Town Geology

But some one may say, and will have a right to say, “Stop—the lower thing may have been thrust under the upper one.”  Quite true: and therefore I said only that the lower one was most probably put there first.  And I said “most probably,” because it is most probable that in nature we should find things done by the method which costs least force, just as you do them.  I will warrant that when you want to hide a thing, you lay something down on it ten times for once that you thrust it under something else.  You may say, “What?  When I want to hide a paper, say, under the sofa-cover, do I not thrust it under?”

No, you lift up the cover, and slip the paper in, and let the cover fall on it again.  And so, even in that case, the paper has got into its place first.

Now why is this?  Simply because in laying one thing on another you only move weight.  In thrusting one thing under another, you have not only to move weight, but to overcome friction.  That is why you do it, though you are hardly aware of it: simply because so you employ less force, and take less trouble.

And so do clays and sands and stones.  They are laid down on each other, and not thrust under each other, because thus less force is expended in getting them into place.

There are exceptions.  There are cases in which nature does try to thrust one rock under another.  But to do that she requires a force so enormous, compared with what is employed in laying one rock on another, that (so to speak) she continually fails; and instead of producing a volcanic eruption, produces only an earthquake.  Of that I may speak hereafter, and may tell you, in good time, how to distinguish rocks which have been thrust in from beneath, from rocks which have been laid down from above, as every rock between London and Birmingham or Exeter has been laid down.  That I only assert now.  But I do not wish you to take it on trust from me.  I wish to prove it to you as I go on, or to do what is far better for you: to put you in the way of proving it for yourself, by using your common sense.

At the risk of seeming prolix, I must say a few more words on this matter.  I have special reasons for it.  Until I can get you to “let your thoughts play freely” round this question of the superposition of soils and rocks, there will be no use in my going on with these papers.

Suppose then (to argue from the known to the unknown) that you were watching men cleaning out a pond.  Atop, perhaps, they would come to a layer of soft mud, and under that to a layer of sand.  Would not common sense tell you that the sand was there first, and that the water had laid down the mud on the top of it?  Then, perhaps, they might come to a layer of dead leaves.  Would not common sense tell you that the leaves were there before the sand above them?  Then, perhaps, to a layer of mud again.  Would not common sense tell you that the mud was there before the leaves?  And so on down to the bottom of the pond, where, lastly, I think common sense would tell you that the bottom of the pond was there already, before all the layers which were laid down on it.  Is not that simple common sense?

Then apply that reasoning to the soils and rocks in any spot on earth.  If you made a deep boring, and found, as you would in many parts of this kingdom, that the boring, after passing through the soil of the field, entered clays or loose sands, you would say the clays were there before the soil.  If it then went down into sandstone, you would say—would you not?—that sandstone must have been here before the clay; and however thick—even thousands of feet—it might be, that would make no difference to your judgment.  If next the boring came into quite different rocks; into a different sort of sandstone and shales, and among them beds of coal, would you not say—These coal-beds must have been here before the sandstones?  And if you found in those coal-beds dead leaves and stems of plants, would you not say—Those plants must have been laid down here before the layers above them, just as the dead leaves in the pond were?

If you then came to a layer of limestone, would you not say the same?  And if you found that limestone full of shells and corals, dead, but many of them quite perfect, some of the corals plainly in the very place in which they grew, would you not say—These creatures must have lived down here before the coal was laid on top of them?  And if, lastly, below the limestone you came to a bottom rock quite different again, would you not say—The bottom rock must have been here before the rocks on the top of it?

And if that bottom rock rose up a few miles off, two thousand feet, or any other height, into hills, what would you say then?  Would you say: “Oh, but the rock is not bottom rock; is not under the limestone here, but higher than it.  So perhaps in this part it has made a shift, and the highlands are younger than the lowlands; for see, they rise so much higher?”  Would not that be as wise as to say that the bottom of the pond was not there before the pond mud, because the banks round the pond rose higher than the mud?

Now for the soil of the field.

If we can understand a little about it, what it is made of, and how it got there, we shall perhaps be on the right road toward understanding what all England—and, indeed, the crust of this whole planet—is made of; and how its rocks and soils got there.

But we shall best understand how the soil in the field was made, by reasoning, as I have said, from the known to the unknown.  What do I mean?  This: On the uplands are fields in which the soil is already made.  You do not know how?  Then look for a field in which the soil is still being made.  There are plenty in every lowland.  Learn how it is being made there; apply the knowledge which you learn from them to the upland fields which are already made.

If there is, as there usually is, a river-meadow, or still better, an æstuary, near your town, you have every advantage for seeing soil made.  Thousands of square feet of fresh-made soil spread between your town and the sea; thousands more are in process of being made.

You will see now why I have begun with the soil in the field; because it is the uppermost, and therefore latest, of all the layers; and also for this reason, that, if Sir Charles Lyell’s theory be true—as it is—then the soils and rocks below the soil of the field may have been made in the very same way in which the soil of the field is made.  If so, it is well worth our while to examine it.

You all know from whence the soil comes which has filled up, in the course of ages, the great æstuaries below London, Stirling, Chester, or Cambridge.

It is river mud and sand.  The river, helped by tributary brooks right and left, has brought down from the inland that enormous mass.  You know that.  You know that every flood and freshet brings a fresh load, either of fine mud or of fine sand, or possibly some of it peaty matter out of distant hills.  Here is one indisputable fact from which to start.  Let us look for another.

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1

See “Nature,” No.  XXV.  (Macmillan & Co.)

2

These Lectures were delivered to the members of the Natural Science Class at Chester in 1871.