great majority of the pebbles have nothing to do with the rock on
which the clay happens to lie, but have come, some of them, from
places many miles away.

Now if we find spread over a low land pebbles composed of rocks which
are only found in certain high lands, is it not an act of common
sense to say--These pebbles have come from the highlands? And if the
pebbles are rounded, while the rocks like them in the highlands
always break off in angular shapes, is it not, again, an act of mere
common sense to say--These pebbles were once angular, and have been
rubbed round, either in getting hither or before they started hither?

Does all this seem to you mere truism, my dear reader? If so, I am
sincerely glad to hear it. It was not so very long ago that such
arguments would have been considered not only no truisms, but not
even common sense.

But to return, let us take, as an example, a sample of these boulder
clay pebbles from the neighbourhood of Liverpool and Birkenhead, made
by Mr. De Rance, the government geological surveyor:

Granite, greenstone, felspar porphyry, felstone, quartz rock (all
igneous rocks, that is, either formed by, or altered by volcanic
heat, and almost all found in the Lake mountains), 37 per cent.

Silurian grits (the common stones of the Lake mountains deposited by
water), 43 per cent.

Ironstone, 1 per cent.