of the Rocky Mountains examined in the construction of the accompanying section is thus characterised in its eastern part by a series of great fractures and thrust-faults, in the centre by broad sweeping folds, and in the west by folding and crumpling, accompanied by the development of cleavage planes and a limited amount of metamorphism.

'Among its other more important features may also be noted the absence of recognisable unconformities, the absence of any of the older crystalline schists, the relatively smaller amount of disturbance in the central parts of the range than towards the edges, the want of similarity in the sequence of the formations east and west of the axis, and the marked preponderance of calcareous beds between the Middle Cambrian and the Cretaceous.'1

Plate No. XVIII., representing the effect of simultaneous circumferential compression on two systems of rocks, one-the basal system-having been folded or sheared at a former time and since denuded, the upper series being sedimentary beds laid down upon the basal series, enables us to understand the differential effect of simultaneous pressure on two such systems. The basal series become more compressed and distorted, and this, if the compression were continued, would be likely to end in fracture and shear-planes. These thrusts are eminently characteristic of rigid strata that

I P. 40 D.

have suffered from intense pressure. The upper series are not rigidly bound, and the pressure expends itself in folding. As already pointed out, we must discount the effect of the domical rise which in our models leaves vacuities between the individual domes, for in nature gravitation would keep the strata practically together as solids.

In a memoir of the Geological Survey of India on the Physical Geology of the Sub-Himalaya of Garhwal and Kumaun, C. S. Middlemiss gives a number of interesting sections across the subHimalyan zone in which the hade and succession of reversed faults seem to point to the dominant pressure having acted from the north.1 The crescentic plan of this mountain system, with its concavity to the north, would seem to favour the same conclusion. How far the pre-Tertiary beds. were folded before the uplift of the Himalayan system is too complex a question to enter upon here. Divergence of strike in the folded regions no doubt points to pre-Tertiary compression, but I must maintain that the system as a whole is Tertiary. As Dr. R. D. Oldham penetratingly observes, the occurrence of marine nummulitic beds at a height of many thousand feet on the north face of the main snowy range in Hundes, and at a height of 20,000 feet in Zanskar, shows that the elevation of this part of the Himalayas

1 Suess is of opinion that there has been in Asia a general movement of rock-masses towards the south. For a summary of his views see History of Geology, by Von Zittel, translated by Maria Ogilvie Gordon, D.Sc.

must have taken place entirely within the Tertiary period.'1

Mr. Middlemiss has done me the honour to devote several pages of his memoir to a discussion and attempted refutation of my theory of mountain building. He cannot accept, for one thing, the central fact upon which it is founded, viz. the connection of mountain building with previous sedimentation. This is disputed by very few geologists. Hall, Dana, and Le Conte had pointed it out before my time, though they failed to see the true relation as cause and effect. I do not propose to go into details here, but will merely observe that Mr. Middlemiss, when he thinks he is 'beating a corpse,' in some curious way misunderstands the physical question involved.

With these remarks I must end this chapter. My desire is not to enter into controversy, but to state my views clearly, so that geologists may be in a position to test their validity. I know full well that we can only hope to arrive at partial truth.

What there is of value in these pages will be separated and preserved by the impartial hand of time. As observations are multiplied and recorded future geologists will possess a far broader basis of facts from which to draw their conclusions. The subject is so new and the innumerable relations so complex that it will doubtless be long before anything like unanimity is arrived at.

1 A Manual of the Geology of India, second edition, p. 477.

CHAPTER XVII

INSTANCES OF THE EFFECT OF EXPANSION DUE TO ATMOSPHERIC CHANGES OF TEMPERATURE

EXPERIENCE proves that most important

changes are going on around us of which

we perceive ordinarily no indications.

The linear dimensions and volume of every substance are in a continual state of flux and reflux with every change of temperature. It is only when these movements affect us personally that we pay much attention to them. I have collected a series of examples of the way atmospheric changes of temperature may interfere with our conveniences. Engineers have to pay special attention to these points, as the success or failure of their works is largely dependent upon the provisions which are made to meet them.

The instances I am about to quote are in continuation of Chapter III. of 'The Origin of Mountain Ranges,' and will help the reader to realise what an important part variations of temperature must play in the reactions of the heated interior globe upon the ever, but slowly, changing lithosphere.

Expansion of Streets-Lifting of Street Paring. A street in Terre Haute, Indiana, had been paved with brick five years ago, the joints being grouted up. The work was partly done during the winter and was finished in early spring. The foundation was of broken stone, 8 inches thick, above which was a layer of sand 2 inches thick. At the end of last July-thermometer at 100° F.a section of the pavement rose like an arch from its foundation, and though water was turned on to it and openings made to let out any possible accumulation of gas, it maintained its position unaffected. While this was being repaired another section rose in a similar manner, with a loud report, to a height of 7 to 8 inches.'1

Effect of Hot Days on Concrete and Asphalte Footwalks and Carriage-ways.—Mr. C. R. Strachan, C.E., says: It is no uncommon thing to see the surface of an asphalte path raised crosswise in an irregular line, as though a small tree-root were under it. In every case where the asphalte has been uncovered at these points by the writer he has found the concrete crusted.' This effect is most marked on hot days. In some laid by him in Chelsea the number of these ridges in the hot weather of June was most astonishing. 'Shortly after midday they were most pronounced, and towards night they were less prominent.' He mentions kerbs in the City as being displaced from the perpendicular by pressure of the concrete of

1 Nature, September 3, 1896, p. 425, quoted from Engineering.