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GEOGNOSY AND GEOLOGY. — 93 
Section 2. Clay Slate. 
The rocks of this section present an intimate union of talc, mica, or 
chlorite, with quartz and feldspar. The micaceous minerals predominate ; 
and mica slate, a combination of the common biaxial mica with quartz, is 
the rock of this series which is most abundant. Those modifications which 
contain chlorite and tale occur but rarely. 
Clay Slate. This tends towards chlorite, towards mica, and towards tale. 
in proportion as it contains a superabundance of any one of these three 
substances. Its laminated structure is very distinct, and there are few 
rocks which are as well calculated for a satisfactory study of all the laws 
of lamination as this. Large plane tabular, and very thin layers, which often 
traverse whole mountain masses, alternate with those of the most remarkable 
complication of folding and contortion. 
The common clay slate, containing mica in superabundance, and of a 
grey color, often contains carbonaceous matter, which imparts a black 
color; oxyde of iron communicates a red, or reddish-brown tint. When 
the materials are very intimately combined, only a feeble glimmering will 
be observed; when this combination is less thorough, the scales of mica 
will be very evident on the surface of thin lamine, and produce some 
lustre. Chlorite slate betrays itself by its greenish color, communicated by 
the chlorite. A calcareous variety is of a clear yellow, blue, or greenish- 
white color, often covered by a ferruginous tinge. 
The transitions of clay slate are very various: the principal are into 
whetstone, silicious, and grauwacke slate, and hearthstone. 
Roofing Slate. This’is nothing more than clay slate penetrated by 
carbonaceous or bituminous particles. The principal external peculiarities 
exhibited by clay slate are found in this variety also. When exposed for a 
long time to the air, it becomes covered with a white crust, caused by the 
disappearance of the carbonaceous matter. The rock is very lustrous on 
the planes of lamination. Nearly allied to it is— 
Graphite Slate, an intimate mixture of clay slate with graphite. It possesses 
a metallic shining lustre, caused by the graphite contained in it in plates. 
Alum Slate. This is an intimate combination of clay slate with iron 
pyrites, thoroughly penetrated by coaly or bituminous matters. From the 
former it derives a black, from the latter a brown color. The brown 
varieties burn with a flame. Alum slate is a substance not without its 
importance in the arts, as it furnishes, in a great measure, much of the 
material for the fabrication of alum. The iron pyrites contained in it 
undergoes oxydation, owing to the fine and divided state in which it occurs. 
Both the iron and the sulphur of the pyrites combining with oxygen, a 
sulphate of iron or green vitriol is formed. This is decomposed again, and 
‘the sulphuric acid combines with the alumina and potassa of the alum slate, 
forming alum. For this reason, that portion of the bed of alum slate 
exposed to the atmosphere, soon becomes coated with a white crust, which 
consists of coarse alum. Bituminous alum shale is especially adapted to the 
manufacture of alum, as it not only furnishes as good a material as the 
black, but from its combustibility may be used for fuel. 
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