CHAPTER X

A FURTHER ILLUSTRATION OF THE STRATA-PLATE AND THE CUMULATIVE EFFECT OF SMALL RECURRENT EXPANSIONS

IN

N The Origin of Mountain Ranges' (Plate VI., p. 28) a reproduction of a photograph of a fold in the bottom of a lead-lined butler's pantry sink is given as illustrating the cumulative effect on a plate of metal of the differential expansion caused by changes of temperature. These changes of temperature have arisen from the hot and cold. water used in ordinary washing up, for which such a sink is provided, and the sink had in no way been interfered with for other purposes.

Since this fold was photographed in 1886 from a plaster cast of the bottom of the sink, the cumulative expansion continued, until the inner concave side of the fold became tightly compressed against the opposite and outer side of the fold. It thus became an overfold, leaning towards

the centre of the sink, as in the accompanying figure, and a second

fold began to rise in crescentic form at the opposite side of the sink.

In process of time these continual minute

movements and strains eventually cracked the lead of the fold, and a plumber, being called in to repair the sink, cut the fold out, beat down the lead to a flat surface, and soldered up the gash. Unfortunately I neglected to note the date of this operation, thinking that the sink would now have no scientific value; but it must have been about eight years ago.

So far from the interest being exhausted, it became greater, for a new fold began in course of time to rise on the site of the old one, involving in the movement the soldered joint, which was lifted and twisted without fracture. Several repairs and solderings of further cracks in the fold took place, until I was satisfied that the sink bottom was beyond further repair.

I now had a plaster cast made of the whole area of the bottom of the sink, measuring 23 inches by 14 inches, and from this Plate III. was photographed. Then I had the lead lining carefully taken out without disturbing the form of the bottom, and had the underside photographed and reproduced in Plate IV.

Furthermore, I cut out the large fold from the bottom (fig. 1, Plate V.), to reduce it to a convenient size, and dissected it with a saw along the lines a b and c d. These three sections were nailed to a board and photographed (fig. 1, Plate V.) in connection with the plaster mould (fig. 2) from which the original Plate VI. in 'The Origin of Mountain Ranges' was taken, and these,

being to the same scale, enable us to see the precise difference between the first fold and the second fold.

The section disclosed by the saw-cut along the line a b is especially interesting from the development of overfolding which had taken place, and as showing the thinning out the metal of the unfolded part of the sink had undergone. It is well shown in Plate VI., which gives the section of the overfold natural size.

The lesson taught by the history of this lead vessel is extremely interesting, and shows very clearly that successive changes of temperature, however small in amount, eventually strain and distort a plate in such a way that the recurrent internal movements in time force up a fold bearing a striking resemblance to a mountain range.

Not only does this happen, but we see by the section at e, Plate VI., that the movement in the centre part of the fold-that is, between the two ends--was continued so as to produce an overfold, a characteristic feature in the structure of mountain ranges.

A careful consideration will also convince us that this movement, or internal strain, satisfied by the rising of the lead in the form of a fold, is due to differential heating of the plate. This is shown by the rate of movement being differential also. The greatest expansion has taken place in the direction of a to b (fig. 1, Plate V.), along the line of section shown natural size in Plate VI., where

the overfolding has developed itself in a remark-
able manner.
Plates III. and IV. also show that
the movement was to a certain extent radial,
which accounts for the crescentic plan of the fold,
further evidenced by the lower and minor folds at
the opposite corners in the same illustrations. The
spreading out and bifurcation of the terminations
of the crescentic fold are partially, I think, due to
the expansion of the plate on the convex side of
the fold.

An examination of Plate V., which enables a comparison to be made between the original fold (fig. 2) and the second fold (fig. 1), shows how this bifurcation commenced with a spreading out of the fold in semi-domical terminations. The history and development of the final form of the fold is recorded well in these series of photographs.

It is very instructive to contrast the perfect moulding and harmonious curves of the original fold (fig. 2, Plate V.) with the gnarled character, irregular ridges, and terminal bifurcations of fig. 1, so representative of the folding of an old mountain range.

Not less interesting to the student of mountain building is the fact that the second fold (fig. 1, Plate V.) rose up on the site of the original fold (fig. 2). A comparison of the two figures will show how closely the second fold followed the lines of the old one, its irregularities arising from the greater stiffness of the portions locally soldered, these solderings rising with the rest of the ridge.

Not only so, but the metal by repeated small strains became less ductile, and so from time to time cracked. One of these cracks or fractures is seen in the photograph (fig. 1, Plate V.) on the inside edge of the ridge, along its central portion.

A measurement of the folding and overfolding along the line a b (fig. 1, Plate V.) shows that the sheet of lead expanded along that line 0.63 inch, and the ridge rose 0.46 inch above the bottom.

This is the maximum lengthening that took place since the initiation of the second fold; but for the actual lengthening of the sheet of lead forming the bottom of the sink from first to last must be added the expansion that produced the first fold, estimated by me at about 0.5 inch, making the total 1.13 inch.

It is therefore mathematically demonstrable that this expansion-creep must be accompanied by a thinning of the sheet of lead, and a transference of lead to the site of the fold.

In sawing through the sink bottom it was found that in the centre part the sheet was obviously thinner than in and near the folds. Some of this diminution was, no doubt, due to extra wear at that particular spot, but in part also to the thinning by expansion over the central area. The whole plate of lead, including the portion forming the folds, must also have lost thickness not only by wear, but by long-continued chemical action of the water.

The fact that the edges of the plate were