Pennsylvania Geology Final Report Volume 1 1981

16 GEOLOGICAL SURVEY OF PENNSYLVANIA.
opportunity for examining the facts which must be used
in making the calculation. Witli this end in
two such calculations will now be given.
view one or
The western coast of South America has been lifted from
the ocean to a great height in the air by successive earthquakes,
one of which suddenly lifted it three feet since the
settlement of Chili by the whites. Marine shells can be
broken out of the rocks at a height of 16,000 feet above the
sea. The average rate of this upheaval
is of course
unknown; but should we base a calculation upon the
observed rise of the land of northern Scandinavia, namely,
live feet in a century, the rocks containing these fossil
shells would be 320,000 years old. From the character of
the shells we know that the rocks which hold them .were
deposited in what is called the Jurassic age. But if all
known geological time were represented by the twelve hour
divisions on the dial of a clock, the Jurassic age would be
at about nine or ten o'clock, and therefore the highest
antiquity we could give to the mountains of South America
would represent but a portion of geological time.
While parts of the crust of the earth are slowly elevated
other regions are slowly sinking into the sea. In middle
Pennsylvania we have a series of great formations lying one
upon another, all of them originally deposited in succession
in a great water basin which in early times occupied the
area of the United States. Some of these formations were
spread upon the bottom in deep water; some of them in
water so shallow that they exhibit mud cracks, ripple
marks and foot-i)rints such as travelers notice ever3nvhere
on sea beaches. They hold both shore-living shells and coral
reefs. These facts compel us to believe that the bottom of
the Pennsylvanian sea kept on sinking through all the ages
during which these deposits of limestone, sand and clay
were made in it ; and probably at a rate proportionate to the
inflow of the solid materials from the rivers around it. The
rate of sinking is of course unknown, but must have been
as slow as the w^earing away of the surrounding lands.
The total thickness of these deposits, measured from the top
of the coal m^^asures down to the bottom of the great lime-