124 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 2lj 
Cretaceous, but thus far no large bodies of ore commercially workable 
have been observed in the sedimentary rocks, although such rocks 
occur in the neighborhood of some of the mines. In the Afterthought 
and Bully Hill districts the nearest sedimentary rocks are chiefly shales 
and limestones of Triassic age; in the Black Diamond district, shales 
and limestones of Carboniferous age; and in the Iron Mountain dis- 
trict, shales and limestones of Devonian age. Neither the kind of 
sediment (except the limestone at Black Diamond, to be noted later) 
nor its age is of special importance in relation to the ore bodies. 
Igneous rocks. — The more important rocks of the copper region, so 
tar as the ore deposits are concerned, are of igneous origin, and of 
these there is a great variety occurring in various forms. They may 
be most conveniently treated in this connection as lavas or surface 
flows, granitic rocks, and dike rocks. 
Lavas. — The most important body of igneous rocks of this type is 
an extensive series of lavas which penetrate the older formations and 
lie to a large extent between the Triassic and Carboniferous strata. 
The volcanoes from which they flowed burst forth during the closing 
stages of the Carboniferous, for the tuffs resulting from the earliest 
eruptions contain carboniferous fossils. The thick mass of volcanic! 
made up of tuffs and sheets of lava extends into the Triassic, for fos- 
sils of that group are found in the later tuffs. Many of the inter- 
bedded tuffs contain minute fossils of marine organisms, suggesting 
that the eruptions were largely submarine. 
M uch of t lie lava contains porphyritic quartz, and in general may be 
designated metarhyolite, but a large part, being without- free quartz 
and less siliceous, has the appearance of metaandesite. A peculiarity 
of many of these rocks is that they are rich in soda. 
A great belt of these ancient volcanic rocks lies east of the Car- 
boniferous limestone, between Squaw Creek and the McCloud, and 
forms a succession of prominent peaks from Bollibokka Mountain, 
through Salt Creek Mountain, Minnesota Mountain, Town Mountain, 
and Horse Mountain to Pit River and beyond a lower ridge to the 
Sacramento Valley. A second belt west of this lies about the Sacra- 
mento River, embracing the Iron Mountain district and extending as 
far north as Backbone Creek. These two areas of ancient lavas, with 
associated dikes, include all the productive copper mines and the 
most active prospects of the region. 
Ancient lavas occur also about Bagley Mountain, west of the Great 
Bend of Pit River, but they are usually of more basic types than 
those mentioned above. 
Granitic rocks. — Two areas of granitic rocks occur, one about 
Shasta, between Keswick and Iron Mountain, and the other about, 
Bayha and Pit River ferry, but neither of these masses is yet known 
to contain important bodies of copper ores, although \fo.&y contain 
some auriferous quartz veins. Dikes from these granitic masses cut 
