Arkell.1956.Jurassic..

554 CORDILLERAS OF NORTH AMERICA
carbonaceous layers passing to impure coal. In the middle and upper
parts of the formation are great thicknesses of volcanic rocks: pillow
lavas, vesicular basalts, andesites and dacites, tuffs and agglomerates
interbedded with limestones and radiolarian cherts; and the whole is
intruded by a variety of hypabyssals. The cherts are best developed where
the volcanic rocks are at their maximum, and the whole is clearly a
shallow-water succession. Many of the Knoxville fossils, including
ammonites, were collected from shales below pillow lavas and cherts of
Franciscan type (Taliaferro, 1943, p. 197). From boulders of the chert
have been obtained two snouts of Ichthyosaurus which compare best
with /. posthumus of the Solnhofen Slates (Upper Kimeridgian, Lower
Tithonian).
According to Taliaferro (1943, pp. 190-195) both Franciscan and
Knoxville are overlain unconformably throughout the Coast Ranges by
the Paskenta formation, which contains a marine Valanginian fauna.
Part of the Franciscan at the type locality in San Francisco, however, is
Cretaceous, for it has yielded the Albian ammonite Douvilleiceras
(Schlocker, Bonilla & Imlay, 1954).
LOWER (AND MIDDLE?) KIMERIDGIAN; AND UPPER OXFORDIAN?
Along the west side of the Sierra Nevada the Mariposa Slates, which
pass into the Galice formation in Oregon, have yielded Amoeboceras
(Amoebites) dubium (Hyatt) (Reeside, 1919, p. 38, pi. xxiv, figs. 5-8)
a Lower Kimeridgian form; also poorly-preserved Perisphinctids, P.
virgulatiformis Hyatt and P. miihlbachi Hyatt, which Crickmay (1933,
pp. 56-7, pis. 16-18) interprets respectively as Virgatosphinctoides [Middle
Kimeridgian] and Dichotomoceras [Upper Oxfordian—Lower Kimeridgian].
The preservation of the material, however, renders generic
placing hazardous. From agglomerates at Longtown Ridge, ? Divisosphinctes
or ? Pachysphinctes is recorded (Imlay, 1952, p. 976).
MIDDLE AND LOWER CALLOVIAN
The Mariposa Slate apparently reaches down into the late Lower
Callovian, for it has yielded Kepplerites (Gowericeras ? or Seymourites)
(})lindgreni Hyatt sp. (Crickmay, 1933, p. 57, pi. 17, figs.9. 10) and Macrocephalitids
or Cadoceratids. ('Catacephalites' was based on a squashed
Cadoceras.) K. lindgreni seem to be much like Seymourites figured from
Canada by McLearn (1929, pis. i-viii). The equivalent Amador formation
of the Sierra Nevada is partly volcanic in the lower layers, containing
pillow basalts, tuffs and agglomerates; it has yielded ? Grossouvria (Imlay,
1952, p. 975). At Mount Jura in the northern Sierra Nevada, Reineckeia
(Reineckeites) dilleri (Crickmay 19336, p. 914, pis. 32, 34) is sufficiently
common to have been proposed as a zonal index. Some hundreds of
feet lower was found Choffatia hyatti Crickmay sp. (19336, pi. 33),
which much resembles some European forms of the Macrocephalus
Zone (subbakeriae group). In central Oregon Lilloetia and Kepplerites
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