; they follow the inequahties of the foil ; 

 and feparate on meeting with any obilrudling eminences in 

 their paffage. The hiftory of thefe currents of lava, fays 

 M. Daubuiffon, is complete, and there is nothing left for 

 the imagination to lupply. We behold the orifice from 

 whence they iffucd, the courfe they purfued, and the 

 country they occupy. As for the period when thefe lavas 

 flowed, though anterior to the hiftory of man, it is never- 

 thelefs recent, compared with thofe vaft changes which the 

 furface of the globe prefents, and was pofterior to the ex- 

 cavation of the valhes, fince it flowed through them, and 

 occupied their bottoms. 



The 



