232 Revieus — T. Mellard Beads — On Mountain Ranges. 



surface in volcanic manifestations. Such is an outline, as I understand 

 it, of the theory. 



If the two preliminary difficulties can be disposed of, the theory 

 seems well suited to explain the formation of elevated plateaux. 

 But for producing the intense corrugation, which characterizes most 

 mountain ranges, the amount of horizontal expansion which it aifords 

 appears inadequate ; especially when we consider, that much of the 

 compression would be expended in deforming the material upwards, 

 without rotation of the parts. The scale is made appreciable if one 

 takes a metre rule, and compares five millimetres with the entire 

 length — or a yard measure with less than a quarter of an inch. 

 The corrugation arising from compression on such a scale must be 

 very small. Many of the numerous and interesting sketches given 

 from nature make one feel rather the inadequacy of the theory to 

 account for the disturbances shown. Altogether this part of the 

 theory needs more reasoning out quantitatively. 



Chapter xxi. on the connection of volcanic action with mountain- 

 building, is worth studying. The subject is an obscure one, but it is 

 so intimately connected with every manifestation of force in the 

 Earth's crust, as cause with effect, that they cannot be separated. 

 Mr. Eeade's leading idea seems to be, that the reservoir of magma 

 is solid, but that it exerts expansive stress to find escape. Being 

 solid, it does not transmit liquid pressure from one vent to another; 

 so that lava can stand at different levels in them. 



The experiment represented in figs. 1 and 2 of plate xlii. and the 

 description of it (p. 431) are interesting. Several strips of paper 

 were laid upon each other, and lines were drawn across the edge of 

 the block so formed. It was then bent into folds. The inclination 

 assumed by the lines now indicated the manner in which the strips 

 had slidden longitudinally over one another in the process of folding. 

 I may mention that the position taken by these cross lines exactly 

 agrees with that of the "planes of less perfect cleavage" in Mr. 

 Sorby's well-known diagram of a contorted bed in slate rock at 

 llfracombe.^ This shows that the cleavage follows the lines of 

 distortion, not those of shear ; for the latter would, in the experi- 

 ment, have been along the surfaces of the sheets of paper.^ 



Mr. Reade, in plate xxxviii., gives a good demonstration of the 

 extension that necessarily accompanies the compression of strata 

 into close chevron folds, which, from what has been said in the 

 preceding paragraph, would lead one to expect such folds to be in 

 general affected by cleavage inclined at an angle across them. 



There is throughout the book a certain want of precision in 

 scientific language. "Heat" is sometimes used where the word 

 should be " temperature " ; " strain " where it should be " stress." 

 At p. 303, having been told that mechanical denudation from the 

 Mississippi basin is one foot in 6000 years, we read, " If we add the 



1 Edin. New PMl. Journ. vol. Iv. p. 138, 1853 ; and Lyell's Students' Elements, 

 p. 577 ; also Prestwich's Geology, vol. i. p. 264. 



2 Rev. 0. Fisher on Cleavage and Distortion, Geol. Mag. Decade III. "Vol. I. 

 p. 400, line 5, 1884. 



