SLATE DEPOSITS AND SLATE INDUSTRY OF THE UNITED STATES. 
By T. Nelson Dale and others. 
. PART I— ORIGIN, COMPOSITION, AND STRUCTURE OF SLATE, 
By T. Nelson Dale. 
DEFINITION AND CLASSIFICATION OF SLATE. 
Definition. — The term slate, in ordinary usage, denotes a rock which has more or 
less perfect cleavage, adapting it to various commercial uses, and in which the con- 
stituent particles, with very few exceptions, can not be distinguished except in thin 
section under a microscope. In contradistinction a schist is a rock of sometimes 
identical chemical and mineralogical composition, but is either made up of coarser 
particles or possesses a wavy structure, or else is marked by both of these features. 
Both slates and schists may have originated in deposits of identical character, but 
they have undergone different processes. 
Slates as above denned vary greatly in color — from black through various shades 
of gray to greens, reds, and purples of different hues. They vary also in luster — 
from having none to being almost as bright as mica itself. They vary greatly not 
only in grade of fissility, but in surface texture, as seen by the unaided eye or deter- 
mined by touch, and still more in microscopic texture, as seen in thin section. They 
differ also in mineralogical and chemical composition and in physical properties. 
On account of their peculiar properties slates are adapted to a great variety of uses — 
roofing, flooring, electric switch boards, blackboards, hand slates, billiard and labo- 
ratory table tops, vats, mantels, grave linings, wainscoting, hearths, chimney and 
well caps, memorial tablets, bread boards, refrigerator shelves, etc. This stone has 
thus become linked with some of the principal necessaries of life and death. 
Slates divide themselves naturally into (1) those derived from aqueous sediments 
and (2) those of igneous origin. The latter, however, are very exceptional. For 
commercial purposes the basis of classification of the aqueous sedimentary slates must 
be, first, structural, for the cost of their production and the degree of their strength 
depend primarily upon that; secondly, it must be mineralogical, for their durability 
depends upon their content of certain mineral constituents. 
Those slates in which the particles have been merely compressed by weight or pres- 
sure and cemented by carbonates of lime and magnesia, by kaolin or different combi- 
nations of iron, and whose grade of fissility, strength, and elasticity are therefore low, 
must be distinguished from those in which, under metamorphic processes, the kaolin 
and feldspar have passed into mica, forming a more or less dense and regular crystal- 
line fabric of overlapping scales and fibers inclosing any remaining sedimentary par- 
ticles. Such slates necessarily possess a high grade of fissility and considerable 
strength and elasticity. The first group includes clay slates, the second mica slates 
or phyllite slates. Those in which the micaceous matrix is but partially formed, 
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