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Hornblende. It is fometimes a congeries oflarge imperfect cryftals, but more generally ofgrains, which laft are often fo minute as to givethe whole rock a compaft appearance.

Schijlofe Hornblende rock is for the mod partcompofed of the fubdance from which itsname is derived; it is however 'occafionallymixed with quartz, aftynolite and pyrites.This rock does not exclufively belong to thetrap formation as it forms fubordinate beds ingneifs, micaceous fchidus, and argillite. Itpafl'es fometimes into griindein.

Primitive grutijlein, or green done, fo calledfrom its colour, is an aggregate of hornblendeand felfpar. It differs in little or nothing ex-cept its fituation from fienite. There are threevarieties of this rock; in one the hornblendeand felfpar are intimately and immediately ag-gregated and its texture is limply granular; inthe iecond variety, the grains of which it iscompofed are fmaller than in the preceding,and in this aggregate, as in a pade, are im-bedded crydals of felfpar; hence its texture isat the fame time finely granular and porphy-ritic; in the third variety the particles of fel-fpar and hornblende compofing the pade, are fominute as to be indidinguifhable, hence itsfracture is compact and even fplintery ; in thisare imbedded crydals of felfpar. '1 he greenantique porphyry belongs to this variety.

Schijlofe grutijlein is compofed of compaftfelfpar and hornblende in nearly equal propor-tions, mixed with a little mica, and morerarely, with quartz: it has a fchidofe texture.

This rock is not peculiar to the primitivetrap formation, being found fometimes to alter-nate with argillite.

Griindein is not unfrequently traverfed bymetalliferous veins.

j 2. Traps of Tranfition.

The principal material of the rocks that be-long to this formation is granular griindein;but in the tranfition traps the mixture of theingredients is more intimate, the grain is finer,and the niafs appears more homogeneous. Thetwo following are its principal varieties.’

Amygdaloid. —Toaddone, Mandeldein.

The mafs of this rock is fchidofe hornblendeor fchidofe griindein in a date of femi decom-pofition refembling Wakke or fine ferruginousclay. It contains a number of globular cavitiesfrom the fize ot a pea to that of a frnall apple ;of thefe cavities lome contain nothing but air,and are coated on the infide with a kind ofvarniih; others contain balls of calcareous fpar,

quartz, and chalcedony, the latter compofed ofconcentric layers round a hollow nucleus linedwith crydals of quartz. The toaddone ofDerbydure is donfidfered by Werner to belong tothis variety. V* .

Globular Trap, ftugeltrap.

This is compofed of fchidofe griindein in adate of femi-decompofition,. arranged in fphe-roids of various magnitudes,Sand compofed ofthick concentric lamellar didindh concretions.

The traps of tranfition appear noftto ty; dra-tifiecl; they form folitary conical mountains inthe vicinity of tranfition limeftone, and containvery few metallic veins.

§ 3. Secondary, or Floeiz Traps.

The rocks or drata belonging to the fecond-ary trap formation may be divided into thofcwhich are peculiar and charadleridic of it, andthofe which are accidental. The former! arebafalt, wakke, bafaltic trap, fecondary amyg-daloid, klingdein porphyry, graudein, and fe-condary griindein. Among the latter may beclaffed, rubble and fanddone, clay, coal, andbituminous wood.

Bafaltic rod, is compofed for the mod partof the mineral already deferibed in the articlebafalt. It is ufually porphyritic, and then con-tains crydals or grains of felfpar, olivine, au-gite, bafaltic hornblende, magnetical iron, leu-cite, quartz, and, rarely, mica, adlynolite, andchalcedony. Sometimes it affumes the amygda-loidal dructure, and its cavities are then eitherempty or filled with zeolite, deatite, calcareousfpar, and even fometimes with water. Of allthe rocks belonging to the fecondary trap form-ation, bafalt is of the mod frequent occurrence;it forms by itfelf whole mountains, and the en-tire feries of rocks with which it is afiociated,may be with much propriety, and is athullyby fome mineralogids, denominated the bafalticformation.

IVakke rock is compofed of the mineral bear-ing this name, mixed with, crydals of bafaltichornblende, and particularly with hexagonalplates of black mica, by which, and by theabfcnce of olivine and augite, it is didinguifhedfrom decompofing bafalt.

Bafaltic Tufa is the refult of the decompofitionor rather of the difintegration of certain bedsof bafalt; it confids of pieces of bafalt, frar-ments of olivine, and vegetable rernaips<agglu-tinated together by an argillaceous ccn?^;t.^

Secondary amygdaloid, appears to haVexfonhabafe decompofed grundcin, which is oftenpenetrated by filiceous particles; fometimes it'