8 SLATE DEPOSITS AND INDUSTRY OF UNITED STATES. 
scales took such a position that their flat sides became parallel to the direction of 
the pressure, and the longer axes of the other scales also lay in that direction. As 
mica crystallizes in columnar crystals, and as the plates or scales due to its molecular 
structure are transverse to the crystal column, and as a slab of slate consists largely 
of parallel scales of mica it may be said to correspond when held horizontally to 
such a crystal held vertically. When a mica-slate is cut in thin section across the 
cleavage its optical behavior under polarized light is like that of a mica crystal cut 
across its crystal cleavage. Yet as not only a considerable number of the mica 
scales in slate lie across the cleavage, but as some scales of chlorite and crystals of 
other minerals do also, the texture of a mica-slate combines some of the features of a 
crystal with some of those of a tissue. Extremely thin sections of slate transverse 
to the cleavage sometimes show this interlacing of the two sets of scales on their 
attenuated edges. It is to this microscopic texture that slate largely owes its peculiar 
properties. 
This crystalline fabric may inclose in its meshes any sedimentary particles of quartz, 
zircon, feldspar, kaolin, or other minerals which were not or could not be made over 
into mica or secondary quartz, but whose alignment became more or less parallel to 
that of the major part of the new mica. During this metamorphism other chemical 
combinations arose from the constituents of the shale. These crystallized in isolated 
scales or crystals of chlorite, biotite (magnesia mica), various carbonates, pyrite, mag- 
netite, graphite, tourmaline, andalusite, etc. These arranged themselves variously — 
some in the cleavage direction, some in the grain direction. Various lenses, consist- 
ing of one mineral surrounded by one or two others, were also formed and arranged. 
The limonite became hematite. And during these changes in the fine sediments 
the intercalated beds of sandstone passed into quartzite and metamorphic grit by the 
formation of siliceous and micaceous cement between their particles. 
The essential part of the process of the transformation of a granitic rock into clay 
and of that into mica may be made generally intelligible by recalling two well-known 
experiments of Daubree, the French experimental geologist. In the first, angular 
fragments of granite and quartz were rapidly rotated in a horizontal iron cylinder 
filled with water and moving on an axis. The trituration resulted in producing a 
very fine clay. The second consisted in inclosing clay and water in a closed wrought- 
iron tube, which was then placed in a gas furnace and exposed for several weeks to 
a very high temperature. The result was the production of scales of mica. In the 
first experiment the factors of stream and wave erosion were duplicated, and in the 
second the elements of moisture, pressure, and heat involved in the crystallization 
of new minerals from clay were reproduced. 
Two other experiments illustrating the arrangement of the mica thus formed, as 
well as the sedimentary mica and other minerals, may well be cited here, also. The 
familiar one of Tyndall, which consisted in subjecting wax to pressure between glass 
plates and, after cooling it in ice and salt, breaking it edgewise with a hammer. The 
fractures showed that it had acquired slaty cleavage. The other experiment was 
made by Jannetaz, and consisted in subjecting a mass of clay in a wrought-iron box, 
open at one side, to hydraulic pressure, which resulted in the formation not only of 
slaty cleavage but also of the grain structure. 
The general microscopic structure of slate is such as to warrant the assumption 
that the compression which produced it operated not only with great uniformity but 
also very gradually. 
The microscopic features of slate will be found more fully considered in the follow- 
ing section on the petrographic characters of slate. The above will suffice, however, 
to convey, with as few technical terms as possible, a general idea of the principal steps 
in the formation of a mica-slate of aqueous-sedimentary origin. The history of clay- 
slate is similar, but metamorphism has been insufficient to produce the micaceous 
texture in it. After the slate was formed the mass was subjected to various stresses 
