it. It is 1500 ft. deep and its
waters are of extraordinary purity (containing only three grains of
solid matter to the gallon). Clear Lake, in the Coast Range, is another
beautiful sheet of water. It is estimated by John Muir that on an
average "perhaps more than a mile" of degradation took place in the last
glacial period; but with regard to the whole subject of glacial action
in California as in other fields, there is considerable difference of
opinion. The same authority counted 65 small residual glaciers between
36 deg. 30' and 39 deg.; two-thirds of them lie between 37 deg. and 38
deg., on some of the highest peaks in the district of the San Joaquin,
Merced, Tuolumne and Owen's rivers. They do not descend, on an average,
below 11,000 ft.; the largest of all, on Mt. Shasta, descends to 9500
ft. above the sea.

Volcanic action has likewise left abundant traces, especially in the
northern half of the range, whereas the evidences of glacial action are
most perfect (though not most abundant) in the south. Lava covers most
of the northern half of the range, and there are many craters and
ash-cones, some recent and of perfect form. Of these the most remarkable
is Mt. Shasta. In Owen's Valley is a fine group of extinct or dormant
volcanoes.

Among the other indications of great geological disturbances on the
Pacific Coast may also be mentioned the earthquakes to which California
like the rest of the coast is liable. From 1850 to 1887 almost 800 were
catalogued by Professor E.H. Holden for California, Oregon and
Washington. They occur in all seasons, scores of slight tremors being
recorded every year by the Weather Bureau; but they are of no
importance, and even of these the number affecting any particular
locality is small. From 1769 to 1887 there were 10 "destructive" and 24
other "extremely severe" shocks according to the Rossi Forel
nomenclatural scale of intensity. In 1812 great destruction was wrought
by an earthquake that affected all the southern part of the state; in
1865 the region about San Francisco was violently disturbed; in 1872 the
whole Sierra and the state of Nevada were violently shaken; and in 1906
San Francisco (q.v.) was in large part destroyed by a shock that caused
great damage elsewhere in the state.

North of 40 deg. N. lat. the Coast Range and Sierra systems unite,
forming a country extremely rough. The eastern half of this area is
covered chiefly with volcanic plains, very dry and barren, ly