TRAP. 



! notwithftanding the difference of their cryflallization. Be 

 ' this as it may, wc confider it to be a queftion yet undecided, 

 ; whether augite [pyroxene of Haiiy) is confined to rocks of 

 I undoubted igneous origin, though Cordier, in the memoir 

 j before alluded to, has proved that this mineral is a confti- 

 j tuent part of a confiderable number of lavas, and of many 

 : rocks denominated trap ; and we know that this opinion is 

 ' further confirmed by the extenfive obfervations of thai emi- 

 I nent geologid Von Buch. 



The claflification of trap-rocks adopted by Werner having 

 been extenfively admitted, we (hall endeavour to ftate it as 

 concifely as is confident with perfpicuity. He divides trap- 

 rocks into three formations : primitive trap, tranfition trap, 

 and the neweft fleet z trap. 



Primitive trap is almoft always diftinguifhed by a great 

 predominance of hornblende, fo that fome of thefe rocks are 

 entirely compofed of this mineral, or nearly fo. There ai'e 

 three principal fpecies of primitive trap ; common hornblende- 

 rock, hornblende mixed with felfpar, and hornblende mixed 

 with mica. 



Common hornblende-rock is principally compofed of 

 hornblende ; it contains two fubordinate kinds, granular 

 hornblende-rock and hornblende -flate ; the latter differs 

 from the firft in having a flaty ftrufture. Hornblende- 

 flate paflfes fometimes into gneifs, and fometimes into talcous 

 flate. ( See Gneiss, and Talcous Slate. ) Thefe two 

 kinds of hornblende-rock occur in gneifs and mica-flate, 

 forming beds which are but few in number, and of incon- 

 fiderable magnitude ; but in clay-flate thpy occur fre- 

 quently, and in vory thick beds. 



Hornblende mixed with felfpar. This fpecies Werner 

 divides into two fubordinate kinds ; greenftone and green- 

 ftone-flate. The term greenftone [grunjle'in, German) is 

 ufed by Werner to denote a rock compofed of cryftals of 

 felfpar and hornblende, and nearly refembling fienite (fee 

 Sienite) ; but in greenftone the hornblende generally 

 predominates, and the felfpar is commonly white, whereas 

 in fienite it is red. When the greenftone includes large 

 cryftals of felfpar, it is denominated porphyritic greenftone. 

 When the granular baCs is very fine, it is called greenftone 

 porphyry. According to Mr. Jamefon, this is the black por- 

 phyry of the ancients. When the granular nature of the 

 bafe is no longer vifible to the naked eye, it has a blackifti 

 green colour, and includes cryftals of 'compaft felfpar. It 

 is the porphyre verle, or antique green porphyry of fculp- 

 tors. 



Geenftone-flate is compofed of hornblende and compaft 

 felfpar, and has a diftinft flaty ftrufture. The felfpar is ge- 

 nerally more abundant than the hornblende, and it fome- 

 times contains fcales of mica. It occurs in great beds and 

 mafles in clay-flate. 



Hornblende mixed with mica is the third fpecies of pri- 

 mitive trap : it is compofed of an intimate intermixture of 

 hornblende and felfpar, and includes fcales of mica. It oc- 

 curs in beds in gneifs and mica-flate. Jamefon's Mineral, 

 vol. iii. 



Tranfition trap is divided into tranfition greenftone and 

 tranfition amygdaloid ; but it does not appear that thefe 

 rocks have any very diftinft character to diftinguiHi them 

 from the greenftone and amygdaloid of the other trap-forma- 

 f.ions. According to Mr. Jamefon, tranfition greenftone is 

 3 fine granular mixture of hornblende and felfpar ; but fome 

 of the primitive and floetz greenftones are alfo defcribed by 

 the fame geologift as being finely granular. Tranfition 

 amygdaloid is ftated to have a bafe of wacke, (fee Wacke,) 

 and approaches more or lefs to the nature of greenftone, 

 fometimes inclining to bafalt, and fometimes to iron-clay. 

 Vol. XXXVI. 



The veficles are filled with clay, or with chalcedony and 

 agate : it alternates with tranfition lime-ttone. 



The neweft flcetz trap-rocks of Werner comprife green- 

 ftone, bafalt, wacke, porphyry-flate or chnk-ftone porphyry, 

 trap-tuff, greyftone, amygdaloid, with compact felfpar and 

 clay-ftone. He alfo includes pearl-ftone, pitch-ftone,obfidiaii, 

 and pumice in this formation of trap. ( See the various ar- 

 ticles here enumerated. ) It is thefe rocks, which Werner 

 calls the neweft flcEtz trap, that prefent the ftrongt-ft indica- 

 tions of their igneous origin ; and fome of the lubftances 

 here enumerated, fuch as obfidian and pumice, are now ge- 

 nerally clafted with volcanic produfts. 



Some geologifts are of opinion, that there is no foundation 

 in nature for the divifion of trap-rocks into three diftincl 

 formations, introduced by Werner. 



There are, however, certain peculiarities of pofition, which 

 feem, to indicate a diff^erence in the mode of formation 

 between the trap-rocks that occur imbedded in other rocks, 

 and thofe which cover tlie fecondary ftrata. To form a 

 more diftindt idea of this, we muft refer to the :jeneral 

 ftrufture of the furface of the globe. The beds of rock 

 which form the cruft of the globe, are commonly arranged 

 over each other in what is denominated a comformable pofi- 

 tion, as reprcfented Plate II Geology, Jig. 6. where G is the 

 foundation, or loweft rock, on which the different beds a, I, c, 

 and d are laid, and may be fuppofed to cover the mountain 

 with a general conformity to the ftiape of the lower rock. 

 This is more diftinftly feen in the fame plate [jig. 3.), where 

 the upper beds havt the fame elevations, deprefRons, and 

 undulations as thofe of the lower. The other pofition in 

 which rocks occur, is called unconformable. " If a thick 

 ftream of lava, as frequently happens, were to flow over a 

 range of conformable rocks, filling up the cavities and 

 inequalities of the furface, when it hardens by cooling, it 

 would form a bed of fuperincumbent uncoriformable rod. 

 Such inftances are common in volcanic countries. Very 

 extenfive ranges of rocks occur in this pofition in various 

 parts of the world, covering not only the primary, but the 

 fecondary rocks. Thefe unconformable rocks, compofed of 

 different kinds of trap, porphyry, and fienite, frequently 

 affume the columnar ftrufture, and form vaft ranges of 

 natural pillars, as at Staffa and many of the Hebrides, on 

 the northern coaft of Ireland, and in Iceland, Sicily, and 

 various volcanic countries. Now whatever theory we adopt 

 refpefting the formation of unconformable rocks in this 

 pofition, we muft admit that they are of more recent origin 

 than the rocks which they cover ; the lower rocks muft have 

 been hard and unyielding, when the upper were thrown 

 upon them." Bakewell's Introduftion to Geology. 



The unconformable pofition of bafaltic rocks, or rocks of 

 the trap-formation, is reprefented Plate TV. Geology, jg. 2. 

 a, a, a, being a feries of regularly ftratified rocks, covered 

 by a mafs of unconformable bafalt, b b. The trap in this 

 pofition is often divided into diftinft beds, fome of which 

 are columnar, as reprefented at c c, and thefe columnar beds 

 are frequently feparated by beds of compadl trap. 



Mafles of trap alfo occur, forming perpendicular walls 

 (called dykes in the northei'n part of England) : thefe in- 

 terfeft tlie ftrata and frequently change their pofition, as 

 reprefented Plate II. Geology, fg. 8. where a feries of 

 ftrata a, b, c, d, e, f, g, which were once continuous, are 

 broken by a dyke of bafalt. The fimilar letters on each fide 

 of the dyke reprefent the fame ftratum, as d d, e e, broken 

 by the intervention of the bafalt, Li fuch inftances, the 

 ftrata in the immediate vicinity of the bafalt have under- 

 gone a confiderable degree of change ; and where the 

 ftratum e e h coal, it is frequently found converted into 



XJ coak, 



