A. J. Juhes-Browne — Mellarcl Reads' s Mountain Building. 27 



these conditions, it is impossible to say how such a mass would 

 accommodate itself to the stresses set up by expansion, but it does 

 not seem likely that a mass which is thickest in the centre would 

 " buckle " or ridge itself up along a continuous diametric line ; one 

 would think that it was more likely to develope a set of concentric 

 ridges near the edges, if any such ridges were produced at all. 



It is quite possible that the crust below such a mass, being held 

 circumferentially by the solid crust around it, would expand within 

 its own area, and the internal compression resulting from this 

 expansion might well produce some plication of the component rock- 

 beds ; the expansion would be greatest where the depression was 

 greatest and the combined crust and sediment were thickest, but the. 

 plication would be greatest where the mass was thinnest and weakest, 

 and there is no apparent reason why any part should throw itself 

 into a great surface earth- wave ; yet that is what Mr. Mellard Eeade 

 assumes it would do ! 



Again, if the expansion could be, or were likely to be, so localized 

 as to cause a gigantic and continuous plication of the crust, the 

 bending is not likely to be entirely upwards. Mr. Eeade begins 

 with assuming a plastic substratum ; but when he comes to consider 

 the expansion of the crust, he appears to assume a foundation which 

 is rigid enough to support that crust without yielding to pressure. 

 In all probability the compression which would cause an upward 

 anticlinal swelling would force some other part of the bending crust 

 still further downwards into the plastic substratum, and the most 

 probable result would be a central anticline flanked by smaller 

 synclinal plications. It is true that the total extent of the down- 

 ward displacements would probably be less than that of the upward 

 bulge, but any downward displacement will detract from the avail- 

 able upward expansion, and Mr. Eeade requires all he can get for 

 his mountain-building. 



One of the most suggestive parts of Mr. Eeade's book is his theory 

 of plication by internal expansion (chapter xv.). He points out 

 that if the corrugations of mountain chains have been formed by a 

 compressive force acting from outside the orographic area, then the 

 corrugated beds must originally have occupied a much wider space, 

 the space in fact which they would now cover if pulled out straight. 

 This is assumed by most geologists, and, taking it for granted, they 

 have calculated that the breadth of country now occupied by the 

 Alps has been shortened by 72 miles, and that occupied by the 

 Appalachians has been shortened by 88 miles, that is to say, two 

 spots, one on each side of the Appalachian Mountains, have been 

 brought nearer to one another by 88 miles, and Mr. Eeade asks 

 whether geologists have fully realized what this means. 



If, on the other hand, the plications are due or even largely due 

 to the compression produced by the internal expansion of the mass, 

 they result from actual lengthening of the strata throughout the 

 whole breadth of the orographic area, and consequently they do 

 not involve any lateral movement in space. This seems to me a 

 point that is well worth consideration. 



