238 The American Geologist. ^p^'*'- i»oi. 
continuous deposition from the Carboniferous through the 
Triassic into the Jurassic. My acquaintance with that region 
is sHght, but when I visited it I could not so clearly separate 
the Carboniferous and Mesozoic slates as in Trinity county. 
My impression is that the latter formations are entirely unrep- 
resented in Trinity county, their place being taken by a marked 
interval of erosion. In the Pitt river valley I found the slates 
grading into the greenstone by interstratification, while in Trin- 
ity county they are distinct and a short interval of erosion be- 
longs between them. My studies lead me to think that 
throughout the Klamath region west of the Sacramento river, 
sedimentation ceased at the close of the Paleozoic era and the 
Devono-Carboniferous series was elevated into dry land, some- 
what folded and metamorphosed and then greatly eroded, while 
sedimentation continued through the Triassic and Jurassic per- 
iods in the region of the McClovid and Pitt rivers, and thence 
northeastward through an undetermined area. Sedimentation 
in the country west of the Sacramento river was only resumed 
well on in the ]\Iesozoic era, after the Clear Creek volcanic per- 
iod, and hence the greenstone and Bragdon slates (which 
rightfully belong together as a series) came to rest unconform- 
ably upon the Devono-Carboniferous. 
Now, there is a remarkable resemblance in the lithology and 
structure of the Clear Creek greenstone and the main diabase 
and porpliA'ryte formation of the Sierra Nevada region. Both 
seem to represent a time when diabasic tuffs and lavas spread 
over wide areas much as the Neocene volcanoes cover the older 
formations in the Cascade region. In the Sierra Nevada re- 
gion also this extrusive series of eruptives is intimately asso- 
ciated with a black slate formation — the Mariposa slates of 
late Jurassic age. Not only are the Bragdon slates litholog- 
ically similar to the Alariposa slates, but the general make-up 
of the two series is identical. This parallelism of conditions 
on opposite sides of the comparatively narrow Sacramento val- 
ley is too remarkable to be ignored. It seems to warrant the 
correlation in a general way of the two series. As the Mari- 
posa slates with their associated diabase and porphyryte are 
considered late Jurassic in age, I shall provisionally class the 
Clear Creek greenstone and the Bragdon slate as also Jurassic. 
