Page 81 - califmining1923

STATISTICS OF ANNUAL PRODUCTION. 79

Grinding Mill Pebbles.
Production of pebbles for tube and grinding mills began commer-
cially in California in 1915. Owing to the decreased iniports and
higher prices of Belgium and other European flint pebbles, due to the
war, there was a serious inquiry for domestic sources of supply. One of
the shipments made in that year was of pebbles selected from gold-
dredger tailings in Sacramento County, for use in a gold mill in
Amador County emplwing Hardinge mills.
The important development in this item, however, took place in San
Diego County. At several points along the ocean shore from Encinitas
south to near San Diego, there are beaches of washed pebbles varying
from 1 inch to 6 inches in diameter, w^hich come from conglomerate
beds made up of well-rounded water-worn pebbles of various granitic
and porphyritic rocks with some felsite and flint. The wave action has
broken down portions of the cliffs for considerable distances and
formed beaches of the pebbles which are well washed and cleaned of
the softer materials. The rocks sorted out for shipment are mainly
basalt and diabase, with an occasional felsite and flint pebble. There
is a tough black basalt which is stated to give satisfactory results. In
Fresno County pebbles have been selected from the gravel beds of the
San Joaquin River near Friant. Shipments have been made to
metallurgical plants in California, Nevada, Montana and Utah.
Imports in 1923 amounted to 14,243 long tons, valued at $130,974
compared with 14,321 tons and $145,805 in 1922'.
California output for 1923 was 2650 tons, valued at $14,936, an
increase over the 1922 figures.
The amount and value of grinding mill pebbles, annually, follows:

Tear Tons Value
1915 . 340 $2,810
1916 . . _ - 20.232 107,567
1917 __ __ 21,450 90,538
1918 _. 8.628 61,268
191D 2,607 19,272
1920 ._ 2,104 17,988
1921 . 247 1,418
1923 ______ . 1,571 14,986
7,628
1922
2,650
Totals 59,829 $253,425