AIDS IN PRACTICAL GEOLOGY.

PART I.

THE SAMPLING OF THE EARTH'S CRUST.

"A fossil shell may interest a conchologist, though he be ignorant of the locality from which it came; but it will be of more value when he learns with what other species it was associated, whether they were marine or freshwater, whether the strata containing them were at a certain elevation above the sea, and what relative position they held in regard to other groups of strata."-CHARLES LYELL, Principles of Geology, vol. i., 1830.

CHAPTER I.

ON CERTAIN OBSERVATIONS IN THE FIELD.

THE examination of the features presented by the Earth's crust in any locality, with the object of learning something of its past history, must always be one of the most delightful of occupations; and the material advantages arising from a correct determination of minerals and rocks are obvious to every traveller. Such aids in determinative geology as are given in the following pages may be applied in any halting-place, or in cities after the return from an expedition; but in any case observations made on specimens are of slight importance if uncoupled with knowledge of their true position in the field.

The Museum-Curator, for instance, has duties of an invaluable character. He brings together, collates, and arranges the types and varieties described by authors, adding to them by his own special knowledge, and thus forming a series with which any new specimen can be easily and accurately compared.

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him the Earth becomes a great reality, for he surveys it through the extent of his collections and his studies; but the ordinary student, gathering together a few relics from the curiositycabinets of relatives and friends, finds that they appeal to him but little; they have no associations, they have long been separated from their kindred, they are "fossil" in the worst of senses. But let him, having a knowledge of first principles and of museum-types, go out to see things for himself. Furnished with the maps and books of experienced workers, let him re-examine the evidence on which they have relied. A week's holiday thus spent amid varied surroundings, as on the Welsh border, or in Antrim, or around Edinburgh or Bristol, will provide material for long and careful study. Once in the field, the complexity of the subject will dawn upon him; but at the same time he becomes assured that, wherever he may wander, he will find congenial work. The first visit to a district commonly raises numberless questions, when the specimens gathered are examined at his leisure; and the suggestions of the laboratory or the microscope must be tested in a second or third excursion by re-examination of the relations of the rock-masses in the field.

In the field itself broad names are assigned to objects, detailed determination being left for comparative and instrumental work; but in these after-hours of study every scene comes back vividly before us, and even the lichens that may yet cling in hollows and betray the collection of an imperfect and weathered specimen, serve their turn with the naturalist and remind him of the wide, open-air, and eminently natural character of his work.

The art of observing in the field, and of balancing the evidence of various exposures, must be to a great extent learnt by oral tradition and personal guidance; and the study of any geological map, with its outliers, its sinuous outcrops, its inliers, its repetitions by faults or foldings, should be carried on, wherever possible, in the actual district that has been mapped. The practical construction of maps, and of sections from them, is discussed in Sir A. Geikie's Outlines of Field Geology (Macmillan & Co.), and Penning's Field Geology (Baillière, Tindall & Cox).

The examination of a country like Britain, or any part of western Europe, from the point of view of a student anxious to grasp the salient features, the connexion between underground and surface characters, has been immensely facilitated by the modern development of cycling. Traverse after traverse of a country may be made with some handy geological map carried in a capacious tool-bag, together with a hammer, heavy enough

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to investigate each wayside exposure. The physical geology, dipslopes and escarpments, alluvial plains or mountain-gorges, become very real to travellers on the road; and the repetition of the same features in the same order in successive traverses comes upon one with admirable distinctness, and gives a key to the structure of wide areas. When the general grouping of the strata has been grasped, attention can be paid to some limited district. even here a bicycle or tricycle ready to hand is of considerable value. Though days and weeks may be spent on foot up and down rugged exposures, or across broad cultivated lands set with little quarries, as in the Cotteswolds, every now and then comparison becomes desirable with some distant point, and the road is taken without delay. The intervening country may be reviewed in passing; when local work is completed, a different route can be taken in returning; and thus hints are received and sections examined which otherwise might have entirely escaped.

And this matter applies also to geological observations in districts inviting by their wildness. If the student of the Grampians, the Juras, or the Alps, can find time to approach them mile by mile along the highways, following up the rivers that flow from them, tracing afar off the limits of the lowland, the first curvings of the foot-hills, the change from pasture to moorland, from moor to desolate crag, he gains a most vivid appreciation of his surroundings when he arrives at the locality of his work. And just as the district in which he finally settles acquires dignity from its wide associations, so his very specimens and chips, whenever studied, come to have a truly geological, not merely mineralogical or palæontological significance. Even a microscopic slide, amid such memories, seems to assume its place in nature.

The instruments used in the field should be noticed here. First in importance is the hammer, which may vary much in size and weight with the work proposed, and which may easily err in being too heavy, as well as in being too light. Before under

taking any long expedition, the head and shaft should be well tested, and the form of handle that cramps the hand least should be selected. A handle too small in circumference is liable to cause blisters, or at any rate to pain the hand, during long use on refractory materials.

For most kinds of work, a flat end to the head about 1 inch square seems most suitable; the other end should be chiselshaped, and there are many reasons, easily seen in practice, why the chisel-edge should run horizontally, not vertically, when the

hammer-handle is held upright. A strong side-reason when working in hill-districts is that a secure hold can often be obtained in ascending or crossing steep grass-slopes, by driving the chisel-end at each step into the soil.

A sharp pick-like termination may sometimes be useful, in place of the chisel-edge, for splitting open lumps of soft rock when seeking fossils, or for laying hold of and bringing down materials that are beyond the reach of the arm alone. But for general purposes, trimming of specimens, wedging out blocks, and so forth, the chisel-edge, some 1 inch or so broad, is invaluable.

Where much collecting is to be done, weight becomes an object; and the reduction of specimens to a convenient minimum size in the field is always desirable, since any accidental fracture can be remedied by at once securing another specimen. Hence a light trimming hammer proves a great additional convenience, and the risk to specimens during trimming, particularly when they contain fossils, is thus very much reduced.

Though many geologists prefer to dispense with a chisel, there is no doubt of its convenience where blocks of rock have to be worked out from a cliff-face, or in any place where the hammer fails to get an easy hold. A good "cold chisel," some 4 to 5 inches in length, is suitable. If it is too short, it may become driven in down joint-cracks before its work is done and before the block is wedged away from the parent-mass.

Elaborate hammer-belts seem quite unnecessary. The specimen-bag is commonly slung by a strap passing over the right shoulder, so that it can be steadied and partly supported by the left hand when it becomes full and heavy. An additional strap for the hammer cumbers the chest, and even in a belt the head has to be prevented from touching and wearing through the clothes. It is simple enough to slip the hammer into the sidebag itself, the handle projecting from the forward end under the flap. The left hand, by resting on the handle, can then easily, during long walking, keep the bag from rubbing unpleasantly on the hip.

The bag itself should be light and strong, with two strongly attached buttons, rather than straps, to close the flaps, so that no time is lost in opening. On moving from each collecting-place it must invariably be closed, as a slight slip or twist when climbing may deprive the observer of valuable spoils. In rounding rocky slopes it is best to keep the bag slung well upon the back; if on the outside, it tends to destroy the balance in the wrong direction; if on the inside, it thrusts the body away from the

rocks, and is also liable to catch during any rapid movement. In jumping, as across little stream-cuts, it will soon be found that the hand should steady it from below.

A receptacle that proves extremely light and satisfactory is the rubber "game-bag," covered with thin cloth, which is procurable at some water-proof dealers.* The price is about 6s., and the supporting rings and straps are generally strong. Though liable in old age to be finally cut through by sharp rocks, the material, even when perforated, does not fray away.

It is scarcely necessary to mention a walking-stick as part of the geologist's equipment, for it is indispensable on steep or roughish ground. It should, at any rate, be never left behind where long slopes and taluses are in question, since its use will make observations possible that might otherwise involve genuine risk. Among rocks it may sometimes be necessary to throw it over in advance; but to descend dry grass-slopes without a stick is undoubtedly time-consuming and vexatious. Mountaineers will forgive our reminding the geological student, who will often find himself in situations all the more pleasant for being unfamiliar, that a steep hill-side should be traversed with the stick in the inside, not in the outside hand.

A compass is a necessity for the pedestrian. It may be combined with the clinometer, as in the convenient box-instruments often made. Many of these, however, do not allow sufficient length in the edge which is to be held co-incident with the line of dip observed. Any one can construct a clinometer from an ordinary protractor; a swinging index, or even a weighted thread, being hung from the centre of the straight edge so as to reach the graduated arc. Of course the 90° marked on the protractor reads as 0° when a dip is to be taken; thus, if the index points to 84°, the dip is 6o, and so on.

In observing a dip, the plane of the graduated arc of the clinometer must be held parallel to a vertical rock-face on which the beds appear exposed, and the distance between the eye and the rocks should be reasonable, in order that the straight edge may appear coincident with a considerable length of the dipping strata. The instrument is tilted until this edge appears to lie along some well-marked line of stratification; the plummet or index then points to an angle equal to the angle of dip observed. Several observations are desirable as checks to one another; any evidences of lenticular or current-bedding must be noted; and the compassbearing of the face of rock utilised must also be observed.

* Messrs. Walkley & Co., 5 Strand, London, supply these.