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.

Carboniferous limestone, 5 per cent.

Permian or Triassic sandstones, i.e. rocks immediately round
Liverpool, 12 per cent.

Now, does not this sample show, as far as human common sense can be
depended on, that the great majority of these stones come from the
Lake mountains, sixty or seventy miles north of Liverpool? I think
your common sense will tell you that these pebbles are not mere
concretions; that is, formed out of the substance of the clay after
it was deposited. The least knowledge of mineralogy would prove
that. But, even if you are no mineralogist, common sense will tell
you, that if they were all concreted out of the same clay, it is most
likely that they would be all of the same kind, and not of a dozen or
more different kinds. Common sense will tell you, also, that if they
were all concreted out of the same clay, it is a most extraordinary
coincidence, indeed one too strange to be believed, if any less
strange explanation can be found--that they should have taken the
composition of different rocks which are found all together in one
group of mountains to the northward. You will surely say--If this be
granite, it has most probably come from a granite mountain; if this
be grit, from a grit-stone mountain, a