* SAN LUIS OBISrO DISTRICT. 381 



o-round was a vein-like formation from two to five feet in width, consisting 

 of oartliy decomposed rock, in whicli occurred threads and bunches of cin- 

 nabar. Bowlders, too, were included in the matrix; they were impregnated 

 with cinnabar to a greater or less extent. 



San Luis Obispo. — The nilncs of the San Luis Obispo are on the Santa Lucia 

 Range, which extends from near Monterey southward along the coast. 

 Tl is range is chiefly composed of metamorphic rocks and Miocene beds. 

 The metamorphic rocks are lithologically identical with those farther north 

 and are also of the same age. Five miles from the town of San Luis 

 Obi.spo, on the road to Santa Mai'garila, in a patch of comparatively little 

 altered shale inclosed in tiie metamorphic series, Mr. Turner found an ex- 

 cellent specimen 0*1 Amelia mosqiieiisis. This is the most southerly locality 

 at which this cliaracteristic fossil of tlie Knoxville series has been encoun- 

 tered, and it is nearly three hundred miles in a direct line from the Man- 

 zanita mine, in CJolusa Count}', where Aucella was also found. This inter- 

 val is equal to three-fifths of the entire length of the Coast Ranges from 

 Ft. Tcjon to Shasta City. 



There is a considerable extent of Miocene rocks in this region, as well 

 as a volcanic range which trends in a westerly direction and ends, according 

 to Professor Whitney, in the Moro rock oft' the coast. It seems to consist 

 of asperites. Hot springs are said to exist in abundance in the quicksilver 

 district of San Luis Obispo, and extinct springs, evidently similar to those 

 now active, are stated to occur close to the most productive of tiie mines, 

 the Oceanic. The hot sulphur springs of Paso Robles are not far from 

 this group of mines. 



The Rincoiuida mine is sunk on a deposit in metamorphic rocks among 

 which is serpentine. These rocks are not distinguishable from those at 

 New Almaden and other more northern districts. The deposit is partly in 

 slaty material, accompanied by black opal, and partly in indurated sand- 

 stones. Pyrite, calcite, dolomite, quartz, and organic matter accompany 

 the ore. It is to some extent disseminated, but usually occupies cracks in 

 the rocks, which it often only partially fills. The crevices sometimes cross 

 the beds and sometimes follow them. The deposit has not as yet proved of 

 any commercial value. It is interesting because its general character is so 



