Page 78 - califmining1923

'6 MINERAL INDUSTRY OF CALIFORNIA.

Serpentine Production of California, by Years.
The following table shows the amount and value of serpentine from
1895 as recorded by this Bureau:

Tear Cubic feet Value Year Cubic feet Value

1895 4.000 $4,000 1904 200 $2,310
1896 1,500 6,000 1905
1897 2,500 2,500 1906 - _ 847 1.694
1898 750 3,000 1907 1,000 3.00O
1899 _„ _ 500 2,000 1917 a a
1900 350 2.000 1918 b b
1901 89 890 1919 -
1902 612 5,065
1903 99 800 Totals 12,347 $33,259
" Under 'Unapportioned.
^See under Marble.
SLATE.
Bibliography : State Mineralogist Reports XV, XVIII. Bulletin
'
38. U. S. Geol. Surv., Bull. 586. U. S. Bur. of Mines, Bull. 218.
Slate was first produced in California in 1889. Up to and including
1910 such production was continuous, but since then it has been irregu-
lar. Large deposits of excellent quality are known in the state, espe-
cially in El Dorado, Calaveras and Mariposa counties, but the demand
has been light owing principally to competition of chapter roofing
materials.
'Slate' is a term applied to a fine-grained rock that has a more or
less perfect cleavage, permitting it to be readily split into thin, smooth
sheets. Varieties differ widely in color and have a considerable range
in chemical and mineralogical composition. Excepting certain rare
slates of igneous origin (of which the green slate of the Eureka quarry,
El Dorado County, California, is an example) formed from volcanic ash
or igneous dikes, slates have originated from sedimentary deposits con-
sisting largely of clay. By consolidation, and the pressure of super-
imposed materials, clays become bedded deposits of shale. By further
consolidation under intense pressure and high temperature incident to
mountain-building forces, shales are metamorphosed to slates. The
principal mineral constituents are mica, quartz, and chloride, with
smaller varying amounts of hematite, rutile, kaolin, graphite, feldspar,
tourmaline, calcite, and others.
The color of slate is of economic importance. The common colors are
gray, bluish gray, and black, though reds and various shades of green
are occasionally found.
The permanency of slate for roofing is well known. It is stated that
there are slate roofs in Pennsylvania and Maryland over 100 years old.
I'ln England and Wales, and in France, many buildings constructed in the 15th and
16th centuries were roofed with slate, and the roofs are still in excellent condition.
There is a record of a chapel in Bedford-on-Avon in Wiltshire, England, roofed with
slate in the 8th century, and after 1200 year^ of climatic exposure is moss-covered
but m good condition."
Contrary to the general impression, however, the major portion of
the slate produced in the United States is used on the inside rather than
^ Bowles, O., Slate as a permanent roofing material : U.
Investigations, Serial No. 2267, July, 1921, p. 4. S. Bur. of M., Reports of