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CHAPTER XXII

TIME AS A GEOLOGICAL FACTOR

IE importance of time as a factor in geology has, in relation to denudation, been more or less understood and admitted by geologists since the days of Hutton and Playfair. Not so, however, is it realised that time plays an equally important part in geological dynamics.

It is well known to those who have to deal with practical mechanics that time is an element which has to be considered and contended with. A beam carrying a static load may be perfectly safe when newly subjected to it, but let the load remain long enough and a permanent set is given to the beam, ending in collapse. In the same way chains, ropes, and similar materials subject to tensile stresses weaken independently of actual wear or attrition.

The molecular changes that take place in all materials under constant stresses are not well understood, but it is within the practical experience of most of us that a bar too rigid to bend on the application of a comparatively sudden force will be pliable to one gradually applied.

In the same way rocks act in the heart of the

earth's crust, and even the most rigid are moulded by the complex stresses they have to contend with. Subjected as they are to enormous vertical pressure, increasing with the depth, when lateral pressure is developed, the various beds are moulded into forms frequently having considerable symmetry. On the other hand, fractures often take place due to the differences of rigidity of the several associated beds.

Again, with normal faulting a new set of stresses are introduced—namely, tension and those due to vertical shearing and compression-which have already been discussed under the head of 'Contraction Faults.'

The proof that all these forces have been applied gradually is to be found in the beds of rigid rock often symmetrically bent as if they were sheets of a ductile metal. Every material is plastic to competent pressure applied sufficiently slowly.

The catastrophic elevation of a mountain range is an event that could only happen were it possible that energy could accumulate in the earth's crust sufficient to bring about suddenly such a vast upheaval. The sudden upheaval of even a small mountain chain would shake the earth to its foundations, while that of, say, the Alps or Alleghanies would go far to destroy the whole habitable globe. But this event is in my philosophy an impossibility. Long before such a vast store of energy could accumulate, the earth's crust

would adapt itself by recurrent small movements to the forces acting upon it. Everything in geology leads us to think that mountain ranges are built up by gradual and successive creeps, and that a sudden release of pent-up forces takes place on a scale not larger than what is experienced in a great earthquake.

The symmetry of the folds seen in the Appalachians and other mountains of that type are eloquent proofs of the extremely gradual way in which they have been built up.

That these forms of upheaval are mainly due to lateral pressure is admitted by nearly all geologists. What, then, does this great fact mean? It means that for a sudden upheaval to take place sections of the earth's crust of areas extending to thousands of square miles must simultaneously move in a horizontal direction one way or the other over distances to be measured by milesdistances, be it observed, sufficient by the compression of the strata to raise up parallel ridges having the cubic contents of these great ranges.

Let us try to follow out mentally the consequences of such an extraordinary event as the sudden creation of a mountain range.

The movement of such an enormous weight over the earth's surface, were such a thing possible, would doubtless be accompanied by earth waves of appalling violence, shaking the whole lithosphere of the globe. Not only would this occur, but the redistribution of weight on the earth's crust and

the piling up of materials in the range itself would profoundly influence the structure of the earth's crust. The weight of the piled-up materials would press the crust down into the subcrust. It is almost needless to tell a geologist of the present day that there is no record to be found in mountain ranges of any such vast and sudden movements having taken place. Instead of a tumbled mass of materials, such as we may conceive would result from a sudden release of dormant forces competent to carry out such changes, we find order and symmetry and a perfect refitting together of the beds, to which operations we may suitably apply the term building' or 'mountain-building.'

The fact also must not be lost sight of that in many cases the material removed from the range by denudation is in excess of what remains, so that properly to appreciate the vastness of the operations involved in the sudden creation of a mountain range we must at least double the load that has to be acted upon and piled up in a restricted area.

And here it may be well to note that in the early days of geology, when catastrophic views were not unnaturally in the ascendant, it was supposed that mountains were created by simple upthrows or upheavals.'

Exactly what upheaval' meant it is difficult to find out. Probably it was one of those words that help to soothe the inquiring mind, even if they do not satisfy it.

We now know that, though uprise of compressed materials of the crust underlying the stratified lithosphere does take place, the principal forces have acted horizontally, and redistributed the load on mother earth by folding, compression, and by over-faulting.

This illustration is given as an extreme case of the application of catastrophic principles-or want of principles-to mountain building properly so called. Doubtless those who now consider that such events have constituted a considerable part of the geological history of our globe will divide each upheaval' into periods, and instead of making one event of it will make many. It is evident to any one possessing elementary mechanical knowledge that such periods must be limited by the stresses the lithosphere is competent to bear without moving; when this is exceeded the change takes place from the sudden relief afforded.

Therefore, looked at in its proper light, the difference in the views of the modern and the earliest geologists are those of degree only. Catastrophes may happen, but not on the stupendous scale invoked by the pioneer reasoners on geology, who, having few facts to go upon, imagined the rest.

The progress of geology is due to the careful way in which Nature has been examined and questioned, and to the application of principles established by kindred and maturer sciences to explain phenomena now more accurately recorded.

The tendency of this closer study of the records