SLATE, a clay rock which has been sub jected to compression during mountain-making processes in the earth's crust, and which as a result splits readily into thin sheets or cleavage lamina in a direction at right angles to the direction of compression. According to Sorby, cleavage is produced in such rocks by the re arrangement during compression of the plastic particles comprising the original rock mass, in such a manner that their longer axes come to lie perpendicular to the direction of com pression, and that planes of easy fracture were thus produced parallel to the long axes of the particles. This was illustrated by Sorby, with a cube of clay through which mica scales or scales of oxide of iron were sprinkled, which was then subjected to powerful compression and drying. A perfect cleavage at right angles to the line of pressure was produced, and micro scopic examination showed that the mica scales lay in the direction of cleavage. Tyndall found by experiment that cleavage is more perfect in proportion in which the cleaved material is free from foreign particles, such as mica scales, and he considered that cleavage was produced in homogeneous rocks (except the vitreous) by the compression and flattening of the minute discrete granules or particles of which all such matter consists. The structure is changed by the compression from granulous to scaly. Enor mous vertical swelling of the rock masses re sults through such lateral compression, and this swelling is an important factor in the formation of mountains.
Ordinarily cleavage exists in rocks in a po tential manner only — the rock is cleavable in a certain direction, but no actual separation takes place. This, however, is rapidly developed when the rock mass is exposed to atmospheric influences. Slates are quarried in solid blocks and split up in thin sheets, usually by hammer and chisel after quarrying. The important slate producing horizons of eastern United States are the Cambrian and Ordovician beds of the Taconic range of eastern New York and its extension in Vermont, and the corresponding formations of Pennsylvania and other regions in the northern Appalachians. In composition slates vary greatly, especially in the proportion of silica. An analysis of common roofing-slate gave the following: Silica, 48; alumina, 25.50; oxide of iron, 11.03; potash, 4.70; magnesia, 1.60; carbon, .30; water, 7.60. Since slate is in reality a structural term, denoting the property of easy splitting so characteristic of roofing slates, rocks of almost any composition may be indexed under this term, provided they show the characteristic slaty cleavage. Thus it has
become customary to speak of anthracite slate, in which a considerable percentage of carbo naceous material is present ; whet slates — clay slates, with a large percentage of silica, often nearly pure silica (Novaculytes) ; talc slates and chlorite slates, where those minerals are prominent ; gray-wacke slates, more or less arenaceous and micaceous slates. Roofing slates, however, are the typical slates, and the ones in which the slaty cleavage is best devel oped. The most important localities for these in the United States are Vermont and Penn sylvania.
The slate belt of Pennsylvania first appears in the eastern corner of York County and "then sweeps around in a gradually narrowing curve to the Susquehanna River." It reappears on the eastern bank of the Susquehanna in Lan caster County, but can be traced for a compara tively short distance from the river. While this belt is quite limited in extent, being but six miles long and less than one mile in width, it is nevertheless producing a very high grade prod uct. It is from this belt that the well-known °Peach Bottom Slate" has come. It has re markable strength and durability, exceedingly fine in grain and texture and the very desirable quality of retaining its original color even after continuous exposure for many years. It lacks the high degree of fissility, however, that char acterizes other localities. Microscopic study of thin sections from this belt shows, according to Merrill, that the slate is not fragmental in character, but on the other hand is a °highly carbonaceous, crystalline schist." This is at tributed to the high degree of metamorphism to which the belt has been subjected. Bangor and West Bangor are the centres of greatest activity. The belt continues for a short dis tance into Maryland where a few smaller cen tres of production are located. Maine follows Vermont with a valuation of $202,325 for 1901. There are five areas of note in the State. One of the two larger belts extends from Franklin County northeastward to Aroostook County, thence northward into New Brunswick. The second passes from the western boundary of Somerset County, in an easterly direction to Houlton, Aroostook County. °Of the three smaller areas, one is on the Kennebec River, south- of Skowhegan and the two others in Washington County, about Baskahegan Lake and near Princeton." Only portions of the two larger areas produce slate of a medium or high grade.