The lowest strata in the Isle of Wight are the coloured marls and blue-grey shales we have already observed in Sandown Bay, which run through the Island to Brook Bay. They are known as the Wealden Strata, because the same strata cover the part of Kent and Sussex called the Weald. They consist of marls and shales with bands of sandstone and limestone. The marls and shales in wet weather become very soft, and flow out on to the shore, causing large slips of land.[1] Now, what we want to find out is what the world was like ages ago, when these Wealden Strata were being formed. We have learnt something of how clays and sandstones and limestones are formed: to learn more we must see what sort of fossils we can find in these rocks. "Fossil" means something dug up; and the word is generally used for remains of animals or plants which we find buried in the rocks. We have seen shells in these strata. These we must examine more closely. And as we walk on the shore we shall find other fossils. In the marls and shales exposed on the shore we are pretty sure to see pieces of wood, black as coal, sometimes quite large logs, often partly covered with shining iron pyrites. Perhaps you say—I hope you do—there must have been land not far away when these marls and shales were forming. Always try to see what the things we find have to tell us. The sort of place where we should be most likely to find wood floating in the sea to-day would be near the mouth of a great river like the Mississippi or the Amazon,—rivers which bring down numerous logs of wood from the forest country through which they flow.
Examine the shales and limestone bands. On the surface of some of the paper-shales are numbers of small round or oval white spots. They are the remains of shells of a very minute crustacean, Cypris and Cypridea, from which the shales are known as Cyprid shales. In other bands of shale are quantities of a bivalve shell called Cyrena. There is a band of limestone made up of Cyrena shells, containing also little roundish spiral shells called Paludina.[2] This limestone resembles that called Sussex or Petworth Marble, which is mainly composed of shells of Paludina, but some layers also contain bivalve shells. It is hard enough to take a good polish, and may be seen, like the similar Purbeck marble, in some of our grand old churches. Another band of limestone running through the shales is made up of small oysters (Ostrea distorta).
We shall see fossil shells best on the weathered surfaces of rocks, i.e., surfaces which have been exposed to the weather. One beginning geological study will probably think we shall find fossils best by looking at fresh broken surfaces of rock. This is not so. If you want to find fossils, look at the rock where it has been exposed to the weather. The action of the weather—rain, carbonic dioxide in the rain water, etc.—is to sculpture the surface of the rock, so that the fossils stand out in relief. A weathered surface is often seen covered with fossils, when a new broken one shows none at all.
Many of the shells in the limestones are very like shells which are found at the present day. We must know where they are found now. Well, these Paludinas are a kind of freshwater snail; and, in fact, all the shells we find in the Wealden strata are freshwater shells, till we come near the top, and find the oysters, which live in salt or brackish water. There were quantities in Brading Harbour in old days, before it was reclaimed from the sea. Now, this is a very important point, that our Wealden shells are freshwater shells. For what does it tell us? Why, we see that the first strata we have come to examine were not laid down in the sea at all. Then where were they formed? They seem to be the Delta of a great river, long since passed away, like the Nile, the Amazon, or the Niger at the present day. When these great rivers near the sea, they spread out in many channels, and deposit the mud they have brought down over a wide area shaped like a V, or like the Greek letter [Δ]). Hence we speak of the Delta of the Nile. Some river deltas are of immense size. That of the Niger, for instance, is 170 miles long, and the line where it meets the sea is 300 miles long. Our old Wealden river must have been a great river like the Niger, for the Wealden strata stretch,—often covered up for a long way by later rocks, then appearing again,—as far as Lulworth on the Dorset coast to the west, into Buckinghamshire on the north, while to the north east they not only cover the Weald, but pass under the Straits of Dover into Belgium, and very similar strata are found in Westphalia and Hanover. The ancient river delta must have been 200 miles or more across.
You must not think this great river flowed in the Island of England as it is to-day. England was being made then. This must have been part of a great continent in those days, for such a great river to flow through, and form a delta of such size. We cannot tell quite what was the course of this river. But to the north of where we are now must have stretched a great continent, with chains of lofty mountains far away, from which the head waters of the river flowed. Near its mouth the river broke up into many streams, separated by marsh land; while inside the sand banks of the sea shore would be large lagoons as in the Nile delta at the present day. In these waters lived the shellfish whose shells we are finding. And flowing through great forests the river carried down with it logs of wood and whole trees, and left them stuck in the mud near its mouths for us to find to-day.
What kind of trees grew in the country the river came from? Well, there were no oaks or beeches, no flowering chestnuts or apples or mays. But there were great forests of coniferous trees; that is trees like our pines and firs, cedars and yews, and araucarias; and there were cycads—a very different kind of tree, but also bearing cones—which you may see in a greenhouse in botanical gardens. They have usually a short trunk, sometimes nearly hemispherical, with leaves like the long leaves of a date palm. They are sometimes called sago trees, for the trunk has a large pith, which, like some palms, gives us sago. Stems of cycads, covered with diamond-shaped scars, where the leaf stalks have dropped off, are found in the Wealden deposits. Most of the wood we find is black and brittle. Some, however, is hard as stone, where the actual substance of the wood has been replaced by silica, preserving beautifully the structure of the wood. Specially noteworthy are fragments of a tree called Endogenites (or Tempskya) erosa, because it was at first supposed to belong to the endogens,—the class to which the palm bamboo belong; it is now considered to be a tree-fern. Many specimens of this wood are remarkably beautiful, when polished, or in their natural condition. Here, by the way, it may be well to explain how we name animals and plants scientifically. We have English names only for the commoner varieties. So we have to invent names for the greater number of living and extinct animals and plants. And the best way is found to be this. We give a name, generally formed from the Latin—or the Greek—to a group of animals or plants, which closely resemble one another; the group we call a genus. Then for the species, the particular kind of animal or plant of the group, we add a second name to the first. Thus, if we are studying the apple and pear group of fruit trees, we call the general name of the group Pyrus. Then the crab apple is Pyrus malus, the wild pear P. communis, and so on. So that when you arrange any of your species, and put down the scientific names, you are really doing a bit of classification as well. You are arranging your specimens with their nearest relations.
To return to our ancient river. With the logs and trunks of trees, which the river brought down, came floating down also the bodies of animals, which had lived in the country the river flowed through. What kind of animals? Very wonderful animals, some of them, not like any living creature that lives to-day. By the time they reached the mouth of the river the bodies had come to pieces, and their bones were scattered about the river mouth. On the shore where we are walking we may find some of these bones. But it is rather a chance whether we find any in any one walk we take. The best time to find them is when rough seas in winter have washed some out of the clay, and left them on the shore. It is only rarely that large bones are found here; but you should be able to find some small ones fairly often. The bones are quite as heavy as stone, for all the pores and cavities have been filled with stone, generally carbonate of lime, in the way we explained in describing the formation of beds of limestone. This makes them quite different from any present-day bones that may happen to lie on the shore. So that you cannot mistake them, if once you have seen them. They are bones of great reptiles,—the class of creatures to which lizards and crocodiles belong. But these were much larger than crocodiles, and quite peculiar in their appearance. The principal one was the Iguanodon. He stood on his hind legs like a kangaroo, with a great thick tail, which may have helped to support him. When full grown he stood about 14 ft. high. You may find on the shore vertebræ, i.e., joints of the backbone, sometimes large, sometimes quite small if they come from the end of the tail. I have found several here about 5 inches long by 4 or 5 across. A few years ago I found the end of a leg bone almost a foot in diameter. Dr. Mantell, a great geological explorer in the days when these reptiles were first discovered about 80 years ago, estimated from the size of part of a bone found in Sandown Bay that one of these reptiles must have had a leg 9 ft. long. It was a long time after the bones of these creatures were first found before it was known what they really looked like. The animals lived a long way from here, and by the time the river had washed them down to its mouth the skeletons were broken up, and the bones scattered. At last a discovery was made, which told us what the animals were like. In a coal mine at Bernissart in Belgium the miners found the coal seam they were following suddenly come to an end, and they got into a mass of clay. After a while it was seen what had happened. They had struck the buried channel of an old river, which in the Wealden days had flowed through and cut its channel in the coal strata, which are much older still than the Wealden. And in the mud of the ancient buried river what should they come upon but whole skeletons of Iguanodons. In the days of long ago the great beasts had come down to the river to drink, and had got "bogged" in the soft clay. The skeletons were carefully got out, and set up in the Museum at Brussels. Without going so far as that, you may see in the Natural History Museum in London, or the Geological Museum at Oxford, a facsimile of one of these skeletons, large as life, and have some idea of the sort of beast the Iguanodon was. I should tell you why he was so named. Before it was known what he was like in general form, it was found that his teeth, which are of a remarkable character, were similar to those of the Iguana, a little lizard of the West Indies. So he was called Iguanodon,—an animal with teeth like the Iguana (fr. Iguana, and Gk. [όδούς] g. [όδόντυς] a tooth). He was quite a harmless beast, though he was so large. He was a vegetarian. There were other great reptiles, more or less like him, which were also vegetable feeders. But there were also carnivorous reptiles, generally smaller than the herbivorous, whose teeth tell us that they preyed on other animals.
PL. I
Perna Mulleti | Meyeria Vectensis (Atherfield Lobster) |
Panopæa Plicata | Terebratula Sella |
Cyrena Limestone | Iguanodon Vertebra |