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, and so on with the whole list. Why—are we to go out of our way to seek improbable explanations, when there is a probable one staring us in the face?
Next—and this is well worth your notice—if you will examine the pebbles carefully, especially the larger ones, you will find that they are not only more or less rounded, but often scratched; and often, too, in more than one direction, two or even three sets of scratches crossing each other; marked, as a cat marks an elder stem when she sharpens her claws upon it; and that these scratches have not been made by the quarrymen’s tools, but are old marks which exist—as you may easily prove for yourself—while the stone is still lying in its bed of clay. Would it not be an act of mere common sense to say—These scratches have been made by the sharp points of other stones which have rubbed against the pebbles somewhere, and somewhen, with great force?
So far so good. The next question is—How did these stones get into the clay? If we can discover that, we may also discover how they wore rounded and scratched. We must find a theory which will answer our question; and one which, as Professor Huxley would say, “will go on all-fours,” that is, will explain all the facts of the case, and not only a few of them.