418 



MINERALOGY. 



Geognosy, tains occur frequently in beds, but more rarely in 

 X T" ' veins, which is directly the reverse of gneiss, where 

 ores occur more frequently in veins than in beds. The 

 ores that occur in beds are the following : magnetic 

 ironstone, iron-pyrites, copper-pyrites, arsenic-pyrites, 

 red-iron ore, lead-glance, blende, gold, and glance- 

 cobalt; and these ores are accompanied with actyno- 

 lite, garnet, and asbestus. 



The veins that occur in mica-slate contain in ge- 

 neral the same ores as those in gneiss. 



The gold mines at the foot of Monte Rose are prin- 

 cipally in mica-slate ; and this is also the case with 

 some of those in the country of Salzburg. The silver 

 mines of Johan-Georgenstadt and Braunsdof in Saxo- 

 ny ; those of Sweden and Norway are in these rocks. 

 Many of the mines in Silesia and Bohemia are in mica- 

 slate. 



The .most important mines in Sweden, as those of 

 Dalecarlia and Fahlun ; those of Roraas in Norway ; 

 many in Hungary and Salzburg, Saxony and Bohemia, 

 are situated in this rock. 



8. It occurs in great abundance in Scotland ; as in 

 the valley between Dunkeld and Blair-in-Athol ; the 

 mountain of Schihallion, and the neighbouring coun- 

 try ; island of Arran; islands of Jura and Isla, &c. 

 It is also very widely distributed in the continent of 

 Europe; as in Saxony, Bohemia, Silesia, France, 

 Spain, the Bannat, Transylvania, Switzerland, Salz- 

 burg. It also occurs in the United States, in South 

 America, and in the continents of Africa and Asia. 



IV. Clay-Slate. 



Thonschiefer. Werner. 



Primitive Argillaceous Schistus, Kirwan. 



Clay-slate. Jameson. 



Schiste argileux, Brochant. 



Phyllade. Daubuisson. 



CUy-slate. 



Varieties. 



1. Characters Clay-Slate is a simple mountain- 

 rock, having a slaty structure, with a fine grained, and 

 dull cross fracture ; it is opaque and soft, with a 

 grey streak, whatever may be its colour. Its most 

 frequent colours are grey, or bluish black ; very often 

 greenish-grey ; more rarely yellowish-grey and brown- 

 ish-red. Oxide of iron is the general colouring in- 

 gredient ; but in the black varieties it is carbon, 

 which gives the tint of colour. The greater number 

 of varieties split easily into slates, which are either 

 plain or variously convoluted. Their surface is some- 

 times smooth, in other instances it is traversed by 

 deepstrise; and it is sometimes dull, sometimes shin- 

 ing with a silky or pearly lustre. It is intimately 

 connected with mica-slate, and there is a distinct tran- 

 sition from it into that rock, by the gradual disap- 

 pearance of the quartz, thus shewing that it is en- 

 tirely composed of mica in very minute and closely ag- 

 gregated scales. 



2. Varieties There are four varieties of clay-slate. 

 The first kind has a yellowish-grey colour, and a 

 shining lustre : it is the oldest kind, to use the lan- 

 guage of Werner, and is that which reposes imme- 

 diately on mica-slate; it is in short the link that 

 connects clay-slate with mica-slate. The second kind 

 is dark-grey; sometimes even bluish- grey and grey- 

 ish-black, forming what is denominated roof-slate, 

 from the circumstance of its splitting into thin and 

 large tables. We must be careful, however, not 

 to consider all roof-slate as of primitive formation. 

 To this follows, in the order of succession, the third 



kind, which has a greenish-grey colour. The fourth 

 and last, which is the newest kind of clay- slate, is 

 bluish-gray, and reddish : it contains a very few in- 

 termixed scales of mica; possesses but little lustre; 

 and is the link that connects the primitive clay-slate 

 with the transition clay-slate. 



3. Stratification. It is distinctly stratified, and its 

 slaty structure is generally parallel to the seams of the 

 strata ; in some cases, however, a double cleavage is 

 observable ; and Count de Bournon observes, that many 

 clay-slates break under angles of 60 and 120, which he 

 supposes may be owing to the presence of mica. The 

 strata are in general much inclined, and are often va- 

 riously convoluted and waved, and sometimes they ap- 

 pear to be composed of different concretions. 



4. Imbedded minerals. Independent of the grains of 

 quartz and scales of mica irregularly distributed through 

 it, we find it containing large imbedded masses of 

 quartz, of hornblende, and crystals of chiastolite. 



5. Subordinate Beds. It contains a greater variety 

 and number of foreign beds than gneiss or mica-slate ; 

 and of these some are nearly peculiar to it, and cha- 

 racterise the whole formation. We shall first mention 

 those which are common to gneiss and mica-slate, as 

 well as clay-slate, and then those that are peculiar to 

 clay-slate. 



(I.) Rocks that occur in gneiss, mica-slate, and clay- 

 slate. 1, Limestone. 2. Hornblende-reck. 3. Primi- 

 tive Greenstone. 4. Hornblende-slate. 5. Porphyry. 6. 

 Quartz. 7. Aclynolile. 



(2.) Rocks peculiar to the day-slate formation, or 

 which occur very frequently in it. 1. Whet-slate. It 

 occurs in beds, in Saxony, Bavaria, Silesia, Stiria, and 

 other countries. 2. Roof-slate. This is but a variety 

 of clay-slate, distinguished by its bluish or ash-grey 

 colour ; its straight slaty fracture ; its splitting into 

 large tables, and its being nearly pure and unmixed. 

 It seldom or never forms whole mountains, but occurs 

 usually in single thick beds with other kinds of clay- 

 slate. 3. Chlorite-slate. This usually follows the pre- 

 ceding. It forms whole beds, and includes garnets, 

 crystallized magnetic iron-stone, iron-pyrites, common 

 schorl, tourmaline, and quartz. 4. Talc-slate. This 

 is usually the next in the order of succession. 5. Alum- 

 date. It occurs in considerable beds in clay-slate ; and 

 the two sub-species, the common and shining, alternate 

 with each other. It contains a portion of carbon, and 

 also iron-pyrites. 6. Drawing-slate. It occurs usually 

 in the vicinity of alum-slate, and |is very nearly allied 

 to it. It contains more carbon than alum-slate, but less 

 iron-pyrites. 7- Polstone occurs in considerable beds. 

 8. Flinty-slate occurs in considerable beds in this great 

 formation. 9. Lydian-stone occurs in beds and imbedded 

 masses. 



6. Formations. This rock occurs along with mica- 

 slate, and sometimes in beds in gneiss, and even in 

 granite. 



7. Form of mountains. It sometimes forms whole 

 mountains, and even chains of mountains. Its moun- 

 tains have usually a gentle acclivity ; and its cliffs are 

 not so steep and rough as those of mica-slate or gneiss. 

 It is more favourable to vegetation than any of the 

 rocks we have hitherto described ; and it is observed 

 that the quantity of vegetation increases from granite 

 to clay-slate ; and this appears to depend, not so much 

 on the lower level of the outgoings of its strata, as on 

 the nature of the rock itself. 



We can thus observe a gradual change in the shape 

 of mountains, also of their cliffs and valleys, from gra- 



Geognosy- 



Stratifica- 



tion. 



Imbedded 

 minerals. 



Subordi- 

 nate beds. 



Rocks pe- 

 culiar to 

 the clay- 

 slate for- 

 mation, or 

 which oc- 

 cur very 

 frequently 

 in it. 



Forma- 

 tions. 



Form of 

 mountains. 



