NOVEMBEK 1, IfcCS] 



KNO WL E DGE. 



205 



Now, unless all the elementary presumptions of geology 

 are wrong, these folded strata must originally have been 

 deposited horizontally. Since their deposition, therefore, 

 their extremes have been brought nearer together. This 

 pucivering points unmistakably to a squeezing in from the 

 sides. It has been calculated, in the case of the Alps, that 



Section of Mont ]?lnnc. 



points on either side of this mountain mass have been 

 brought closer to one another by as much as seventy-two 

 miles. In the case of the Appalachian Mountains the 

 estimate is eighty-eight miles. We seem to have here, then, 

 the clear record of the successive stages in such a process as 

 is indicated in a simplified fashion by our Fig. 2 (A, B, C, D), 

 in which A, B and C represent phases in a steady lateral 

 compression, and D repeats C, with some allowance for 

 the action of sub-aerial denudation. 



It is upon this aspect of mountain structure that Prof. 

 Lapworth laid particular stress in his memorable address 

 to the Geological Section of the British Association. He 

 insisted upon the horizontal pressure and upon the strata 

 girin<i to this strain at their weakest points, bulging up 

 into ridges and furrows, and with further compression 

 folding over, so that we get at last "oyer folds "(Fig. 1), with 

 an upthrust or arcli limh, a middle portion, and a down- 

 thrust or troitjili limb. The middle portion must especially 

 be under great pressure, and it may undergo crushing, or 

 the fold may rupture and the arch slide forward over the 

 fault to form a reversed fault or over-fault or thrust plane 

 as in Fig. 5. The final result of this folding will be to 

 strengthen the crust at the original weak point by more 



^ A 



(vs. 



than doubling its thickness, and adjacent portions of the 

 strata will then begin to pucker. So that m the flanks of 

 the original fold fresh folding; will arise until we get either 

 a fan-like series (as in Mont Blanc), or a one-sided arrange- 

 ment (Fig. 8) such as is displayed in the Appalachian 

 and Jura Mountains. 



The causes of these mountain foldings may possibly be 

 the lateral stress due to local horizontal expansion, if the 

 theory of Mr. Mellard Eeade is correct. But a great 

 number of geologists consider that the prime cause of these 

 foldings, and indeed of mountain upheavals, is the con- 

 traction of the earth due to its secular cooling. As this 



contraction goes on, the cold crust has to accommodate 

 itself to the shrinking interior, and in doing this it is 

 necessarily crumpled and wrinkled. The great land masses 

 and the great oceanic troughs of our earth, moreover, lie 

 along lines of longitude. Winchell has attributed this 

 north and south trend of the chief lines of crumpling to the 

 directive inliuence of the tidal stress. 



Prof. Lapworth, in his address, stated the case for the 

 contraction theory of moimtain origin in a remarkably 

 vivid way. He called attention to the manner in whicli 

 trough and ridge everywhere corresponded. For the 

 upthrust of America, with its Mississippi valley and its 

 unilateral ridges of the Eockies and Appalachians, we have 

 the Atlantic with its division by the Dolphin ridge into 

 two parallel troughs ; and corresponding to the broad uprise 

 of the older continents we have the great depression of 

 the Pacific. Coming to the shorter transverse foldings, 

 the Alpine mass had for its trough the Mediterranean ; 

 and Central Asia the southward deej) of the Indian Ocean. 

 This " undulation " of the surface of the solid earth is 

 far more in agreement with the theory of secular cooling 

 than the theory of Mr. Mellard Eeade. 



On the other hand, there are those who consider the 

 amount of folding we find in mountain masses, which 

 must amount altogether to a diminution of the earth's 

 circumference by many hundred miles, too great for their 

 conception of the amount of contraction the world has 

 undergone since the rocks in question were solidified. 

 Moreover, in certain localities in Sweden and elsewhere, 

 cnoitpled rocks are found lying on an undisturbed base. 

 Prof. Eeyer has recently propounded some novel and 

 remarkably suggestive views in this matter. 



He has conducted a series of experiments upon the 

 behaviour of artificial strata made of muddy material or 

 plaster of Paris mixed with glue and variously tinted. 

 These before complete consolidation were placed on boards 

 slightly tilted (5^ to 15°) , and the arrangement was occasion- 

 ally tapped to imitate earthquake shocks. There was u 

 general sliding down and crumpling of the mass, such as 

 might conceivably happen in the case of sedimentary rocks, 

 and sections taken after hardening showed, in consequence 

 of this gliding, beautiful imitations of folding, contortion 



and faulting such as are seen in mountain chains. Such 

 experiments as this might very easily be repeated by 

 teachers of geology or physiography. They certainly aid 

 the imagination very greatly in thinking out these 

 physiographic problems. 



Prof. Eeyer's conception of the development of a 

 series of mountain folds, based on experiments of this kind, 

 may be illustrated by the three figures above. While 

 it harmonizes with Mr. Mellard Reade's hypothesis, 

 it seems perhaps a little better adapted to explain complex 

 crumpling of strata than does that supposition. A repre- 

 sents a continental mass, from which the sediments C, D, 



