MINERALOGY. 



421 



S. Strttfifcati-m. It is scarcely ever stratified, and 

 when traces of stratification do appear they ore very 

 indistinct. 



4. Meinl/iferoiu minerals. It always contains magne- 



mountain 



copper. 



occurs in beds and 

 clay slate. 



-On exposure to the weather, its 

 >1 the colour change* from 

 ng to the change of the pro. 

 of iron. It re-Ut.- tin- de- 

 le weather more obstinately than 

 or clay-slate, with which it is 

 t inir forms 



iciss and 



Slratifica- 

 lion. 



*' tic ironstone, either in imbedded grains and masses, or in 

 e " snail veins, and these are sometimes so considerable as 

 to be worthy of being worked as mines. There are 

 mines of this description in the Alps. The chromate of 

 iron, so much valued in the arts, occurs disseminated, also 

 in imbedded masses and in veins in this mountain-rock, 

 in the Shetland Islands, and at Portsoy. On the con. 

 tinent of Europe, in Provence, and in Sliria, and 

 in the New World in the United States of America. 

 With exception of iron, this rock contains but few me. 

 talliferous minerals ; nevertheless near to Joachim.-thal 

 in Bohemia, it contains so much galena, that a mine is 

 established in it, and in Cornwall, and in the Shetland 

 Islands, it contai: 



Formation. 5. Formal inn. It 

 manes in 



Dompo- 



i .; surface K 



thegne 



rice peaks anil 

 of er|XM>! 



the softer and less durable surroun 

 Other r<>- 



Like all other magnesian rocks, it i.i inimical to ve- 



getation. The mountains of which it is composed, are 



and bleak ; and tin nakedness, joined to the 



sombre colour, gives a dreary and monotonous aspect 



to most serpentine- di-t: 



C*ofTph'- 7- ( if graphical dulrit,uiion. It occurs in great beds 

 in th- ! i-lands along with gneiss, mica-elate. 



*> .lartz-rock; in beds at 1 



along with (pi.irt/. rock, trap- nt k 

 and limestone ; near Cortachie in An^u-liin- ; in . 

 ncsshirc, and other parts of Scotland ; and abundantly 

 in Cornwall in England. 



It is very abundant in the A!i 



hut not - 



i, Saxony, tin 



It is common in the mountains of the United State* 

 of America. It occur-, in the mountain* of ' 

 in Mexico, when-it alternates with besb of stroke, pri- 

 mitive-trap, and r'.ay- slate ; and in die inland at < ul>a 

 it is associated with sin 



V**. 8. r.'t. It; and 



namental stone. 



VIII. 



r.n])ln>tidr. // 



hotidc.- Jameson. 

 Gabbro of the Italians. 

 DiaJlage Rock of others. 



f'fnitilurnt Parti. \i is compound of sauasurite 

 '1 those two minerals are sometimes in- 

 Ulc, hornblende, actynolite, garnets, 

 . flee. 



tlittriLuliu*. It occurs along with ser- 

 pentine in the Shetland islands and in Cornwall. 



found it in great abundance in Norway, even as 

 I. r north .-,- the North ( .,, . . .,,:,! ,t |,a- l>..-.-ii met with 

 in Germany, Switzerland, and Italy. 



I 



an or. 



* saM ka. 



enl p.rt^ 



GfOfrtphi 



r, ,: ::. 



Utet. When cut and polished, it presents a very Geognosy. 

 beautiful surface, hence it is much esteemed in some ^ 

 countries, as Italy, as an ornamental stone. The ntro 

 di prato, verde dipralo, granito di gabbro of the Italians 

 are either varieties of euphotide, or serpentines with 

 disseminated metalloidal diallage. 



IX. Porphyry. 



Porphir. Jl 'erner. 



Purphery. Kim-ait. 



Porphyry. Jameson. 



Porphyre Brochanl and Daubuisson. 



1. Name. The Grecian word from which the name Name, 

 porphyry i derived, signifies red; hence the name of 



tin- formation is borrowed from that kind, which is de- 

 nominated Antique Ited Porphyry. It is worthy of re- 

 mark, that red, or colours lx>rderin on it, or passing 

 into it, prevail in rocks belonging to the porphyry for- 

 mation. 



2. Composition and varieties. It is a compound rock, Compost- 

 baring a basis, fn which the other cotemporaneous con- """ ""' 

 stituent parts are imbedded, either in the form of ri ti ' Ifc 

 grains or crystals. Neither the base nor the iml>edded 



parts are always of the same kind. On the differences 

 of the first, depends the distinction of the different kinds 

 of porphyry. The base is sometimes clay stone, forming 

 clayttotu porphyry, hornstone forming horns/one par- 

 phyry, compact felspar forming feltptr porphyry, pitch- 

 stone, when it is named ptlchslone porphyry; ,-.\. 

 tains much hornblende, it has been named ttrrntic ;>r- 

 phyrtf. The imbedded parts are most commonly felspar 

 and quartz, which are usually more or less pert 



-tallixed. The quarts is usually crystallized, and in 

 double six-sided pyramids. The feUpar crystals are 

 broid six-sided prism*, but usually wry indistinct. Tin- 

 felspar is more or less fresh, sometimes even glassy, 

 sometimes completely disintegrated ami earthy, or only 

 in white specks. The frequency and magnitude of these 

 mixed parts, of quart/, and felspar, modify the an. 

 Maranceof the different kind* of porphyry very much. 

 Sometimes inie, sometimes the other, but more Ire- 

 qnently Ixith occur together, and along with several 

 otlu-r minerals which are less frequent, as crystals of 

 ibleiidi- and mica. The basis and the mixed parts 

 of the jxirphyry aUo differ in colour and several other 

 properties. It sometimes contains chalcedony and ngate, 

 which are in massive pieces,. or iti Mii;ill layerx or plates. 

 Further, there sometimes occur balls ot a greater or 

 less size; in clay- porphyry the centre of these balls is 

 chalcedony. Imt their exterior is hornstone porphyry ; 

 but in pitfhtone-porphvry, thewt)alfor* composed of 

 a particular kind of conchoidal hum-tine. Imt i he centre 

 is quarts. It sometimes also contains previous .-mil com- 

 mon opal, and these are either di emulated through 

 it, or traverse it in the form of very small vi 



S. .Structure. Porphyry is seldom stratified, and Structure. 

 when it is stratified, the strata are very inrli-tnut. It 

 is usually either massive, and merely traversed by nu. 

 merous accidental rent*, or disposed in distinct co'ncre. 

 tions, which are tabular and columnar, or they are glo- 

 Inilar, and these, again, are composed of concentric 

 lamellar concretions. 



4. r'orrit:* />!*. It contains few foreign beds, with 

 exception of granite, gneiw, and greenstone, which are 

 intermixed uiti -t examples of j>or- 



phyry with foreign beds, is that given hy H, ndant . who 

 describes t' rent sorts of lie.U in the xicnitic 



porphyry, in the environs of Schcninitz in Hungary. 



