es are
joined together at the lower ends, forming loops, some of which are
close and vertical, while others are quite open. These lines are due to
the folding of the strata in consequence of the lateral pressure they
are subjected to, when crowded into the lower course of the valleys, and
the difference in their dip is due to the greater or less force of that
pressure. The wood-cut on the next page represents a transverse cut
across the Lauter-Aar and the Finster-Aar, the two principal tributaries
to the great Aar glacier, and includes also a number of small lateral
glaciers which join them. The beds on the left, which dip least, and are
only folded gently downward, forming very open loops, are those of the
Lauter-Aar, where the lateral pressure is comparatively slight. Those
which are almost vertical belong in part to the several small tributary
glaciers, which have been crowded together and very strongly compressed,
and partly to the Finster-Aar. The close uniform vertical lines in this
wood-cut represent a different feature in the structure of the glacier,
called blue bands, to which I shall refer presently. These loops or
lines dipping into the internal mass of the glacier have been the
subject of much discussion, and various theories have been recently
proposed respecting them. I believe them to be caused, as I have said,
by the snow-layers, originally deposited horizontally, but afterward
folded into a more or less vertical position, in consequence of the
lateral pressure brought to bear upon them. The sheets of dust and of
ice alternating with the snow-strata are of course subjected to the same
action, and are contorted, bent, and folded by the same lateral
pressure.

[Illustration]

Dr. Tyndall has advanced the view that the lines of apparent
stratification, and especially the dirt-bands across the surface of the
glacier, are due to ice-cascades: that is, the glacier, passing over a
sharp angle, is cracked across transversely in consequence of the
tension, and these rents, where the back of the glacier has been
successively broken, when recompacted, cause the transverse lines, the
dirt being collected in the furrow formed between the successive ridges.
Unfortunately for his theory, the lines of stratification constantly
occur in glaciers where no such ice-falls are found. His principal
observations upon this subject were made on the Glacier du Geant, where
the ice-cascade is very remarkable. The lines may perh