slates--are found infolded into the Sierra Nevada Mountains. Hence
during Jurassic times the Sierra trough continued to subside, and
enormous deposits of mud were washed into it from the land lying
to the east. Contemporaneous lava flows interbedded with the
strata show that volcanic action accompanied the downwarp, and
that molten rock was driven upward through fissures in the crust
and outspread over the sea floor in sheets of lava.

THE SIERRA DEFORMATION. Ever since the middle of the Silurian, the
Sierra trough had been sinking, though no doubt with halts and
interruptions, until it contained nearly twenty-five thousand feet
of sediment. At the close of the Jurassic it yielded to lateral
pressure and the vast pile of strata was crumpled and upheaved
into towering mountains. The Mesozoic muds were hardened and
squeezed into slates. The rocks were wrenched and broken, and
underground waters began the work of filling their fissures with
gold-bearing quartz, which was yet to wait millions of years
before the arrival of man to mine it. Immense bodies of molten
rock were intruded into the crust as it suffered deformation, and
these appear in the large areas of granite which the later
denudation of the range has brought to light.

The same movements probably uplifted the rocks of the Coast Range
in a chain of islands. The whole western part of the continent was
raised and its seas and lakes were for the most part drained away.

THE BRITISH ISLES. The Triassic strata of the British Isles are
continental, and include breccia beds of cemented talus, deposits
of salt and gypsum, and sandstones whose rounded and polished
grains are those of the wind-blown sands of deserts. In Triassic