o the sedimentary bedding.

Clays are used principally for building and paving brick and tile,
sewer-pipe, railroad ballast, road material, puddle, Portland cement,
and pottery. Clay is mined in almost every state. Ohio, Pennsylvania,
New Jersey, and Illinois have the largest production. There has been a
considerable importation of high-grade clays, principally from England,
for special purposes--such as the filling and coating of paper; the
manufacture of china, of porcelain for electrical purposes, and of
crucibles; and for use in ultramarine pigments, in sanitary ware, in
oilcloth, and as fillers in cotton bleacheries. War experience showed
the possibility of substitution of domestic clays for most of these
uses; but results were not in all cases satisfactory, and the United
States will doubtless continue to use imported clays for some of these
special purposes.

Shales, because of their thinly bedded character and softness, are of no
value as building stones, but are used in the manufacture of brick,
tile, pottery, and Portland cement.

Slates owe their commercial value primarily to their cleavage, which
gives well-defined planes of splitting. The principal uses are for
roofing and, in the form of so-called mill stock for sanitary,
structural, and electrical purposes. Small amounts are used for
tombstones, roads, slate granules for patent roofing, school slates,
blackboard material, billiard table material, etc. The color, fineness
of the cleavage, and size of the flakes are the principal features
determining the use of any particular slate. Ten states produce slate,
the principal production coming from Pennsylvania and Vermont.

THE FELDSPARS

Feldspars are minerals, not rocks, but mention of them is made here
because, with quartz, they make up such an overwhelming percentage of
earth materials. It is estimated that the feldspars make up 50 per cent
of all the igneous rocks and 16 per cent of the sedimentary rocks. As
the igneous rocks are so much more abundant than the sedimentary rocks,
the percentage of feldspars in the earth approaches the former rather
than the latter figure. In most rocks feldspar is in too small grains
and is too intimately associated with other minerals to be of commercial
importance; in only one type of rock, pegmatite, which is an igneous
rock of extremely coarse and irregular texture, are the feldspar
crystals sufficiently large and concentrated to be commercially
available.

F