VOLCANO. 



mult have poUtlTed bill a low deforce of lieat, as the fame 

 cryftals may be melted in a common furnace. The forma- 

 tion of cryftals does not depend upon the degree of heat, 

 but on the circumftances under which the fubftancc cools ; 

 a long ftate of q\iiefcent fluidity being a» neceflary to the for- 

 mation of perfeft cryftals by igneous fufion, a3 it is known 

 to be in aqueous folutions. Inattention to tliis ciicumftance 

 has rendered many of the conclufions from the laborious re- 

 fearches of Spallanzani invalid. M. Cordier, an ingenious 

 geologift in France, has devifed a new mode of analyfing 

 lavas. He very properly obferves, that the attention of 

 geologifts has been hitherto direfted more to the imbedded 

 cryftals in lava, than to the pafte or bafe of the lava itfelf ; 

 and it has been admitted, without fufficicrt proof, that the 

 bafe of lava was either hornblende or felfpar, or a mixture 

 of tlicfc two minerals. 



On attentively examining the fubftance of lava and vol- 

 canic fcoria;, with a very high magnifying power of the 

 microfcope, he difcovered that it was not homogeneous, but 

 confilled of a congeries of minute cryitals of different mi- 

 nerals, which were principally fimilar to the larger imbedded 

 cryftals. To afcertain more decidedly the nature of thefe 

 fmall cryftals, he endeavoured to difunite them by com- 

 prellion, then felefting particles of the fame fize feparated 

 them, according to their relative denfity, by wafhing. The 

 ifolated particles were afterwards examined with the micro- 

 fcope, and compared with the particles of the cryftals moft 

 commonly found in volcanic rocks, fuch as felfpar, cryfo- 

 lite, olivine, iron-fand, and menakanite. He commenced 

 with the examination of compaft or ftony lavas, beginning 

 with thofe from burning volcanoes, then proceeding to thofe 

 from extinft volcanoes, and laftly to thofe whofe volcanic 

 origin has been doubted by geologifts, fuch as bafalt and 

 wacke. The refult of thefe examinations have led him to 

 conclude, that all thefe rocks, from whatever diftrift they 

 come, are compofed nearly in the fame manner, and are all 

 granular, confifting of very different diftinft cryftalline 

 grains, interlaced with each other, fo that all ftony lavas may 

 be regarded as minutely granitic, when viewed with the mi- 

 crofcope. There fometimes exift minute pores between the 

 grains, which however do not occupy one-fixtieth part of 

 the bulk : thefe pores are more common in modern than in 

 ancient lavas. 



There are five forts of thefe grains diftinguifhable by 

 their colour ; i. white more or lefi tranfparent ; 2. bottle- 

 green ; 3. black and pcrfeftly opaque ; 4. a clear brown ; 

 5. and laftly, very fmall grains of reddifh-brown. Thefe 

 five forts of grains are fufceptible of further fubdivifion, 

 according to their phyfical or chemical properties. The 

 white grains belong to three diftinft minerals. The moft 

 common are thofe which melt into a white enamel ; thefe 

 are felfpar. The more infufible are cryfolite, and thofe 

 which are perfeftly infufible are leucite. 



According to the prevalence of felfpar, the lava poflcfTes 

 different charadlcrs. Thofe which contain from forty-live 

 to fifty-five per cmt. of felfpar, melt into a black glafs, the 

 minute edges of which are bottle-green, black, or greyilh- 

 black ; bafalts arc of this kind. 



Thofe lavas which contain from fifty-five to feventy^^r 

 :ent. of felfpar, melt into a bottle-green enamel, fuch are 

 the greeiiifh, greyifh, and dark-coloured bafalts. 



Stony lavas, which co:rain ninety per cent, of felfpar, 

 vnelt into a white glafs. Such are the petrofiliceous or com- 

 pact felfpar lavas and clink-ftonc. 



The yellowilh or greenifti grains belong to augite or to 

 hornblende, which are fometimes difficult to be diftinguifhcd 

 from each other. According to Cordier, the grains o\ .lugitc 



are rounded and iircgbl..r, with a viirtousfratlure and fplen. 

 dent hiftre. The grains of hornblende are long, and affume 

 a prifmatic form : they prefent indications of their laminsr 

 ftrufture, and have little luftre except in the direftion of 

 the laminse. 



The greateft proportion of augite in lava is forty -five per 

 cent. Thefe lavas melt into a black glafs. Thofe lavas 

 which melt into a white glafs only contain one per cent, of 

 augite. The black opaque grains confift of titanium 

 combined with iron, as iron-fand, fer titnne, or as me- 

 nakanite. The iron-fand contains only 0.5 of titanium, 

 the particles have a perfcA metallic luftre, and conchoi- 

 dal fiafture, and are attraftcd by the magnet. The 

 greatcil proportion in which they exift in ftony lavas that 

 melt into a black glafs, is fifteen per cent. The grains of 

 menakanite exift in a much fmaller proportion, they are dif- 

 ficult to melt, and are not attrafted by the magnet. 



The grains of iron ore, fer oligifte, may be known by the 

 red colour of the powder when they are pounded. Thefe 

 are very rare in lavas. 



From an examination of a great number of lavas, it ap- 

 pears that there are only two prevailing minerals which com- 

 pofe the greater part of their bafe. Thefe are augite 

 and felfpar. All the reft are in a very fmall proportion ; 

 and hornblende, which has been admitted without examina- 

 tion into all volcanic rocks, exifts but in a very few, and 

 thofe are fuch as abound in felfpar. In the latter the cryi- 

 tals of hornblende, which are diffeminated, are very diftinft. 

 Bafaltic rocks, which have hitherto been ttatcd to confift of 

 hornblende and felfpar, according to Cordier, are principally 

 compofed of augite and felfpar. 



Stony lavas may therefore be clafTed into two kimls, thofe 

 which melt into a white glafs, and thofe which melt into a 

 black glafs. The former M. Cordier denominates leu- 

 coftine, the latter bafalt. Leucoftine comprifes thofe fub- 

 ftances called, by Dolomieu, petrofiliceous lavas ; by Hauy, 

 compa8 fonorous felfpar ; by Karften, domite, and lava 'with 

 a horn-flone bafe ; and by Werner, clink flone. The latter 

 comprifes the ferruginous lavas of Dolomieu ; the bafaltic 

 lava of Hauy, les laves bafaltiques uniformcs ; and the bafalt 

 and lava of Werner. The refult of thefe obfervations con- 

 firms the fimilarity of compofition between ftony lavas of 

 recent volcanoes and bafaltic rocks, whofe igneous origin 

 has been contefterf. 



In the fame manner M. Cordier has examined the com- 

 pofition of volcanic fcorix and volcanic glafs, volcanic cin- 

 ders and tufa. Thefe arc all compofed of the fame fub- 

 ftances as the ftony lava. 



Obfidians, or volcanic glaffes, may be divided into two 

 kinds like lava, according as they yield a black or white 

 glafs to the blow-pipe. In the vitreous pafte of both may 

 be difcovered by the microfcope, the fame cryftals as in 

 lava, grains of felfpar are feen in thofe glaffes which become 

 white before the blow-pipe ; grains of augite in thofe which 

 melt into a black glafs. In certain inftaiices, we fee the 

 tranfition of obfidian into a compaA black bafalt, and alfo 

 into pumice. 



Thefe obfervations of Cordier tend to eftablifh the iden- 

 tity of bafaltic rocks with thofe of volcanic origin, whilft 

 at the fame time tlicy diftinguifli them from the beds of 

 liornblendc and trap, which occur in primary mountains. 

 The latter ditlcr in compofition from bafalts and lavas, and 

 alfo in the nature of the imbedded cryftals which they con- 

 tain. All volcanic rocks, even thofe which appear the moft 

 homogeneous, are compofed in a groat part of microfcopic 

 cryftals, belonging to 3 fmall number of mi'ierals, parti- 

 cularly augite, felfpar, olivine, and iron-fand. Volcanic 



rocks 



