THE ORIGIN OF MOUNTAIN-RANGES.* IT is little mare than half-a-century
since Elie de Beaumont announced his doctrine of the upheaval of mountain-chains. Influenced by the prevailing catastrophism of his day, this dis- tinguished geologist taught that mountain-ranges were thrown up by a sudden movement, that a number of chains originated contemporaneously, and that all those of the same epoch were parallel to each other. This hypothesis, whether as regards the suddenness or the parallelism, has received little support. No geologist of repute now believes that mountain-ranges originated in catastrophes. It is agreed that they have been evolved by a process as gradual as that which deposits a delta or builds up a coral-island. Some internal force has uplifted the earth's crust along a certain line. The slowly rising ridge, as it emerged from the waves of the ocean, was set upon by the earth-powers which dwell in the billows, the rain, the frost, and the air. The hammering of the waves gave place to the action of the atmo- spheric agents as the elevatory movement continued. The frost pulverised, the gases of the atmosphere caused softening and decay, the rivers excavated ; and thus, as by the tools of myriads of sculptors, the great linear mass of rock was carved into the variety of mountain•shapes which forms the glory of alpine regions.
So far there is no dispute. There is peace on the summits. The war that disturbs the minds of Mr. Mellard Reade and his antagonists is waged concerning the interior of the globe. A few miles under our feet, the thermal forces are working with enormous energy. Temperatures running up to thousands of degrees Fahrenheit are acting upon the rocky crust. Variations of temperature must cause fusion and congelation, expansion and contraction, upheaval and depression. But how do the thermal laws behave under the pressure of twenty or thirty miles of solid rock P Mr. Mallard Reade gives one answer, and his opponents give another. But who shall decide ? Where is the Ulysses who will venture into the realms of Pluto to investigate temperatures and pressures ? Yet the matter is of fundamental importance in any theory of the origin of mountain- ranges.
It is not disputed that the puokerings of the earth's cruet which are modified into mountain-chains result from lateral pressure. Geological discovery has within the last few years greatly enlarged our views of the effects of this force. In the North-West of Scotland, the earth-thrust has not only produced excessive contortion of the rocks, but has pushed up the oldest series, the " fundamental " gneiss, over younger formations, and sent it sliding up slopes for miles. This astonishing result is seen along a line extending from the mainland opposite Skye right up to Loch Erribol, on the Northern coast of Sutherland. Similar effects have been observed in the Swiss Alps and other great chains ; so that lateral thrusts of enormous potency are coming to be recognised as the chief factors in mountain-building, when it occurs on a large scale. In great ranges, there are numerous parallel wrinklings of the crust arranged along a certain band, and usually increasing in height towards the -central axis ; but in small ridges, each as the English Pennine Chain, the lateral pressure has been comparatively feeble, and has merely given rise to a single arch-like fold of rock. But what has caused the lateral thrust ? This is the crux of the difficulty.
The explanation current in the text-books is called the Con- traction Theory. It is supposed that, as the globe cooled down from a state of fusion, a solid crust was formed by congelation. The nucleus continued to give off heat, and, contracting, shrank inwards from the solid shell, which thus left unsupported, was compelled to adapt itself to a continually shortening diameter. This gave rise to lateral pressures and consequent corrugation. The corrugation being produced along definite lines, mountain- thains were formed. Mr. Mellard Reade declines to accept this hypothesis. He considers that the shrinking of the earth's nucleus would not cause contortions of any importance. His reasonings are largely based upon the rate of contraction at varying depths, and being necessarily rather speculative, do not produce absolute conviction. We hesitate to apply measured results obtained at the earth's surface, or formulas deduced from such results, to the behaviour of rocks under pressures and at temperatures which we cannot reproduce. We turn with more satisfaction to the constructive part of Mr. Iteade's book.
The author has accumulated a great wealth of information out of the works of other writers, and has made valuable additions • The Origin of Mountain-Ranges. By T. Mallard Heade, C.E., F.G.S., F.R.I.B.A. London: Tadlor and Francis. from his own experiences as a civil engineer and a geologist. He has personally verified many of the published data, and out of his voluminous material he has built up a theory possessing several points of originality. He regards the necessary plication and elevation as the result of expansion acting within the shell of the globe. Numerous examples are given of the effects of expansion in metals and masonry. Many experiments are described. Some of the more important of these were made with slabs of stone. A bar of sandstone 71 in. long, and about " II in. by 1 in. scantling, was placed with its ends resting flat- ways on two supports. The flame of a blow-pipe was brought to bear upon its upper surface," which became "hot enough to melt solder, but not lead,—perhaps 500° F. on the top surface." After a second heating, the stone arched upwards against gravity to the extent of " I'r in., at which it took a permanent set." Of still greater interest were experiments with sheets of lead fixed round the margins, so that lateral expansion was prevented. In one case, the author caused the flame from a blow-pipe to play over the surface of a plate measuring 18 in. by 6 in., when the " lead rose in a longitudinal ridge traversing the centre of the plate, with transverse ridges or spurs branching from it." With a circular disc, the protuberance formed "after repeated heatings and coolings," had a " domical form." This puckering upwards of metal plates fixed round the edges is, of course, intended to illustrate the probable effects produced by heat upon a given area of the earth's crust.
But the degree in which rocks expand is also an important factor in the inquiry. The figures obtained by Mr. Mellen]. Reads under this head agree in the main with those of Mr. Adie. Summing up these results, the author concludes that the average expansion of a mass in the earth's cruet is about 21 ft. per mile for every 100° F. He then applies this co-efficient to " a given area and depth of rock. Take a volume, for example, equal to 500 x 500 x 20 miles. If this were heated to a mean of 1,000° F., a temperature that must have occurred over and over again in the local heating of the earth's crust, there would be a linear expansion in two directions of 500 x 2.75 ft. x 10 = 13,750 ft., or 2-6 miles." These figures look imposing; but do they supply us with adequate data I, Ten yards in a mile is not much. If a rod 176 yards long were compelled to adapt itself to a length of 175 yards, the contor- tion would be very slight indeed. In mountain-chains, the lateral thrust has sometimes bent the strata into zigzags almost as sharp as the folds of a closed fan ; and this not along a mere line, but in bands several miles in breadth. It is common on the flanks of great ranges to find the very oldest rocks, forming the nucleus of the chain, bent outwards on both sides so as to overlie the younger deposits to a considerable distance. The enormous earth-thrusts of the Highlands, referred to above, also imply expansion (or contraction) on a far greater scale than is expressed by the humble ratio of ten yards to one mile. Of course it may be urged that the pressure is concentrated or localised, so that a mass of the earth's shell many miles in breadth may, when expanding, produce plication along a very narrow zone. Let ns assume that a band one hundred miles broad causes corrugation in a strip of one mile in breadth. This would give, at 271 ft. per mile, a compression of 2,750 ft. within the one-mile zone,—say that the mile is squeezed into haH- a.mile. This compression would perhaps be adequate to produce all the effects described; but are we justified in making such large assumptions P In his second edition, perhaps our author will grapple with the difficulty here indicated.
Although we can hardly admit that Mr. Mellard Reade has furnished a complete solution of the origin of mountain-ranges, we must congratulate him upon a solid advance. His book brings into prominence a sera causa which has not hitherto re- ceived adequate attention. A mass of rock, when heated, must expand in all directions, whether its bulk be a thousand cubic miles or one cubic inch. In the earth's crust the resistance to expansion must act laterally, causing elevation; and this must be added to the vertical expansion produced directly by the heat. It is therefore difficult to resist the conclusion that expansion has taken an important part in the production of mountain- ranges. But may we not combine the theories of Mr. Mallard Reade and his opponents P Contraction of the nucleus may well be accompanied by local expansion in the shell. The two causes may thus combine to produce.an effect for which each by itself would be inadequate.
Perhaps geologists would have made more satisfactory pro- gress towards a true and complete solution of the problem had
they extended their inquiries beyond the limits of mechanical and thermal laws. We have scarcely yet come to recognise the great share which chemical forces have taken in causing expan- sion and contraction. The earth's crust, we know not to what depth, is one vast chemical laboratory, in which the enormous temperatures and pressures can hardly fail to accelerate and intensify mineral changes. Combinations and dissociations of which our crucibles and re-agents know little or nothing, may cause great and rapid variations in volume. Water, soaking down into the interior, becomes a most potent agent of trans- formation. The minute liquid cavities so commonly found in the quartz of granite, demonstrate the energetic part taken by water in rearranging the mineral constituents of the globe. But the time for the adequate discussion of the chemical causes of earth-movements has not yet come.
Mr. Mellard Reade's book would be worth reading, even if his hypothesis were entirely baseless. The geological expert, behold- ing the long array of facts marshalled before him by the author, may cry, with Macbeth,— "What! will the line stretch oat to the crack of doom ?"
Yet even the expert, to whom many of the data are already known, will often perceive a new significance in the familiar faces. To the general reader, The Origin of Mountain-Ranges will be a rich and varied gallery of illustration. Many of the most striking results of modern investigation are described with clear- ness, and further elucidated by well-drawn figures. A large number of these are from Mr. Reade's own pencil, and are sketched from Nature. There are in all forty-two plates, many of them with five or six figures. Amongst these illustrations are pictures of sea-cliffs displaying massive strata crumpled up like a folded stair-carpet ; sections through mountain-chains, with huge bands of crystalline rock several miles in length squeezed into a tangle of loops ; sketches of the Matterhorn, Mont Blanc, Snowdon, and other famous peaks ; an ink- photograph of part of the moon's surface ; with maps, sections, plans, charts, and diagrams ad libitum. We trust that Mr. Mellard Reads will be rewarded by the public appreciation which he deserves for his laborious and excellent book.
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