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mitive and tranfition argillaceous fchiftus. Theprimitive appears generally to reft upon mica-ceous fchiftus, and is occafionally mixed withquartz, mica, hornblende, black fchorl, garnet,pyrites, granular limeftone, and calcareousfpar. It alfo contains various metallic ores,either in veins or beds. The quickfilver minesof Idria are in primitive argillite, fo is the vaftbed of copper and iron pyrites forming theParys mine in Anglefey.
Tranfition argillite has not been much exami-ned : the greater part of the Hate of NorthWales appears to belong to this formation,where it refts upon griinftein, and appears tocontain no extraneous minerals, except veins ofquartz and calcareous fpar, with blende and gale-na. Secondary argillite contains impreffions oforganized bodies and figured pyrites : it oftenrefts upon fecondary limeftone, as is the cafein Derbyfhire.
THUMERSTEIN. Thumerftone, Kina.Pierre de I hum, Broch. Axinite, Hnuy.
The colour of this mineral is clove-brown,paffing into violet-blue, and yellowifh or green-ilh grey. It occurs in rnafs, difleminated andcryftallized. The primitive form of its cryftalsis a ftrait rhomboidal prifm, the alternate anglesof which meafure refpectively ioif° and 785°.The vertical edges of the prifm, correfpondingwith the acute angles, are generally, however,more or lefs truncated. Sometimes the prifmsare fo fhort as to be nearly tabular, and are thenoften cellularly aggregated. The faces of theprimitive cryftal are ftriated, but all the fecon-dary planes are fmooth. Externally the cryftalshave a bright vitreous luftre ; internally they aregenerally gliftening. The fraCture is fmallconchoidal, in fome varieties paffing intofplintery and fine-grained uneven. When inmafs, it is ufually compofed of thin teftaceousdiftinft concretions. It varies from femi-tranfparent to tranflucent on the edges. Itshardnefs is nearly equal to that of quartz ; it isbrittle and very eafily frangible. Sp.gr.3.2103.3.
It melts before the blowpipe without addi-tion into a greenifh white femitranfparent glafs;but on charcoal to a' black glafs. It has beenanalyfed by Klaproth a and Vauquelin b , with
the following refults. Klapr. Vauq.Silex - - - 52.7 — 44*
Alumine - - 25.6 — 18.
Lime - - - 9.4 — 19.
Oxyd of Iron - 9. — 14*
Ditto Manganefe 0.6 — 4.
97-3 99
*_An»l. E£T. Vol. I.p. 413.
The proportions of this mineral as ftated bythe two able chemifts juft mentioned, differfrom each other very cotifiderably, and on com-paring the methods purfued by each in perform-ing the analylis, it will we believe, be generallyallowed that Vauquelin’s is the leaft fubjetft toerror. With regard, however, to the oxyd ofiron, the difference is more apparent than real,Klaproth ftating it as magnetic oxyd, and Vau-quelin as red oxyd ; whereas, if in each ana-lyfis it was brought to the latter ftate, Klap-roth ’s 9 grs. fhould be raifed to 12.6 grs.
Thumerftein occurs only in primitive moun-tains, and is accompanied by afbeftos, a£ty-nolite, calcareous fpar, quartz, felfpar, andfometimes with fulphur, arfenical pyrites, andnative bifmuth. The maflive variety is metwith at Thum in Saxony (whence is derived theGerman name of this mineral.) It occurscryftallized near Oifans in Dauphine, at Kongf-berg in Norway; alfo in Spain , Savoy, andCornwall .
Tin is a metal of a filver-white colour, veryductile and malleable, gives out while bendinga peculiar crackling noife, is fufible at a heatmuch lefs than that of ignition : is flowly fo-luble in muriatic acid, and by dilute nitric acidis rapidly converted into a white oxyd.
§ 1. Ores.
Sp. 1. Tinstone . Zinnftein, Wern. Pierrede Etain. Mine d’Etain commune, Broch.
Its ufual colour is brownifh-black, pafling intoclove-brown, blackifh, redifhor yellowifh-brown:fometimes alfo it occurs wine-yellow, fmoke-grey, yellowifh-grey and greyifh-white. Itoccurs in mafs, difleminated, in rounded frag-ments, and cryftallized. Its primitive form isfufpeCted by Hauy to be the cube : the formsunder which it afiually appears are,
1. A rectangular tetrahedral prifm, termi-nated at each extremity by a tetrahedral pyra-mid, the faces of which are ifofceles triangles.
2. The preceding, with the longitudinaledges of the prifm truncated.
3. Var. 1. with the longitudinal edges bothof the prifm and pyramid truncated, whencerefults an eight fided prifm, with eight fidedfummits, the faces of which latter are alter-nately triangles and pentagons. Sometimes theprifm is doubly truncated, and then prefentsfixteen fides, the terminating pyramids remain-ing as before.
4. A ftrait eight-fided prifm fimilar to theforegoing, with the terminal pyramids verydeeply truncated.
Jovirn. des Mines, No.23. p. I