Page images
PDF
EPUB
[graphic][subsumed][merged small]

bending layers of cloth and clay. The omission may well seem strange to us, but an explanation is to be found in Hutton's teaching. Uniformitarian as he was, Hutton did not believe that great internal changes had taken place in strata after their solidification; nor did his pupil, when seeking to establish the Huttonian theory by direct experiment, think it needful to investigate the properties of consolidated rock.

In 1866 I began some simple experiments, taking up the points that had been disregarded by Hall. First of all, I took a thin slab of marble and placed it on the edge of a mantelpiece, so that the end projected. A few books kept the slab in its place and then I placed a letter weight of one ounce on the free end and left it for some weeks. On testing it by a straight edge it was found to be deflected to a trifling extent. Other plates of different materials, two or three inches long and as thin as possible, were next procured and subjected to the same treatment. But no accurate results were obtained, and the form of the experiment was inconvenient.

Subsequently I tried an improved plan. Two wooden slabs, ten inches by four, were fitted together by a hinge so that they could be set at any angle from 0° to 180°, just as you might open a book, keeping the letterpress always downwards. One slab was screwed to the table, the other could be adjusted at pleasure. The angle made by the two surfaces was indicated by a graduated semicircle. Upon the ridge various thin plates of stone were placed and attached at one end to the fixed slab by heavy weights. The other end of the plate of rock, projecting over, but not at first in contact with the movable slab of wood, was lightly weighted and allowed time for bending. The angle of sudden fracture could be obtained by setting the machine at a low angle and forcibly bending down the lamina of rock until it touched both surfaces. If it yielded thus far, the angle was slightly increased and the experiment was repeated.

This apparatus had some advantages but many defects. The most serious was that the pressure exerted at any time was difficult to estimate. A weight placed upon a surface of gradually increasing inclination exerts a diminishing pressure which changes appreciably even at angles of 3° or 4°. It was difficult to read the small deflections obtained with any accuracy, and the apparatus was liable to disturbance and accident.

At length I tried the machine represented in Plate LXXX.* Here a thin plate of limestone or other rock is screwed down to a block f travelling in a horizontal groove h. Upon this

• For the construction of this machine, and for many useful suggestions, I am indebted to Mr. Thomas Prince, of Bradford.

descends a vertical plate c, terminating in a hinged knife-edge e. We get in this way pressure applied always to the same line upon the lamina of rock, for when deflection begins the knifeedge inclines forwards out of the perpendicular at its lower edge without sliding over the rock specimen. By pushing the block f along the groove h perpendicularity is restored. An index m connected with e makes any deviation more apparent. When a piece of rock is to be tested, shot is poured into the cylinder a, which is in direct communication with the vertical plate c, and the pressure is taken by a steelyard or balance. The index l is set at zero by the screw i, and its motion along the graduated scale enables the observer to record with precision a deflection of less than •01 in. If, in adjusting the knife-edge, the index is displaced, it can be restored by this screw independently of other parts of the machine.

With this apparatus I began a long series of observations on limestone. Thin plates of various thickness from •1 to .05 in. were subjected to low but protracted pressures. Experience taught the best form of plate and the time required to produce a given result. I succeeded in one case in bending a plate •07 in. in thickness to an angle (reckoned as rectilinear) of 12°. This took three months to accomplish. On removal from the machine the plate cracked near the apex of the angle of deflection in three days, or I should have operated upon it again. The pressure was applied so gently and uniformly that sudden fracture seldom occurred except when intentionally produced. The bent slabs were, however, very fragile, and could seldom be kept many days after released from strain, cracks slowly extending themselves transversely across the part where the deflection was greatest. From this circumstance, which caused much annoyance at the time, some useful lessons were learned. Details of the experiments with limestone have already appeared.* It will now suffice to say that thin plates of mountain limestone (especially a certain bituminous kind, occurring in thin beds with partings of shale) proved indefinitely plastic. The elasticity of the rock was greater than I had expected, but the set or permanent deflection produced by long-continued pressures of inconsiderable amount far exceeded what I had hoped to find. It may be doubted whether there is any limit to the bending which a careful and patient observer can produce. I found that magnesian limestones, while usually much more elastic than specimens of pure carbonate of lime, were slightly more plastic. The two properties are not connected in any direct or inverse ratio that I can discover. Some of the

• " British Association Report,” 1869; “Geological Magazine,” November, 1869.

one the two edibility, does

“ flexible limestones” are very difficult to bend permanently. One specimen exhibited for many years in a public museum, with the two ends supported and the centre slightly depressed to show its flexibility, does not present any visible deflection when placed on its edge.

Thin natural laminæ of flagstone from the coal measures were also tried. Various specimens were selected according to their texture and mineralogical character, but none yielded important results. It will be seen further on, from other evidence, that considerable deflection has been unintentionally produced in these flagstones, but as yet I have never succeeded in bending thin plates more than 8'. Slates of various kinds have also proved very intractable, an interesting and not unexpected result. No material has yet done so well in my hands as carefully cut slabs of mountain limestone 4 in. x 3 in. and •07 in thickness.

The frequent destruction by spontaneous fracture of bent plates when removed from the machine seems to imply that an indefinitely protracted and uniformly contorting force is needed to produce unbroken curvature, such as that on the coast of Berwickshire. The experiments next to be related tend to show that resistance on all sides diminishes the risk of fracture. While designed to answer other purposes, the precautions described and the result attained serve to strengthen the opinion that unbroken anticlinals and synclinals are only formed under a considerable weight of superjacent strata.

Anxious to imitate the natural condition of lateral pressure more closely, and at the same time to preserve well-contorted specimens for reference, I tried another method, which ultimately yielded interesting results. My object was to apply pressure to the edges of a slab of stone and overcome the tendency to fracture by embedding it in a matrix of some tenacious substance. The precaution was especially necessary in this second series of experiments. It is easy to see that when deflection begins the bending force is increased in a high ratio. We have the pressure acting upon the slab, not as a force transmitted through its plane, but concentrated upon the middle point, the two halves acting as levers. As the contortion proceeds the strain increases rapidly, and in practice it is found that no graduated pressure can be contrived sufficiently delicate to avoid sudden fracture.

To overcome such difficulties as these I imbedded thin slabs of limestone in pitch and fitted them into a cast-iron box, the two sides of which were removed. One end was cut to allow a screw to travel through it, and within was a plate of iron which could be moved along by the pressure of the screw so as to tighten the slab within the box. By this means pressure was

« PreviousContinue »