522 



MINERALOGY. 



Drawing. 



slate. 



Oryctogno- Uses. When cut and polished, it is used for sharp- 

 ^_^' ing iron and steel instruments. For these purposes, it 

 *"~ Y ~ ' is necessary that it contain no intermixed hard mine- 

 rals, such as quartz. The light-green coloured varie- 

 ties, from the Levant, are the most highly prized : 

 those from Bohemia are also much esteemed in com- 

 merce. The Levant whet-slate is brought in masses 

 to Marseilles, and is there cut into pieces of various 

 sizes. It is ground by means of sand or sandstone, and 

 polished with pumice and tripoli. These whet-stones, 

 or hones, as they are called, ought to be kept in damp 

 and cool places ; for when much exposed to the sun, 

 they become too hard and dry for many purposes. 



The powder of whet-slate is used for cutting and po- 

 lishing metals, and is by artists considered as a variety 

 of emery. 



DRAWIXO-SLATE, or BLACK CHALK, Jameson. 



Zeichenschiefer, Werner. 



External Characters. Colour intermediate between 

 bluish and greyish black, rather more inclining to the 

 latter. Massive. Lustre of the principal fracture 

 glimmering. Cross fracture dull. Principal fracture 

 slaty, generally straight, sometimes curved ; cross frac- 

 ture fine earthy. Fragments partly tabular, partly long 

 splintery. Opaque. Soils slightly, and writes. Re- 

 tains its colour in the streak, and becomes glistening. 

 Very soft. Sectile. Easily frangible. Adheres slightly 

 to the tongue. Feels fine, but meagre. Specific gra- 

 vity, 2.110, Kirtvan. 2.111, Karsten. 



Chemical Character. Infusible. 



Geognostic Situation. Occurs in beds in primitive 

 and transition clay-slate ; also in secondary or flcetz 

 formations. 



Geographic Situation Found at Marvilla in Spain, 

 Brittany in France, and in Italy ; also in Germany, 

 as in the mountains of Bareuth ; and in the coal forma- 

 tion in Scotland. 



Uses. Used for drawing, and also as a black colour 

 in painting. 



* ALUM SLATE. Alum slate is divided into two 

 kinds, viz. common alum slate, and glossy alum slate. 



First Kind. 



Common COMMON ALUM SLATE, Jameson. Gemeiner Alaun- 



alum-slatc. schiefer, Werner. 



External Characters. Colour intermediate between 

 bluish and iron black. Massive, and sometimes in 

 roundish balls, which are imbedded in the massive va- 

 rieties. Lustre more or less glimmering. Fracture 

 nearly perfect straight slaty. Fragments tabular. 

 Opaque. Does not soil. Retains its colour in the 

 streak, but becomes glistening. Intermediate between 

 soft and semihard. Easily frangible, and rather brittle. 

 Specific gravity 2.384, Kir man. 



Second Kind. 



Glossy GLOSSY'ALUMSLATE, Jameson. Gliinzender Alaun. 



alum-siate. schiefer, Werner. 



External Characters. Colour intermediate between 

 bluish and iron black, and it sometimes exhibits on the 

 surface of fissures the pavonine, columbine, or temper- 

 steel tarnish. Occurs massive. Lustre semi-metallic 

 and splendent, shining, or glistening, on the principal 

 fracture, and glimmering or dull on the cross fracture. 

 Principal fracture is generally undulating curved and 



.A in m- 

 slate. 



Native 



short slaty ; seldom inclines to straight slaty. Cross Oryctogno. 

 fracture earthy. Fragments tabular, and these run into 

 wedge-shaped fragments. Specific gravity 2.588, 2.339, 

 Kinvan. 



In all the other characters it agrees with the preced- 

 ing subspecies. 



Geognostic Situation Both subspecies agree in geog- 

 nostic situation : they occur in primitive, and also in 

 transition clay slate, and more rarely in veins travers- 

 ing these rocks. Some varieties of alum slate have 

 been observed associated with secondary rocks. 



Geographic Situation. It occurs along with grey, 

 wacke and greywacke slate in the vicinity of Moffat, in 

 Dumfriesshire ; in the transition districts of Lanark- 

 shire, particularly in the neighbourhood of Lead Hills ; 

 and near the Ferry-town of Cree in Galloway. 



Uses. This mineral, when roasted and lixiviated, 

 affords alum. 



The following minerals are placed immediately after 

 the genus Mica, on account of the general affinity to 

 it. Their present situation is not to be considered as 

 fixed, but only temporary. 



* 1. Native Magneisa; 2. Magnesite; 3. Meerschaum. 



** 4. Nephrite ; 5. Serpentine ; 6. Fuller's Earth. 



1. NATIVE MAGNESIA. Jameson and Bruce. 



External Characters. Colour snow white, passing magnisU. 

 into greenish white. Occurs massive, and in granular 

 and prismatic concretions. Lustre pearly. Semi- 

 transparent in the mass, transparent in single folia. 

 Soft, and somewhat elastic. Adheres slightly to the 

 tongue. Specific gravity 2.13. 



Chemical Characters. Before the blowpipe it becomes 

 opaque and friable, and loses weight. Soluble in sul- 

 phuric, nitric, and muriatic acids. 



Constituent Parts. Magnesia . . 70 



Water of crystallization, SO 100 

 Bruce, American Min. Jour. vol. i. p. SO. 



Geonoslic and Geographic Situations. Occurs in small 

 veins in serpentine, at Hoboken in New Jersey. 



2. MAGNESITE, Jameson. Reine oder Natiirliche Ma"nesiie. 

 Talkerde, Werner. 



External Characters. Colours grey and white. Mark- 

 ed with yellowish and ash-grey spots, and also with 

 bluish-grey dots, and dendritic delineations. Oc- 

 curs massive, tuberose, reniform, and in a shape inter- 

 mediate between vesicular and perforated ; walls of ve- 

 sicles rough and uneven. Has a rough surface. In- 

 ternally dull. Fracture large and flat cor.choidal, which 

 passes into fine earthy. Fragments rather sharp-edged. 

 Nearly opaque. Scratched by fluor-spar, but it scratches 

 calcareous-spar. Adheres pretty strongly to the tongue. 

 Feels rather meagre. Dull in the streak. Rather easi- 

 ly frangible. Specific gravity, 2.881, Haberle. 



Chemical Characters Infusible ; but before the 

 blowpipe it becomes so hard as to scratch glass. 

 Constituent Parts. Magnesia, 



Carbonic acid, 47.00 



Silica, 4.50 



Alumina, 0.50 



Ferruginous Man- 

 ganese 0.50 



Lime, 0.08 



Water, 2.00Buchols 



Geognostic and Geographical Situations. Found at 



Hrubschitz in Moravia, in serpentine rocks, along with 



meerschaum, common and earthy talc, mountain cork, 



and rhomb spar. 



