Popular Science Monthly/Volume 10/April 1877/Accoutrement of a Field-Geologist
|←World-Creations||Popular Science Monthly Volume 10 April 1877 (1877)
Accoutrement of a Field-Geologist
By James Geikie
|On the Annihilation of the Mind→|
FIELD-GEOLOGY does not mean and need not include the collecting of specimens. Consequently a formidable series of hammers and chisels, a capacious wallet with stores of wrapping-paper and pill-boxes, are not absolutely and always required. Rock-specimens and fossils are best collected after the field-geologist has made some progress with his examination of a district. He can then begin to see what rocks really deserve to be illustrated by specimens, and in what strata the search for fossils may be most advantageously conducted. He may have to do the collecting himself, or he may be able to employ a trained assistant, and direct him to the localities whence specimens are to be taken. But, in the first instance, his own efforts must be directed to the investigation of the geological structure of the region. The specimens required for his purpose in the early stages of his work do not involve much trouble. He can detach them and carry them off as he goes, while he leaves the full collection to be made afterward.
It is of paramount importance that the field-geologist should go to his work as lightly equipped as possible. His accoutrements should be sufficient for their purpose, and eminently portable. You may judge of the portability which may be secured when I tell you that I have on my person at this moment all the instruments necessary for carrying on a geological survey, even in the detailed manner adopted in the Geological Survey of this country. You observe, therefore, that a fully-equipped field-geologist need not betray his occupation by any visible implement. The want of such tokens of his craft often greatly perplexes rustic observers, to whom his movements are a fruitful source of speculation. I shall divest myself of my accoutrements one by one as I have occasion to refer to them, and describe their uses.
The hammer is the chief instrument of the field-geologist. He ought at first to use it constantly, and seldom trust himself to name a rock until he has broken a fragment from it and compared the fresh with the weathered surface. Most rocks yield so much to the action of the weather as to acquire a decomposed, crumbling crust, by which the true color, texture, and composition of the rock itself, may be entirely concealed. Two rocks, of which the outer crusts are similar, may differ greatly from each other in essential characters. Again, two rocks may assume a very different aspect externally, and yet may show an identity of composition on a freshly-fractured internal surface. The hammer, therefore, is required to detach this outer deceptive crust. If heavy enough to do this it is sufficient for your purpose; any additional weight is unnecessary and burdensome. A hammer, of which the head weighs one pound or a few ounces more is quite massive enough for all the ordinary requirements of the field-geologist. When he proceeds to collect specimens he needs a hammer of two or three pounds, or even more, in weight, and a small, light chipping-hammer to trim the specimens and reduce them in bulk without running a too frequent risk of shattering them to pieces.
Hardly any two geologists agree as to the best shape of hammer; much evidently depending upon the individual style in which each observer wields his tool. This (Fig. 1.) is the form which, after long experience, we have found in the Geological Survey to be on the whole the best. A hammer formed after this pattern combines, as you observe, the uses both of a hammer and a chisel. With the broad, heavy, or square end, you can break off a fragment large enough to show the internal grain of a rock. With the thin, wedge-shaped, or chisel-like end, you can split open shales, sandstones, schists, and other fissile rocks. This cutting or splitting edge should be at a right angle to the axis of the shaft. If placed upright or in the same line with the shaft, much of its efficiency is lost, especially in wedging off plates of shale or other fissile rocks.
A hammer shaped as I recommend serves at times for other than purely geological purposes. On steep, grassy slopes, where the footing is precarious, and where there is no available hold for the hand, the wedge-like end of the hammer may be driven firmly into the turf, and the geologist may thereby let himself securely down or pull himself up.
The most generally convenient way of carrying the hammer is to have it in a leather sheath suspended from a waist-belt. The hammer hangs at the left side under the coat, the inside of which is kept from being cut or soiled by the protecting outer flap of the sheath. Some geologists prefer to carry the belt across the shoulders outside, and the hammer suspended at the back. Others provide themselves with strong canvas coat-pockets and carry the hammer there.
Even the most sharp-sighted observer is the better for the aid supplied to him by a good magnifying-glass. For field-work a pocket lens with two powers is usually sufficient. One glass should have a large field for showing the general texture of a rock, its component grains or crystals, and the manner of their arrangement; the other glass should be capable of making visible the fine striæ on a crystal, and the minute ornament on the surface of a fish-scale or other fossil organism. Applied to the weathered crust of a rock, the lens often enables the observer to detect indications of composition and texture which the fresh fracture of the rock does not reveal. It sometimes suffices to decide whether a puzzling fine-grained rock should be referred to the igneous or the aqueous series, and consequently how that rock is to be colored on the map.
Any ordinary pocket-compass will suffice for most of the requirements of the field-geologist. Should he need to take accurate bearings, however, a small portable azimuth compass will be found useful. This is the instrument employed in the Geological Survey. It is carried in a leather case, or pocket, hung from the waist-belt on the side of the body, opposite to the hammer (Fig. 1). The directions of the dip and strike of rocks, the trend of dislocations and dikes, the line of boundaries, escarpments, and other geological features, are observed accurately, and noted on the spot at the time of observation, either on the map or in the note-book. A convenient instrument for light and rapid surveys, or reconnaissances, combines the compass and the next instrument I have to describe—the clinometer. I shall refer to it again.
The clinometer, or dip-measurer, is employed to find the angle at which strata are placed to the horizon—an important observation in the investigation of the geological structure of a country, and one having frequently a special economic value—as, for instance, when it points out the depth to which a well or mine must be sunk. Various patterns have been proposed and used for this instrument. Formerly a spirit-level was commonly employed. But, apart from the difficulty of rapid adjustment for the requirements of the field, the spirit-levels in the clinometers were apt to get broken. A much more portable and serviceable form of clinometer may be made by the geologist himself. It consists of two thin leaves of wood, each two inches broad and six inches long, neatly hinged together, so as to open out and form a foot-rule when required (Fig. 2). On the inside of one of these leaves a small brass pendulum is so fixed that when it swings freely and hangs vertically it forms an angle of 90° with the upper edge of the leaf to which
it is attached. An arc, graduated to 90° on each side of the vertical, is drawn on the wood, or on paper or brass fastened to the wood, so that when the leaf is moved on either side the exact number of degrees of inclination is shown by the pendulum on the graduated arc. The corresponding face of the opposite leaf is hollowed out just enough to let the two leaves fit closely, and keep the pendulum in its place when the instrument is not in use. This form of clinometer, made of boxwood and bound with brass, may be obtained of instrument-makers. It is light and strong, and its durability may be understood from the appearance of the instrument which I hold in my hand, and which, though it has been in constant daily use for more than twenty years, is as true and serviceable as ever.
If at any time the geologist has occasion to lighten his equipment for some long mountain-expedition, where every additional ounce of weight begins to tell by the end of the day, and where, therefore, for the sake of doing as much and holding out as long as possible, he should carry nothing that is not absolutely needful for his purpose, he may advantageously combine the pocket-compass and clinometer in the one instrument to which I have already alluded. This convenient instrument is about the size of an ordinary gold watch. It consists of a thin, round, flat, metal case, shaped like that of a watch, and covered either with a common watch-glass, or, still better, with a flat disk of strong glass. Instead of figures for the hours and minutes, the white enameled face of this geological watch is that of a common pocket-compass. But the interval between each of the four cardinal points is divided into 90°. On the central pivot, just underneath the needle, a small brass pendulum is placed, and a straightedge of metal is soldered on one side of the outer rim of the watchcase in such a position that the instrument will stand on it if need be, and the pendulum will then point to zero. A simple piece of mechanism passing through the handle enables the observer to throw the needle off the pivot, or let it down, as he may require.
As it is impossible for a field-geologist to remember the details of all the observations he makes on the ground, or to insert them on a map, he regards a good note-book as an essential part of his apparatus. From the nature of his work, he has frequently occasion to make rough sections, or diagrams, and, if possessed of the power of sketching, he has abundant opportunity of aiding the progress of his researches by jotting down the outlines of some cliff, mountain, or landscape. Hence, his note-book should not be a mere pocket memorandum-book. A convenient size, uniting the uses of a common note-book and a sketch-book, is seven inches long by four and a quarter inches broad. Let me remark, in passing, that perhaps no accomplishment will be found so useful by the field-geologist as a power of rapid and effective sketching from Nature. If he has this power in any degree, he ought sedulously to cultivate it. Even though he may never produce a picture, he can catch and store up in his note-book impressions and outlines which no mere descriptions could recall, and which may be of the highest value in his subsequent field-work. This is true of ordinary detailed surveys, and still more of rapid reconnaissances, which may have their ultimate usefulness enormously increased if the observer can seize with his pencil and carry away the forms of surface as well as the geological relations of the region through which his traverse lies.
As every device which saves labor and time in the field, or which adds to the clearness of the work, is deserving of attention, I would refer here to the use of variously-colored pencils for expressing at once, upon map or note-book, the different rock-masses which may occur in a district. Water-colors are, of course, ultimately employed for representing the geological formations on the finished map. But a few bits of colored pencils carried in his pocket save the geologist much needless writing in the field. To a red dot or line he attaches a particular meaning, and he places it on his map without further explanation than the local peculiarities of the place may require.
This leads me to remark, also, that he necessarily adopts a system of signs and contractions on his map, not only to save writing, but to prevent the map from being so overcrowded with notes as to become hopelessly confused. Every field-geologist insensibly adopts contractions of his own. For the fundamental facts of geological structure, however, it is eminently desirable that the same signs and symbols should be used with the same meaning on all published geological maps. The subjoined diagram (Fig. 3) shows some of the signs used on the maps of the Geological Survey of Great Britain and Ireland.
Such are the few prime instruments required in field-geology. We may add others from time to time, according to the nature
|Fig. 3.—Some Useful Signs in Geological Surveying.|
of the work, which in each region will naturally suggest the changes that may be most advantageously made. A small bottle of weak hydrochloric acid, carried in a protecting wooden box, or case, is sometimes of use in testing for carbonates, particularly in regions where rocks of different characters come to resemble each other on their weathered surfaces. When Sir William Logan was carrying on the survey of the Laurentian limestones of Canada, he received much help from what he called his "limestone spear." This was a sharp-pointed bit of iron fixed to the end of a pole or a walking stick. He enlisted farmers and others in his operations, instructed them in the use of the spear, and obtained information which gave him a good general notion of the distribution of the limestone. The spear was thrust down through the soil until it struck the rock below. It was then pulled up, and the powder of stone adhering to the iron point was tested with acid. If, after trying a number of places all round, the observer uniformly obtained a brisk effervescence when the acid drop fell on the point of his spear, he inferred that the solid limestone existed below, and noted the fact on his map accordingly.
When the Geological Survey was busy with the great Wealden area of the southeast of England, my colleagues used what they named a "geological cheese-taster." It was, indeed, a kind of large cheese-taster, fixed to the end of a long stick. This implement was thrust down, and portions of the subsoil and of the clays or sands beneath were pulled up and examined. Similar devices must obviously suggest themselves according to the nature of the work in different districts and countries.
In the course of his observations in the field, the geologist will meet with rocks as to the true nature of which he may not be able to satisfy himself at the time. He should in such cases detach a fresh chip from some less weathered part of the mass and examine it further at home. The detailed methods of investigation, which may be pursued with all the conveniences of a laboratory in town, are not possible to him in the country. But he may subject his specimens to analysis in two cases, and obtain valuable, and perhaps sufficient, information as to their characters. He can easily fit up for himself a small and portable blowpipe-box, a machine for slicing and preparing rocks, minerals, and fossils, for examination under the microscope, and a microscope.
The blowpipe-box should contain a common blowpipe, platinum-tipped forceps, platinum wire, small bottles with the ordinary reagents, and as many of the most useful parts of blowpipe apparatus as the space will admit, consistently with the whole box being easily packed into a portmanteau. By means of the blowpipe, it is often possible to determine the nature of a doubtful rock or mineral, and to ascertain the proportion of metal in an ore. A young geologist should take with him to the field only the most essential apparatus and reagents; he will gradually come to see by practice what additions he may best make to his equipment.
A convenient and portable form of the rock-slicing machine is sold by Fuess, of Berlin. Where it cannot be obtained, the field-geologist may succeed in preparing his slices by chipping thin splinters from the rock and reducing them upon a grindstone or whetstone. One side of the splinter is to be made as smooth and free from scratches as possible, which can be effected by polishing on a water-of-Ayr stone. This polished side is then cemented with Canada balsam to a piece of plate-glass. When quite firm, the upper side of the stone is ground down until the requisite degree of transparency is obtained. Considerable practice may be required, and many preparations may be spoiled, before the observer becomes proficient. But the labor is well bestowed, for in no other way can he obtain the same insight into the internal texture and arrangement of the rocks with which he is dealing. He sees what are the component minerals of a rock, and how they are built up to form the mass in which they occur. He likewise can detect many of the changes which these minerals have undergone, and he thus obtains a clew to some of the metamorphic processes by which the rocks of the earth's crust have been altered.
The microscope should be, like the rest, as portable as possible. For most geological purposes high powers are not required, consequently a small microscope is sufficient.
It is sometimes of service, when working in a district where microscopic rock-sections are required, to carry a small collection of microscopic slices of selected or typical rocks or minerals for purposes of comparison. A series of fifty or one hundred slices can be packed in a box a few inches square.—Outlines of Field-Geology.