552 



NA rURE 



[October 5, 1893 



sea-bottoins, and softened by invasion of interior heat. This 

 view is therefore satisfactory as far as it goes, and brings order 

 out of the chaos of mountain phenomena. It has successfully 

 directed geological investigation in the past, and will continue 

 •to do so in the future. 



" But there sliU remains the question, ' What is the cause of 

 the lateral pressure ? ' The answer to this question constitutes 

 ^)^s physical theory. 



'• Thus far I suppose there is little difference of opinion. I 

 have only tried to put in clear condensed form what most 

 geologists hold. But henceforward there are the most widely 

 diverse views, and even! the wildest speculations. But let us 

 not imagine, on that account, that we have made no progress 

 in the science of mountain origin. The formal theory already 

 given is really for the geologist by far the most important part 

 of the theory of mountain origin. For I insist that for the 

 geologist, formal theories are usually more important than 

 physical \):\^ot'\t% of geological phenomena. That slaty cleavage 

 is the result of a mashing of strata by a force at right angles 

 to the cleavage-planes, is of capital importance to the geologist, 

 for it is a guide to all his investigations. To what property of 

 matter this structure is due, is of less importance to him, 

 though of prime importance to the physicist. That the 

 phenomena of the drift is due to the former existence of a 

 moving ice-sheet is the one thing most important to the 

 geologist, guiding all his investigations. Whether this ice- 

 sheet was caused by geographical or astronomical changes, is a 

 question of wider but of less direct interest to him. So in the 

 case of mountain ranges, the most important part of the theory 

 is their origin by lateral pressure under the conditions given 

 above. The cause of lateral pressure, though still of extreme 

 interest, is certainly of less immediate importance in guiding 

 investigations." 



The Contraction Theory. 



"The most obvious view of the cause of lateral pressure refers 

 it to the interior contraction of the earth. This theory is so 

 well known that I will give it only in very brief outline. It 

 assumes that the earth was once an incandescent liquid, and 

 has cooled and solidified to its present condition. At first it 

 cooled most rapidly at the surface, and must have fissured by 

 tension. But there would inevitably come a time when the 

 surface, being substantially cool, and, moreover, receiving heat 

 also from the sun, its temperature would be fixed, or nearly so, 

 while the incandescent interior would be still cooling and con- 

 tracting. Such has probably been the case ever since the 

 commencement of the recorded history of the earth. The hot 

 interior now cooling and contracting more rapidly than the cool 

 crust, the latter, following down the ever-shrinking nucleus, 

 would be thrust upon itself by lateral pressure with a force 

 which is simply irresistible. If the crust were ten times, yea, 

 one hundred times more rigid than it is, it must yield. It does 

 yield along the lines of greatest weakness, i.e. along marginal 

 sea-bottoms, as already explained. As a first attempt at a 

 physical theory, it seems reasonable, and therefore until re- 

 cently has been generally accepted." 



Objections to the Contraction Theory. 



" It is well known that American geologists have taken a very 

 prominent part in the study of mountain structure and mountain 

 origin ; so much so, indeed, that the lateral pressure theory in 

 the form given above and interior contraction as its cause, have 

 sometimes been called the ' American theory.' It is also well 

 known that my name, among others — especially Dana's — has 

 been associated with this view. All I claim is to have put the 

 whole subject, especially the formal theory, in a clearer light 

 and more consistent form.' The formal theory I regard as a 

 permanent acquisition. The contraction theory may not be so. 

 It is natural, from my long association wilh it, that I should be 

 reluctant to give it up. But I am sure that I am willing to do 

 so if a better can be offered. We all dearly love our own 

 intellectual children, especially if born of much labour and 

 thought ; but I am sure that I am willing, like Jephtha of old, 

 to sacrifice, if need be, this my fairest daughter on the sacred 

 altar of Truth. Objections have recently come thick and fast 

 from many directions. Some of these I believe can be removed, 

 but others perhaps cannot in the present condition of science, 



1 " Theory of llle Formation of the Great Features of the Earth's Sur- 

 face." Am. /onriial, vol. iv. p. 345 and 460, 1872 ; and also " Structure and 

 Origin of Mountains," vol. xvi. p. 95. 1878. 



NO. 1249, VOU 48] 



and may indeed eventually prove fatal. Time alone can show. 

 I state briefly some of these objections." 



(i) " Mathematical physicists assure us that on any reason- 

 able premises of initial temperature and rate of cooling of the 

 earth, the amount of lateral thrust produced by interior con- 

 traction would be wholly insufficient to account for the enormous 

 foldings {Cam. Phil. Trans, vol. xii. Part 2, December, 1873). 

 Let us admit — surely a large admission — that this is so. But 

 this conclusion rests on the supposition that the whole cause of 

 interior contraction is cooling. There may be other causes of 

 contraction. If cooling be insufficient, our first duty is to look 

 for other causes. Osmund Fisher has thrown out the sugges- 

 tion (a suggestion, by the way, highly commended by Herschel) 

 that the enormous quantity of water vapour ejected by vol- 

 canoes, and the probable cause of eruptions is; not meteoric 

 in origin as generally supposed, but is original and con- 

 stituent water occluded in the interior Magma. {Cam. Phil 

 Trans, vol. xii. Part 2, February, 1875. ''Physics of the 

 Earth's Crust," p. 87.) Tschermak has connected this escape 

 of constituent water from the earth with the gaseous explosions 

 of the sun {Geol. Mag. vol. iv. p. 569, 1877). Is it not barely 

 possible that we have in this an additional cause of contraction, 

 more powerfully operative in early times, but still continuing ? 

 See the large quantity of water occluded in fused lavas to be 

 ' spit out ' in an act of solidification ! But much still remains 

 in volcanic glass which by refusion intumesces into lightest froth. 

 Here, then, is a second probable cause of contraction. If these 

 two be still insufficient, we must look for still other causes before 

 rejecting the theory. 



(2) "Again, Dutton {Am. Jour. vol. viii. p. 13, 1874; 

 Pinn. Jijionthty, May 1876) has shown that in a rigid earth it 

 is impossible that the effects of interior contraction should be 

 concentrated along certain lines so as to form mountain ranges, 

 because this would require a shearing of the crust on the 

 interior. The yielding would be evenly distributed everywhere, 

 and therefore imperceptible anywhere. This is probably true, 

 and therefore a valid objection in the case of an earth equally 

 rigid in every part. But if there be a subcrust layer of liquid 

 or semiliquid or viscous, or even more movable or more un- 

 stable matter, either universal or over large areas, as there are 

 many reasons to think, then the objection falls to the ground. 

 For in that case there would be no reason why the effects of 

 general contraction should not be concentrated on weakest 

 lines, as we have supposed. 



(3) "But again, it has been objected that the lines of yield- 

 ing to interior contraction ought not to run in definite direc- 

 tions for long distances, but irregularly in all directions. I 

 believe we may find the answer to this objection in the principle 

 of flow of solids under very slow heavy pressure. The flow of 

 the solid earth, under pressure in many directions, might well 

 be conceived as being deflected to the direction of least resist- 

 ance, i.e. of easiest yielding. 



(4) "But again, it will be objected that the amount of cir- 

 cumferential shortening necessary to produce the foldings of 

 some mountains is simply incredible, for it would disarrange the 

 stability of the rotation of the earth itself. According to Clay- 

 pole, in the formation of the Appalachian range the circum- 

 ference of the earth was shortened 88 miles, and in the 

 formation of the Alps 72 miles. Now this would make a de- 

 crease of diameter of the earth of 28 miles in the one case, and 

 23 in the other. This would undoubtedly seriously quicken the 

 rotation and shorten the day. This seems indeed startling at 

 first. But when we remember that the tidal drag is all the 

 time retarding the rotation and lengthening the day, and much 

 more at one time than now, we should not shrink from accep- 

 tance of a counteracting cause hastening the rotation and 

 shortening the day, and thus giving stability instead of destroy- 

 ing it. We must not imagine that there would be anything 

 catastrophic in this readjustment of rotation. Mountains are 

 not formed in a day, nor in a thousand years. It requires 

 hundreds of thousands, or even millions of years — if physicists 

 allow us so much. 



" The objections thus far brought forward, though serious, 

 are by no means unanswerable. But there is one brought 

 forward very recently which we are not yet fully prepared to 

 answer, and may possibly prove fatal. I refer of course to the 

 level of no strain." 



Level oj No Strain. 



" Until recently the interior contraction of the earth was con- 

 sidered only roughly and without analysis. It was seen that the 



