Part 6
"Land and Fresh-water Mollusca indigenous to the British Isles." By Lovell Reeve.
"A Plain and Easy Account of the Land and Fresh-water Mollusca of Great Britain." By Ralph Tate.
FOSSILS.
BY
B. B. WOODWARD.
FOSSILS.
INTRODUCTORY.
Geology is of all "hobbies" the one best calculated not only to develop the physical powers, but also, if pursued with any degree of earnestness, to train and extend the mental faculties. To study geology properly, the rocks themselves must be visited and carefully observed, their appearance noted, and the fossils, if any, which they contain, collected. This necessitates many a pleasant walk into the open country to quarries and cuttings, or rambles along the sea-shore to cliffs which may be worth investigating, whilst botany, entomology, or any other congenial pursuit, may be followed on the way; for natural science in its different branches has so many points of connection that it is impossible to study one of them without increasing one's interest in, and knowledge of, all the others. Again, in arranging, classifying, and studying at home the specimens collected on these expeditions, many an hour may be usefully spent; habits of exactitude and neatness are acquired; whilst in endeavouring to draw correct conclusions as to the way in which particular rocks were formed, and by what agencies brought to their present position, the reasoning faculties are exercised and developed.
The existence of fossil shells and bones in various strata of the earth's crust attracted attention at a very early date of the world's history; the Egyptian priests were aware of the existence of marine shells in the hills bounding the Nile valley, and from this fact Herodotus drew the conclusion that the sea formerly covered the spot. The bones of the larger mammalia (rhinoceros, elephant, etc.), were, however, thought by the ancients to be human, and hence arose the idea of a race of giants having existed at some previous period of the earth's history. It was not, however, until near the end of the last century that geology began to be recognised as a science, and the true bearing of fossils in relation to the rocks in which they were found was conclusively proved. William Smith in England, and Werner in Germany, while working independently of each other, both came to the same conclusion, viz. that the numerous strata invariably rested on each other in a certain order, and that this order was never inverted,[1] whilst William Smith in addition proved that each group of rocks, and even each stratum, had its own peculiar set of fossils, by which it might be recognised wherever it occurred. From that time forth the study of the various fossils began to be considered as a separate science apart from that of the beds containing them; this is now known as Palaeontology, the study of the composition of the rocks themselves being termed Petrology.
[1] Except in such cases where the rocks themselves have been displaced by movements of the earth's crust.
At this moment, however, we are less concerned with the study of rocks and fossils than with the best and simplest way of collecting, preparing, and arranging specimens as a means to this study.
THE CABINET.
With regard to the cabinet for such specimens as you are able to collect, the same advice holds good as that given in a previous Manual (The Young Collector's Shell Book), namely, the simpler the cabinet the better, though of course card-board boxes would not as a rule be strong enough to stand the weight of the specimens, and hence it is advisable to have wooden ones. The boxes in which Oakey's Wellington Knife-powder is sent out (they measure about 15 in. x 10 in. x 3 in.) are on the whole the most convenient size, and are easily obtainable at any oil and colourman's. These, when painted over with Berlin Black, after first removing the external labels, look very neat. The inside may be papered according to taste, when the trays may be arranged in order ready for the reception of your specimens.[2]
[2] For description of trays, see "The Young Collector's Shell-Book."
IMPLEMENTS REQUIRED WHEN COLLECTING.
A certain amount of apparatus is needful in collecting geological specimens. It is necessary to break open the hard rocks in order to get at the fossils within, and for this purpose a strong hammer is required. One end of the hammer-head should be square, tapering, slightly, to a flat striking face; for when thus shaped the edges and corners are less likely to break off; the other side should be produced into a rather long, flat, and slightly curved pick, terminating in a chisel-edge at right-angles to the handle; the total length of the head should not exceed 91/2 in., the striking face being 3 in. from the centre of the eye in which the handle (18 in. long) is inserted; the latter should be made of the toughest ash, American hickory, or "green-heart," and fixed in with an iron wedge ("roughed" to prevent its coming out again), taking care that 1/4 in. of the handle protrudes on the other side. It is the usual practice, but a mistaken one, to cut it off level with the hammer head, which is likely, under these circumstances, to come off after it has been in use for a time, whereas, by leaving a small portion of the wedged-out end projecting, this mischance is avoided, and your weapon will not fail even when used to drag its owner up a stiff ascent. It is better to shape and fix the handle yourself, as by this means you can not only cut it to fit your hand, but may rely upon its being properly fastened in. By filing grooves around it an inch apart, it will serve to take rough measurements with, while a firm grasp may be insured by bees-waxing instead of polishing it. Another and much smaller hammer will also be necessary, chiefly for home use, to trim the specimens before putting them away in the cabinet; the head of this hammer must not be more than 21/2 inches long, the handle springing from the centre; one end has a flat striking face, square in section, the other, instead of being formed like a pick, is wedge-shaped, the sharp edge being at right-angles to the handle. Next to a hammer, a cold chisel is indispensable to the collector, since without its aid many a choice specimen embedded in the middle of a mass of rock too large to break with the hammer would have to be left behind. There is one thing, however, to beware of in using this tool--it has sometimes to be hit with great force, and should you chance to miss it and strike your hand instead, the result may be more serious than even a severe bruise. To prevent this, procure from the shoemaker or saddler a piece of thick leather, about 4 inches in diameter, having a hole cut in the centre through which to pass the shank of the chisel, and, thus protected, you may wield the hammer with impunity.
For digging fossils out of clay, an old, stout knife, such as the worn-down stump of a carver, is handy, and in sandy beds an ordinary garden trowel is very useful, whilst in a chalk-pit a small saw is sometimes of great aid in extricating a desirable specimen. The same may be said of an ordinary carpenter's wood-chisel. For picking up small and delicate specimens, a pair of forceps should be carried, whilst without a pocket lens no true naturalist ever stirs abroad. An ordinary stout canvas satchel, such as is commonly used by schoolboys, is the best thing for carrying home your specimens; this may be made much stronger by the addition of two short strips of leather stitched on the back and running, one from each ring, to which the strap passing over the shoulder is fastened, down to the bottom of the bag; by leaving a small portion unstitched near the bottom of each of these, wide enough for the shoulder-strap to pass through, the satchel may at a moment's notice be slung knapsackwise on the shoulders--a method of carrying it which is, as all who have tried it know, by far the most convenient when it is heavily laden or not in immediate requisition. A stout leather belt may be worn in which to carry all your hammers, supporting it on the side where the heavy hammer hangs by a band passing over the opposite shoulder. Before starting on an excursion, make a practice of seeing that you have everything with you, or when the critical moment comes, and some choice and fragile specimen is ready to be borne off, you may find that you are without the means necessary for taking it home. For ordinary hard specimens, newspaper well crumpled around them is without its equal, but some of the more delicate must be first wrapped in tissue paper or even cotton-wool, whilst the most fragile fossils should be packed in tins with bran or sawdust, the particles of which fill in all the corners and press equally everywhere, a useful faculty which cotton wool does not possess. When neither of these are to be obtained, dry sand will answer quite as well, though it is heavier to carry.
Although not absolutely necessary in the field, it is often useful to have a small bottle of acid in your pocket (nitric acid diluted to 1-12th with distilled water is the best) with which to test for limestones; a drop of acid placed on a rock will, if there be any carbonate of lime in it, immediately begin to fizz. Finally, every young collector should carry a note-book, and carefully record in it what he sees in each pit he visits, while, if it can be procured or borrowed, a geological map of the district you are exploring is a great help, for with its aid and that of a good compass you become practically independent of much extraneous assistance.
HOW TO USE YOUR IMPLEMENTS.
We will suppose by way of illustration that near us flows a river, on the rising ground above which is a pit that we propose to visit for the purpose of putting our apparatus into practical operation. When we have reached the floor of the pit, and stand looking up at the section before us, we are at first rather puzzled as to what the beds, which we see before us, are; for as the pit has not been worked for some time, its sides are partially overgrown with grass, and in places bits and pieces of the upper beds have fallen down and form a heap beneath which the lower ones lie buried. We must therefore make our way to those spots where the beds are left clear, and find out, if possible, what they are. By climbing up one of the heaps of fallen earth (_talus_) we reach the top, where, first of all, under the roots of the grass and shrubs, we find the mould in which these grow, and which is formed of the broken up (_disintegrated_) rocks forming the still higher ground above, and which the rains, frosts and snows, aided afterwards by the earthworms, have converted into mould. This, geologically speaking, is called _surface soil_, and is here about two feet deep. Just below it we find a layer of coarse gravel; the pebbles of which this is composed are of all sorts, sizes, and shapes, and are stained a deep brown by oxide of iron. Most of them are flints, and by diligent search you may find casts and impressions in these of sponges, shells, spines of sea urchins, etc. Flints, whether from gravel or their parent rock the chalk, are easiest broken by a light smart tap of the hammer, though when it is desired to shape them for the cabinet a soft iron hammer should be used, and the piece to be shaped placed on a soft pad on the knee, for when struck with a steel hammer flints splinter in all directions, and often through the very portion you most desire to preserve. In one spot we find a mass of sand included in the gravel; this mass is thickest in the middle, and tapers away towards each end, its total length being about fifty feet. Could we see the whole mass, we should probably find it to be a patch lying on the gravel and thinning out all around its edges; in other words it would be shaped like a lens--"_lenticular_" as geologists term it. When we examine this mass more closely, we find that the layers of sand do not run parallel with the bed, but are inclined in different directions, sometimes lying one way, sometimes another. This _false bedding_ is due to the sand having been thrown down in waters agitated by strong currents that swept over the spot, now in one direction and now in another, scattering at one moment half the sand they had just piled up one way only to redeposit it the next minute in another. In the gravel also may be observed a similar though less marked arrangement, owing to the larger size of its constituents, which of course required a still stronger current action to wash them down.
Amongst the sand we now see some shells, and set to work to dig them out very carefully, for they are exceedingly brittle. The best specimens are to be obtained by throwing down masses of the sandy material and searching in it; but only the stronger and finer examples will bear such usage. We next notice that these shells are precisely similar to those still found with living occupants in the river below, only they are no longer of a brownish colour, but owing to the loss of the animal matter of the shell have an earthy, dirty-white appearance. To carry these home they should be packed in bran in one of your tins with a note as follows made on a piece of paper and placed just inside--"Sand in gravel: topmost bed ---- pit, August 2nd, 188-." Then if you are not able to work them out at once on reaching home, you will not forget whence they came. From the appearance of these sands and gravels, and the presence in them of shells exactly like those in the river below, it may reasonably be inferred that they once formed a portion of the bed of that river long ago, before it had scooped out its valley to the present depth. There is, however, something else in this sand-bed--a piece of bone protruding; clear away the sand above it, and dig back until the whole is visible. It is broken through in one or two places, but otherwise is in fair condition; remove the pieces carefully one by one, and wrap them in separate pieces of paper, and then proceed to search for others. These bones, which are plentiful in some of our river valley gravel-beds, are the remains of animals that once roamed in the forests which at that time covered the country; they were probably either drowned in crossing the water, or got stuck in the mud on the banks on coming down to drink. A fine collection was made at Ilford by the late Sir Antonio Brady, and is now in the British Museum (Natural History) at South Kensington. Besides the bones of animals, you may expect to find examples of all, or nearly all, the different rocks in which the river has cut its valley, and samples of these may be picked out and taken home. Each specimen should be wrapped in a separate piece of paper to prevent its rubbing against others, care being taken to note the locality either by writing it on the paper or by affixing to the specimen a number corresponding to one in your note book against the description you have written of the bed. The gravel, with its accompanying bed of sand, may be traced down, by scraping away the surface, for about ten feet, when you will discover that it rests unevenly upon the beds below, which, instead of being horizontal, slope (_dip_) in a N.N.E. direction, making an angle of about 45 deg. with the floor of the pit; the gravel therefore rests successively upon the upturned ends of the lower beds, and, geologically speaking, is "unconformable" to them. Now as these underlying rocks were of course originally deposited in an horizontal position, they must have been pushed up and the upper parts worn away (_denuded_) before the gravel was deposited on them, for the accomplishment of which process an amount of time must have elapsed that it would be impossible to reckon by years.
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 "_laminae_," 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 } 10 ft. 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-- } 8. Pure Sand, with layers of concretions containing casts } of shells } 15 ft. 9. Dark-Grey Limestone, with numerous fossils 80 ft. seen.
Beds 4 to 9 dip at an angle of 45 deg. 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.