100 J. F, CAMPBELL ON GLACIAL PERIODS. 
density according to temperature, and they are kept in motion out- 
side of the world’s solid surface chiefly by the sun’s rays. That 
power works the engine which carves the world’s surface. Most 
solids float in their fluids; ice floats in water ; cooled lava floats in 
melted lava. The world is hot enough within to vaporize water, 
and to fuse the solids of which the outer crust is made. Steam 
escapes from hot springs, and lava wells up through rents in all 
known latitudes. I believe now that the world was fluid, like 
a grain of shot; that it cooled first outside as shot cools, and that 
it is still cooling in cold space and shrinking as it cools, so that the 
skin of it wrinkles. I hold that felded strata result from the lateral 
crushing of a heavy outer crust which surrounds a shrinking hot 
interior, and that earthquakes and changes of level and volcanic out- 
bursts also result from the cooling of the world in cold space and 
from consequent shrinkage. That is induction and a question for 
argument. It is a question of fact to be solved by observation of 
facts, whether there is or is not upon the surface a record of any 
‘ period,” or “ periods,” during which the world’s outer solid or fluid 
surface was hotter or colder than it is now, and whether the quan- 
tities of vapour, water, and ice were respectively greater or less. 
That is a question of geological periods which can be settled by 
gathering facts in superficial and in deeper geology ; and this paper 
is intended to treat ‘‘ Glacial Periods” as doubtful matters of fact 
in the science. 15. Fresh and worn surfaces are as characteristic as 
any other form. Botanists, conchologists, anatomists, mineralogists, 
and geologists try to distinguish plants, shells, bones, crystals, and 
beds of rock by their shape at sight. All my life I have been striving 
to learn to recognize fresh, fused, or fractured surfaces, and surfaces 
worn by water, by ice, or otherwise, so as to distinguish them at 
sight. At first mountains seemed only bigger hills, and so they 
appear to many observers; but in fact Etna differs from the. Alps 
as much as a growing palm-tree differs from a lot of bent, cross- 
grained, gnarled, worn, warped, tarred oak-planks in the bottom of 
an old, broken, klinker-built boat, keel upwards on a sand bank in a 
snowstorm. ‘lo my eye now Etna isa volcano at sight. Stones dif- 
ferently worn are as characteristic as fossil shells are to a paleeonto- 
logist. After learning my lesson for forty years, I think that I am 
now able to distinguish a volcanic from a glaciated country at sight, 
and to be pretty nearly sure of a surface which retains marks of frac- 
ture or fusion, or of erosion by water, or by ice, when I can examine 
it closely. 
16. Rivers flow from their condensing sources. Ice-action begins 
at colder spots. Shasta bute, in California, is a recent volcanic cone 
like Etna, with a small glacier near the top. Ice-action began 
there when the hot cone had grown high enough and cooled enough 
to condense enough of snow. tna has not yet reached that cooling 
stage, though it condenses snow in abundance. Sneefell, in Iceland, 
is coated with ice. Round Shasta and Sneefell the distance to which 
ice has reached is shown by worn surfaces, and the fact can be as- 
certained by walking round the volcanic cones. The Alps became a 
