410 University of California Publications in Geology [Vol. 9 



that "On stratigraphic grounds the auriferous gravels are regarded 

 as contemporaneous with the lone formation of the Sacramento Valley, 

 but here too as in the earlier auriferous gravels, the fossil plants and 

 shells appear to indicate that they belong to the Miocene." 



Lindgren 58 in his description of the lone formation summarized 

 conditions succinctly as follows : 



During the Miocene period and contemporaneously with the accumula- 

 tion of the later pre-volcanic gravels on the slopes of the Sierra Nevada, 

 there was deposited in the gulf then occupying the Great Valley a sedi- 

 mentary series of clays and sands to which the name lone formation has 

 been given. The water in this gulf was probably brackish; no marine fossils 

 have been found in the lone formation along the foot of the range, but 

 fossil leaves, vegetable material, and in places, coal are abundant. At the 

 mouth of the rivers which descended from the Tertiary Sierra Nevada 

 extensive delta deposits were accumulated, and it is thus difficult in many 

 places to draw any exact line between the lone formation and the river 

 gravels proper. The gravels in the formation are locally auriferous, though 

 generally poor, because spread over large areas. 



The lowest and oldest Tertiary auriferous gravels lie in troughs over 

 which the lone formation has transgressed, in places a depth of more than 

 500 feet. At many localities the sandstones and clays of the formation 

 merge directly into the upper river gravels of the so-called benches. On 

 the other hand, the thick gravels of the rhyolitic are distinctly later than 

 the lone formation. Turner has shown that in the Jackson Quadrangle 

 extensive short or delta gravels of interrhyolitic [these gravels are not 

 interrhyolitic but are andesitic, R. E. D.] age rest on the eroded surface of 

 the lone (see PI. 11, B, p. 72).* The lone formation belongs to the Tertiary 



The greatest thickness of the formation measured is in Calaveras County 

 in the Jackson Quadrangle, where Turner has determined it to be about 1000 

 feet. Post-lone erosion has removed the formation entirely over large areas. 



The most northerly exposures of the lone north of the Sierra Nevada, 

 have been observed by Diller on Little Cow Creek and Pit River in the 

 northwest corner of the Lassen Peak Quadrangle, Shasta County. The clays 

 and sands are here directly overlain by andesitic tuffs and rest on meta- 

 morphosed slates of Jurassic or Triassic age at an elevation of about 2000 

 feet. South of this locality few exposures are seen until the Oroville Table 

 Mountain is reached, a distance of nearly 100 miles. At this place, a capping 

 of basalt, somewhat earlier than the andesitic flows, has preserved the lone 

 intact. The formation here consists of fine gravels, white clays and sands, 

 and reaches to elevations of about 1200 feet. (See PI. IV, B; fig. 4, p. 86; 

 and fig. 5, p. 90.)* With insignificant exceptions, no further exposures occur 

 between this point and Lincoln in Placer County, where some white clays 

 are preserved underneath a capping of andesitic tuff in the midst of Quater- 

 nary gravels and a few miles west of the first outcrops of the pre-Cretaceous 

 rocks, usually referred to by the collective name "Bedrock series". 



About 40 miles northwest of Lincoln, in the late Tertiary andesitic 



ss Lindgren, W., Tertiary Gravels of the Sierra Nevada of California, 

 Professional Paper, No. 73, U. S. Geological Survey, pp. 24 and 25, 1911. 

 *Lindgren's Plates. 



