Bragdon Poiiiiation in N. W. California. — Hcrshcy. 255 
dikes bincling- tngetlier tlic stratified members or perhaps in part 
very thick lava sheets. Certain partially altered, fine-s;rained 
diabasic rocks undoubtedly occur in the form of dikes. 
Where the upper portion of the series is preserved as in the 
vicinity of the Sacramento river and in the Pit River section, 
altered rhyolytes are common. They appear tc be partly in the 
form of sheets, partly fragmental and partly as dikes rising 
through the altered andcsyte. Much of the rhyolyte contained 
quartz phcnocrysts which have escaped destruction. This al- 
tered rhyolyte outcrops in belts which are broad where they oc- 
cur in the east, but narrow in the west where erosion has cut 
deeper into the series. I have never given sufficient attention 
to the Shasta County country to learn just what is the structure 
which causes belts of altered rhyolyte to traverse the altered 
andesyte areas, but I am inclined to believe that it is due partly 
to the infolding of the rhyolyte sheets with the andesyte and 
partly to rhyolyte dikes. Mr. Diller has just completed a thor- 
ough study of that territory and he will give us more precise 
information about it ; for the present purpose it is sufficient to 
know that the altered rhyolyte characterizes the upper portion 
of the series and is areally distributed in narrow belts. 
The principal Clear Creek area lies southeast of the eastern 
Bragdon area, is elongated from northeast to southwest, has a 
length of about twenty-five miles and a width of six to twelve 
miles. The Sacramento river traverses it for over .fifteen miles 
and Clear creek for more than twelve miles. Within the ter- 
ritory indicated, there are several later igneous masses as well 
as several isolated areas of Devonian strata. The latter are in- 
volved by the volcanic material in a way to indicate that they 
are older and apparently this Clear Creek, area is based on De- 
vonian strata largely. East of the McCloud river and south of 
Pit river, as Mr. Diller called to my attention, this same vol- 
canic series rests on Upper Carboniferous strata. Numerous 
greenstone dikes occur in the Paleozoic strata and are evidently 
related to the volcanic series. East of the Carboniferous belt a 
thick sheet of the volcanic series dipping eastwardly as a whole, 
goes down under the Pit shales. From my own observation I 
state that the upper portion of the volcanic material, particu- 
larly the altered rhyolyte, alternates with the lower strata of 
typical Pit shales ; in other words, the volcanic series grades in- 
