There is another species called the oak puceron, which bury themselves in the crevices of the bark when it is a little separated from the wood, and live at their ease on the sap. They are black, and nearly as large as a common house-fly. Their trunk is twice the length of their bodies, and it holds so fast by the wood, that, when pulled away, it frequently brings a small piece along with it. Ants are so fond of this species of puceron, that they are the surest guides where to find it; for whenever we see a number of ants upon an oak, and all creeping into one cleft of the bark, we may be certain, my aunt says, of finding quantities of oak pucerons there.
Mary, two or three days ago, raised the turf in different places in a dry pasture field, and shewed me clusters of ants gathered about some large grey pucerons. My aunt says that these earth pucerons draw the juices from the roots of plants, as the other species do from the stem and branches. It is imagined by some people, that they are only the common pucerons, which in winter creep into the earth to shelter themselves: but this is not the case, as they are usually met with in places distant from the trees or plants on which they might before have fed. And she says, that though many may be killed by the cold, yet numbers escape, and are found early in spring, sucking the buds of the peach and other trees.
14th.—I have not yet found the least difficulty in comprehending what my uncle tells us in our geological conversations. This is partly owing to the clearness with which he teaches; and partly to my immediately writing down the substance of it for you. The habit of writing this journal has been indeed of very great use to me, and I have to thank you, dear mamma, for desiring me to do it. I am afraid Marianne will not be much interested as yet by the present subject, for want of my uncle’s explanations; but when I am once more with you and her, I will try to give her at full length the details of what he has told us; and I am sure that she will then like it for his sake.
We have just had another little chapter on the changes in the globe. My uncle said, that extraordinary as the changes on its surface appear, yet when we have an opportunity of penetrating a little into the interior by means of deep mines, or of viewing a long section of the strata in cliffs or on bare mountains, then our ideas expand into a clearer conception of the extent and grandeur of its ancient revolutions. In examining the more elevated chains of mountains, or in following the beds of their torrents, we can perceive somewhat of its interior structure thus laid open to us.
The low and level parts of the earth, when penetrated to a great depth, generally exhibit parallel strata, composed of various substances, and most of them containing vegetable and animal, and innumerable marine productions. Similar strata, with the same kind of productions, compose the hills even to a great height; and sometimes the shells are so numerous, that an entire stratum seems to be formed of them. These shells are frequently in such perfect preservation, that they retain their sharpest ridges, and their tenderest forms. They are sometimes found incrusted in hard stone, and sometimes inclosed in loose sand or clay; and the nicest comparison cannot detect any difference between the texture of these shells, and those which now inhabit the sea. It is, therefore, fair to conclude, that they also must have formerly lived in the sea, and, consequently, that the sea must once have flowed over those places.
But we must not forget that in some countries none of these remains occur, for instance, in Cornwall, and the highlands of Scotland; while in others, not a well can be sunk, or a pit opened, without presenting them in abundance; as in the south-eastern counties of England. The reason of this difference will, I am sure, have suggested itself, if you recollect our former conversations: Cornwall is composed of the lowest series of rocks, which are therefore called primitive; and they, you know, must be entirely destitute of organic remains. The next series contains them very sparingly, but they abound in the three succeeding series, or what are called the secondary formations; though sometimes there are beds interposed, in which they are still rare. In examining these organic remains, the skill of the botanist and zoologist has discovered that several of the plants and animals are entirely different from any with which we are at present acquainted; and a vast field of inquiry has thus been opened in those departments of nature.
I asked my uncle whether these remains are regularly distributed through the whole of those series in which they are so numerous. He likes that I should ask him questions; he says it doubles his pleasure in giving information, when he sees people really alive to what he tells them.
He replied, that, in one respect, the regularity is surprising, for they are found, as it were, in families; each formation containing a collection of species often peculiar to itself, and differing widely from those of the adjoining one; so that at any two points, in similar formations, however distant, we are sure of meeting the same general assemblage of fossil remains. For instance, if the fossils found in the chalk of Flamborough Head in Yorkshire, or in the cliffs of Dover, or even in Poland, or Paris, be examined, eight or nine species out of ten will be found to be the same. Again, if collections of fossils from the carboniferous limestone, of any of the above places, are compared, they will be found to agree in the same manner with each other: but if you compare the collection from the chalk, with that from the limestone, you would not find one single instance of agreement; indeed very few appearances of it that could deceive even your unpractised eye.”
“I wish, uncle, I could make these curious comparisons with my own eyes.”
“So you shall, my dear Bertha. I have a few specimens of remarkable fossils, though I have no regular collection; and when we reach home, I will endeavour to shew you some instances of these facts, as they interest you and Caroline so much.”