236 The American Geologist. -^p"^- i^^^- 
to thin and become more uniformly a diabase similar to the 
dikes in older formations. Its thickness is unknown, but in 
Trinity valley it is at least 1,000 feet, as that thickness is ex- 
posed and the bottom not seen. 
The Clear Creek greenstone was deposited on land. At the 
close of the epoch it suffered erosion and was levelled oflf either 
by the sea, during the progress of the submergence to which the 
slates are due, or b)' sub-aerial erosion, probably as the result 
of both. This interval of erosion does not appear to have been 
a long one. 
The Bragdon slate. — This has its heaviest development in 
the Trinity mountain (the bulk of which it forms) between the 
high mountains east of Trinity Center and the vicinity of Lew- 
iston. It is cut ofif on the west at the foot of the high Sierra 
Costa mountains by a sharp monocline or a fault. On the east 
it thins out because of erosion and has been complete! v re- 
moved from over the greenstone formation along a broad belt 
lying just west of the Sacramento river. 
It is a series of alternating thin-bedded black slates and 
thick-bedded blue quartzytes. As doubts have been expressed 
that this formation is distinct from the Devono-Carboniferous 
I will lay special stress upon the points of difference. In the 
lower slates, the quartzytes are white and weather red, yellow' 
brown or a bleached white; in the Bragdon slates, the quartz- 
ytes are blue and weather gray. The former formation is schis - 
tose throughout the argillaceous portion because of regional 
shearing: the Bragdon slates have only been sheared along cer- 
tain zones, and most of the formation is merely a well lithified 
or silicified shale and sandstone, without shearing. The Lower 
Slate areas have limestone ; the Bragdon slate areas none. 
The former series abounds in intrusive diabase ; the latter has 
no dikes of this eruptive and all the diabase in its vicinity is 
under it. Conglomerates are rare in the Devono-Carbonifer- 
ous ; in the Bragdon slates thin sheets of well-lithified, moder- 
ately fine conglomerate are rather common, -particularly in Hay 
gulch, three miles north of Bragdon, and near PYench gulch. 
The pebbles are of blue, black, red, yellow, brown and white 
quartz and appear to have been formed principally from the 
quartzytes, cherts and phthanytes of the Devono-Carboniferous. 
Along the Sacramento river, near Elmore and thence east- 
