ORIGIN OF SLATE. 7 
the same source. The repeated alternation of such fine and coarse sediments is 
attributed to the alternation of calm water, favorable to the deposition of fine mate- 
rial, with strong currents that bring coarse sediments more rapidly from the shore. 
These materials consisted largely of quartz, feldspar, and mica, but included also 
zircon and other silicates, various compounds of iron, lime, and magnesia, as well as 
kaolin arising from the decomposition of the feldspar. In such a slate region as that 
of western Vermont and eastern New York, where the slate is interbedded with fos- 
siliferous limestone, it is evident that alternating with such changing conditions in 
the water there were also periods when marine life abounded and the sediments 
were entirely calcareous. Black slates owe their blackness to carbonaceous matter, 
probably derived from the decomposition of marine organisms on the sea floor. Red 
slates owe their color to the access of ferruginous matter from the land, and purplish 
ones to an admixture of this and a green magnesian mineral (chlorite) of secondary 
origin. In both the reddish and purplish slates the iron is supposed to have been 
originally precipitated in the form of the rust-colored limonite (2Fe 2 3 , 3H 2 0) from 
iron-bearing solutions, and afterwards altered by loss of water (H 2 0) to the reddish 
hematite (Fe 2 3 ). « 
An accumulation of several hundred feet of such clayey and sandy sediments 
when buried under several thousand more of other sediments of like origin on a 
gradually subsiding sea bottom must have been subjected to sufficient vertical pres- 
sure, in connection with a small amount of moisture, to have been cemented together 
and hardened — the clay into shale and the sand into sandstone. During this process 
the particles in these sediments retained the general horizontal and parallel arrange- 
ment which they had received from their distribution by sea water, but became 
firmly compacted and thus acquired a bedding foliation. 
The next stage in the formation of slate is attributed ultimately to the radiation 
of heat from the earth's interior interspace, resulting in a contraction of the interior, 
and consequently in a corrugation of the upper portion. This corrugation, for rea- 
sons not yet perfectly evident, took place, as far as observation extends, chiefly 
within certain belts along the axes of our mountain systems, producing lateral com- 
pression in a great mass of parallel strata. The first effect of this compression was 
to bend at least the lower portion of the strata into wave-like folds, and thus to 
shorten its horizontal area in one direction and increase its vertical thickness. The 
character of this folding is well shown in the Pennsylvania and western Vermont 
slate regions (see Pis. Ill, XIII, XIV, XVII, XVIIl/xXII, XXV). But another 
effect of this compression was to metamorphose the shale into slate. This meta- 
morphism probably did not take place until the folding was well initiated. The 
transformation included two processes, and it is uncertain which preceded. There 
was a rotation of each individual sedimentary particle from its original horizontal 
position in the bedding foliation into one forming a considerable angle to the direc- 
tion of pressure. There was also, under the combined presence of moisture and the 
effect of pressure and heat, both that which must have been generated by the pres- 
sure and that which pervaded the strata at the depth at which they were buried, such 
a chemical recombination of the silica, alumina, potash, iron, and water of the feld- 
spar, kaolin, and iron of the shale as to generate new potash-mica in amount suffi- 
cient to constitute, in the case of the mica-slates, over 33 per cent of the resulting 
slate. This muscovite was formed in scales of infinitesimal thinness and other dimen- 
sions, and generally of longish, tapering, or ribbon-like outline. Most of these scales 
arranged themselves with their flat sides parallel to or overlapping one another, but 
facing the direction from which the pressure came, and also with an angle of inclina- 
tion governed by that pressure. A small but variable proportion, however, of these 
"See on these changes Van Hise, C. K A treatise on nietaniorphism: Mon. U. S. Geol. Survey, vol. 
47, pp. 225,232. 
