6 SLATE DEPOSITS AND INDUSTRY OE UNITED STATES. 
and which represent a transition from clay slate to mica slate, are more conveniently 
put into the clay-slate group. Thus the dark-purple ( ' 'red' ' ) roofing slate of Penrhy n, 
in Wales, is a clay slate, and so also is the black slate of Martinsburg, W. Va. ; but 
the French Ardennes slate, the Welsh Festiniog, and the Peach Bottom of Pennsyl- 
vania and Maryland, are all mica slate. 
But mica slate includes slates of very different composition and structure, and 
therefore of properties and value. The first distinction to be made is based upon 
the amount of ferrous carbonate — whether or not it is sufficient to produce discolora- 
tion on continued exposure to the weather — for this is an important factor in the 
value of slates used for roofing. This distinction gives a group of fading and one of 
unfading slates, each of which can be further subdivided as to color. But each of 
these colors of fading and unfading mica slate embraces a wide range of texture. 
Some, like the Arkansas black and light-greenish slates, have a matrix of extremely 
fine sericite fibers, while the black slates of Arvonia, Va., and the Peach Bottom 
slates have a texture approaching that of a schist. Such differences in texture can 
not but result in difference of physical properties. The grades of texture are suffi- 
ciently designated by the words very fine, fine, medium, and coarse. There are also 
distinctions in the grade of fissility which do not always correspond to the grade of 
texture. These are conveniently numbered from 1 to 4, beginning with those of the 
best fissility. Finally there are differences in luster and amount of magnetite — 
differences which are purely specific. The general distinctions are formulated in 
the following table: 
CLASSIFICATION OF SLATE. 
(I) Aqueous sedimentary. 
(A) Clay slates: Matrix without any or with but very faint aggregate polarization. 
(B) Mica slates: Matrix with marked aggregate polarization. 
(1) Fading: With sufficient FeC0 3 to discolor considerably on prolonged 
exposure. 
(a) Carbonaceous or graphitic. 
(b) Chloritic (greenish). 
(c) Hematitic and chloritic (purplish). 
(2) Unfading: Without sufficient FeC0 3 to produce any but very slight 
discoloration on prolonged exposure. 
(«) Graphitic. 
(b) Hematitic (reddish). 
(c) Chloritic (greenish). 
(d) Hematitic and chloritic (purplish). 
(II) Igneous. 
(A) Ash slates. 
(B) Dike slates. 
The scientific basis for these subdivisions will be seen by consulting the table 
facing page 124 and the microscopic and chemical analyses of the slates there 
named. The application of this scheme to 13 kinds of American slates will be found 
on page 125. 
ORIGIN OF SLATE. 
With the exception of the rare slates of igneous origin, referred to in the previous 
section, slates originate in marine deposits of clay and sand. The common occur- 
rence of angular grains of feldspar and of quartz in slate implies the nearness of 
shores or land masses of granitic rocks to such deposits. The alternation of beds of 
slate with beds of quartzite or grit (''ribbons," "hards," altered sandstone) corre- 
sponds to the alternation of extremely fine clayey sediments, derived from the waste 
of granitic land masses, with sandy sediments consisting of coarser material from 
