Part 5

I hope, meanwhile, that you will agree with me, that there is plenty of geological matter to be seen and thought over in the neighbourhood of any town.

Be sure, that wherever there is a river, even a drain; and a stone quarry, or even a roadside bank; much more where there is a sea, or a tidal aestuary, there is geology enough to be learnt, to explain the greater part of the making of all the continents on the globe.

II. THE PEBBLES IN THE STREET

If you, dear reader, dwell in any northern town, you will almost certainly see paving courts and alleys, and sometimes--to the discomfort of your feet--whole streets, or set up as bournestones at corners, or laid in heaps to be broken up for road-metal, certain round pebbles, usually dark brown or speckled gray, and exceedingly tough and hard. Some of them will be very large--boulders of several feet in diameter. If you move from town to town, from the north of Scotland as far down as Essex on the east, or as far down as Shrewsbury and Wolverhampton (at least) on the west, you will still find these pebbles, but fewer and smaller as you go south. It matters not what the rocks and soils of the country round may be. However much they may differ, these pebbles will be, on the whole, the same everywhere.

But if your town be south of the valley of the Thames, you will find, as far as I am aware, no such pebbles there. The gravels round you will be made up entirely of rolled chalk flints, and bits of beds immediately above or below the chalk. The blocks of "Sarsden" sandstone--those of which Stonehenge is built--and the "plum-pudding stones" which are sometimes found with them, have no kindred with the northern pebbles. They belong to beds above the chalk.

Now if, seeing such pebbles about your town, you inquire, like a sensible person who wishes to understand something of the spot on which he lives, whence they come, you will be shown either a gravel- pit or a clay-pit. In the gravel the pebbles and boulders lie mixed with sand, as they do in the railway cutting just south of Shrewsbury; or in huge mounds of fine sweet earth, as they do in the gorge of the Tay about Dunkeld, and all the way up Strathmore, where they form long grassy mounds--tomauns as they call them in some parts of Scotland--askers as they call them in Ireland. These mounds, with their sweet fresh turf rising out of heather and bog, were tenanted-- so Scottish children used to believe--by fairies. He that was lucky might hear inside them fairy music, and, the jingling of the fairy horses' trappings. But woe to him if he fell asleep upon the mound, for he would be spirited away into fairyland for seven years, which would seem to him but one day. A strange fancy; yet not so strange as the actual truth as to what these mounds are, and how they came into their places.

Or again, you might find that your town's pebbles and boulders came out of a pit of clay, in which they were stuck, without any order or bedding, like plums and raisins in a pudding. This clay goes usually by the name of boulder-clay. You would see such near any town in Cheshire and Lancashire; or along Leith shore, near Edinburgh; or, to give one more instance out of hundreds, along the coast at Scarborough. If you walk along the shore southward of that town, you will see, in the gullies of the cliff, great beds of sticky clay, stuffed full of bits of every rock between the Lake mountains and Scarborough, from rounded pebbles of most ancient rock down to great angular fragments of ironstone and coal. There, as elsewhere, the 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.

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.

What, then, brought the stones?

We cannot, I think, answer that question, as some have tried to answer it, by saying that they were brought by Noah's flood. For it is clear, that very violent currents of water would be needed to carry boulders, some of them weighing many tons, for many miles. Now Scripture says nothing of any such violent currents; and we have no right to put currents, or any other imagined facts, into Scripture out of our own heads, and then argue from them as if not we, but the text of Scripture had asserted their existence.

But still, they may have been rolled hither by water. That theory certainly would explain their being rounded; though not their being scratched. But it will not explain their being found in the clay.

Recollect what I said in my first paper: that water drops its pebbles and coarser particles first, while it carries the fine clayey mud onward in solution, and only drops it when the water becomes still. Now currents of such tremendous violence as to carry these boulder stones onward, would have carried the mud for many miles farther still; and we should find the boulders, not in clay, but lying loose together, probably on a hard rock bottom, scoured clean by the current. That is what we find in the beds of streams; that is just what we do not find in this case.

But the boulders may have been brought by a current, and then the water may have become still, and the clay settled quietly round them. What? Under them as well as over them? On that theory also we should find them only at the bottom of the clay. As it is, we find them scattered anywhere and everywhere through it, from top to bottom. So that theory will not do. Indeed, no theory will do which supposes them to have been brought by water alone.

Try yourself, dear reader, and make experiments, with running water, pebbles, and mud. If you try for seven years, I believe, you will never contrive to make your pebbles lie about in your mud, as they lie about in every pit in the boulder clay.

Well then, there we are at fault, it seems. We have no explanation drawn from known facts which will do--unless we are to suppose, which I don't think you will do, that stones, clay, and all were blown hither along the surface of the ground, by primeval hurricanes, ten times worse than those of the West Indies, which certainly will roll a cannon a few yards, but cannot, surely, roll a boulder stone a hundred miles.

Now, suppose that there was a force, an agent, known--luckily for you, not to you--but known too well to sailors and travellers; a force which is at work over the vast sheets of land at both the north and south poles; at work, too, on every high mountain range in the world, and therefore a very common natural force; and suppose that this force would explain all the facts, namely--

How the stones got here;

How they were scratched and rounded;