When we come to examine these lower beds, we find first a stratum of stiff dark-brown clay containing fossils disposed in layers: those near the outer surface have been rendered so brittle by the weather, that it is necessary to make use of the pick end of the hammer and dig a little way into the face of the section before we come upon some which will bear removal by cutting them out with a knife. Pack them in a tin with bran, or, where much clay still adheres to them, wrap them in paper.
The true top of this bed is not visible, being concealed beneath a heap of earth in the corner of the pit, but we can see and measure about six feet of it.
The next bed in order is a light brownish band of sandy clay that splits along its layers into thin pieces or "laminæ," whence we may describe it as a sandy, laminated clay. On the freshly split surface of one piece we see scattered a number of small darker brown fragments; an examination with a pocket lens clearly shows that these are little bits of leaves and stems, with here and there a more perfect specimen. These beds must have been deposited in the still waters just off the main stream of a large river which brought the plants floating down to this spot, where they became water-logged and sunk; so, too, if you examine the shells in the bed immediately above, you will see that they are very like though not the same as those which at the present day love to dwell in the mud off the estuaries of big rivers in warmer parts of the globe; hence we discover that at some far distant period a big river, but one which had no connection with that running close by, once flowed over this very spot. On tracing the leaf-bed down, we come all at once, at about three feet from its upper surface, upon a narrow band one or two inches thick of a substance composed of numerous bits of sticks and stalks closely matted together and partially mineralized. Vegetable matter in this form is known as lignite, and is one of the first stages towards the formation of coal out of plant remains. Below this lignite band we find our leaf-bed getting sandier and sandier, and losing all trace of the plants by degrees till it becomes almost pure sand. Here and there, however, it contains some curiously shaped masses, which, when broken through with the hammer, seem composed of nothing but the same grains of sand cemented together into a hard mass. In one there is, however, a curiously shaped hollow, which, upon examining it closely, you will see is a perfect cast of a small shell that has itself disappeared. A drop of acid on it fizzes away and sinks in between the grains of sand which in this spot become loose. A mass of sand or particles of clay thus cemented together, be it by iron, lime, or any other substance, is termed a "nodule" or "concretion," and in this particular instance has been formed as follows:—The rain-water falling on the sand where it comes to the surface sinks in and filters through the bed. Now there is always a certain amount of carbonic acid in rain-water, and this acid acted on the carbonate of lime of which the shell was composed, dissolving and dispersing it amongst the neighbouring grains of sand where it was re-deposited, cementing them together as we have seen. The bottom of this bed of sand we find to be just fifteen feet from the lignite band when measured at right-angles to the bed, and it is succeeded by a hard greyish rock, which requires a smart blow of the hammer to break it, but the surface of which, where it has been exposed to the weather, is much crumbled ("weathered"), and breaks readily into small pieces. It is easily scratched with the point of a knife, and therefore is not flint; moreover, it fizzes strongly when touched with acid—hence there is a great deal of carbonate of lime in it, and we know that it is limestone.
Limestones are very largely, sometimes almost entirely, made up of the calcareous portions of marine creatures, such as the hard parts of corals, the tests of sea-urchins, the shells of mollusca, etc., welded, so to speak, into one mass by the heat, pressure, and chemical changes which the bed has undergone since its deposition at the bottom of the sea. There would be every reason, therefore, one might suppose, to expect a number of fossils in this bed; but, alas! disappointment awaits the young explorer, for with the exception of chalk and a few other limestones, these rocks are generally of such uniform texture that on being struck with the hammer they split through fossils and all, the fractured surface only too frequently showing nought save a few obscure markings. But what we fail to accomplish in our impatience, nature effects by slow degrees, and if you will turn over the weathered pieces and blocks lying about, you will soon find plenty of fossils sticking out all over them; by a judicious use of hammer and chisel any of these may be detached and added to your stock, each being separately packed in paper and the locality written on the outside. Some seventy or eighty feet is all that is visible of this limestone; the rest is unexcavated.
Before leaving the pit, it will be as well to select such rock specimens as you wish to place in your cabinet, trimming them to the required size on the spot, for should you, as is not unlikely, spoil two or three, you can readily pick a fresh one. Having secured our specimens, we will take a look at our note-book, to see if we have noted all the details we require. If so, our entries should run something as follows:—First, we have made a rough sketch of the position of the beds, carefully numbering each one; then follow our notes on the individual beds, preceded by numbers corresponding with those in the sketch, thus:—
| 1. | Surface Soil | 2 ft. | ||
| 2. | River Gravel, including a lenticular mass of |
| ||
| 3. | Sand, with land and fresh-water shells and bones of animals | |||
| 4. | Stiff dark-brown clay, with estuarine shells | 6 ft. seen. | ||
| 5. | Light-brown sandy clay, with leaves and stems of plants | 3 ft. | ||
| 6. | Band of Lignite | 2 in. | ||
| 7. | Same as 5, passing into— |
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| 8. | Pure Sand, with layers of concretions containing casts of shells | |||
| 9. | Dark-Grey Limestone, with numerous fossils | 80 ft. seen. | ||
| Beds 4 to 9 dip at an angle of 45° to the N.N.E. |
Our imaginary pit is of course only a sort of geological Juan Fernandez, but it will serve in some degree to illustrate the method of dealing with various rocks and fossils when met with in the field, and how they may best be collected and carried home. A few additional suggestions where to look for fossils may, however, be given here. To begin with, I never neglect to search the fallen masses, especially their weathered surfaces, or to look carefully over the heaps of quarried materials, whatever they may happen to be, piled on the floor of the pit. In working at the beds themselves, remember that fossils frequently occur in layers which of course represent the old sea-bottom of the period; to find these, it is necessary to follow the beds in a direction at right angles to their stratification, till you arrive at the sought-for layers, or zones.
Do not be surprised, when collecting from a formation you have never before studied, if the fossils are not at first apparent, though many are known to be present. The eye requires a few days in which to become accustomed to its fresh surroundings, and when the same spot has been carefully hunted over every day for a week, it is astonishing what a quantity of fossils are discernible where not one in the first instance was recognised.
HOW TO PREPARE THE SPECIMENS FOR THE CABINET.