68 TOPOGRAPHY AND GEOLOGY
large
amounts of gravel which have been washed quite extensively. These
gravels are, however, thought to be ordinary river deposits on a large
scale. In the southern part of the State, in Santa Barbara and San
Diego counties, gold-washing has been carried on to some extent, but
under unfavorable conditions and apparently without much profit.
Deposits at La Grange. —
The deposits at La Grange, Stanislaus County, in a distance of one and
a half miles in a northerly and southerly direction, cross four
distinct and widely varying formations (see annexed topographical and
geological section), which, enumerated in accordance with their
relative ages, are: argillaceous slates, occurring north of the
Tuolumne River, probably Jurassic ; diorite ; a thin stratum of
basaltic tufa; and post-pliocene auriferous deposits of sand and gravel.
The
slates have a general strike northwesterly and southeasterly, parallel
to the general trend of the Sierra Nevada Mountains, and dip at an
angle of about sixty degrees. The diorite is occasionally porphyritic,
changing into aphanite and serpentine in places which, so far as
observed, are not on the direct line of the section. It sometimes
contains quartz, and must be classed as syenitic. Where overlaid by
basaltic tufa or gravel it is very much decomposed, presenting the
appearance of clay shale, showing thick-bedded stratification, a
water-worn and undulating surface, with numerous pot-holes similar to
a river bed.
The
basaltic tufa, from two to six feet thick, occurs in more or less
isolated patches, having been washed away in places and cut up by
streams previous to or during the deposition of the gravel. It is
generally of a light greenish or yellowish color, occasionally pink or
of a rusty iron tinge, and frequently contains angular quartz pebbles
and rounded masses of flint-like rock.
The auriferous deposits of sand and gravel rest upon the tufa, and are not capped by any volcanic flow. Bones
