GEOLOGICAL STRUCTURE. 629 
puddingstone—was crossed upon the south fork of the Umpqua. The 
Umpqua Mountains consist of the talcose rock, some portions of which 
are slaty. 
Twenty-five miles to the southward of the Umpqua Range, we 
passed a region of hornblende rocks and syenite. ‘Twelve miles far- 
ther south, as we approached the Shasty, the rock was granite, and 
the same was found on the south side. We here deviated from our 
southerly course, and for fifteen miles followed the banks of the river 
eastward, passing soon into a region of hornblende rocks, some of 
which were imperfect syenite. After making about twelve miles of 
easting, we again turned south, and twenty miles beyond reached a 
wide prairie. A few outcropping basaltic rocks were crossed as we 
entered it, and hills, apparently basaltic, were seen to the eastward ; 
but in our course, after travelling ten or twelve miles, we reached 
a region of granitic hills. A short distance farther south, we crossed 
what we designated the Boundary Range; it consisted of basaltic 
sandstone or conglomerate, and what appeared to be the Astoria 
tertiary sandstone containing fossils. Southward, the basalt and 
sandstone run side by side nearly to the Shasty Mountains. Front- 
ing the Shasty Peak, the plain is covered with hillocks of a por- 
phyritic lava, while the hills to the westward consist of sandstone, 
and then change to serpentine and syenite. 
Entering the Shasty Mountains, we travelled for twenty miles over 
trachytic rocks; then passed to granite, which graduated into syenite, 
hornblende rock, hypersthene rock, protogine, talcose rock, and talcose 
slate. The talcose rocks constitute the greater part of the ridges along 
the upper part of the Sacramento Valley. During one day, in the 
Shasty Mountains, we passed over a formation of shale, sandstone, 
and puddingstone of early origin. 
On the plains of the Sacramento, we met with nothing but alluvial 
deposits, till reaching the Sacramento Bute, eighty miles north of San 
Francisco, (or about one hundred and fifty miles following the course 
of the river.) ‘The Bute is an ancient crater, and consists of trachyte 
and trachytic porphyry. 
Near the head of the bay of San Francisco, the rocks are composed 
of a soft sandstone like that of Astoria. Half way down the bay, 
about Sansalito, and below to the sea, the hills consist of red and 
green chert and shale, which are subordinate members of the talcose 
rock formation. Some beds of soapstone and talcose slate, with acti- 
nolite, and an impure serpentine, exist on the borders of the bay. 
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