570 KANSAS CITY REVIEW OF SCIENCE. 
rock but afterwards seems to be abundantly proved. This pressure was fre- 
quently applied at right angles to the line of bedding. Wax and other plastic 
material can be made laminar in structure by means of lateral pressure. And 
Tyndall has shown that the same thing takes place in the ice of glaciers, at 
points where it is subjected to immense pressure, as at a narrow part of the val- 
ley. In the case of a slaty rock the clay might become laminated, while sand 
or limestone associated with it might show no such structure. 
The largest European slate quarries are in North Wales, though it occurs 
also on the Continent. In this country, slate is extensively worked in Maine, 
Vermont, New York, Pennsylvania, and in the Peach-Bottom district of north- 
ern Maryland. It is found in several other States. 
Its color varies in different localities, from drab through different shades of 
green and red. 
Though essentially a silicate of iron and alumina, slate often contains other 
ingredients which we may term "accidental," that seriously affect its durability. 
A specimen of ordinary slate analyzed by M. Maumene contained of silica 48.6 
per cent; alumina 23.5 ; sesquioxide of iron 11. 3; magnesia 1.6; potash 4.7; 
water 7.6. A Scotch slate contained of silica 50 per cent; alumina 27 ; sulphate 
and oxide of iron 11; magnesia i; potash 4; water 7. Among the accidental 
ingredients are carbonates of lime and magnesia, and iron pyrites or sulphide of 
iron. All of these substances are injurious and increase the tendency of the 
slate to disintegrate or fall to pieces by the action of the weather. One speci- 
men examined by the above author contained 53.5 per cent of carbonate of lime. 
It is needless to say that it had been condemned because it would not weather 
well. Acid liquids readily decompose carbonates, and roofing slate is particu- 
larly exposed to the action of sulphurous fumes from the chimneys. These settle 
on the roof, become oxidized, and combine with the lime, setting the carbonic 
anhydride free, while the sulphate of lime is washed away, and thus the slate is 
decomposed. 
In regard to the pyrites, which is often visible as bright, brassy particles col- 
lected in parallel lines, it is oxidized by action of air and moisture, and the result- 
ing sulphate is washed out by the rain, hence its presence is detrimental. 
Another accidental factor having an influence on the durability, is the por- 
osity. Those slates absorbing the most moisture will be more subjected to the 
disintegrating effects of atmospheric agencies, especially rain and frost, and will 
soonest be destroyed. Dry slate treated with water absorbs from two ten-thou- 
sandths to twenty-five ten-thousandths of its weight. The American slate has a 
density of about 2.80. 
The methods employed in quarrying, and in fact many of the managers and 
workmen engaged in the slate industry in this country come from Wales. The 
pit is often worked to a great depth. In the slate regions visited by the author, 
the rock is dislodged by blasting, the holes being drilled by a steam diamond- 
drill, or the slabs are split off by a line of wedges. Seams usually occur which 
greatly facilitate the quarrying. The pieces of, say eight feet in length by two in 
