Henry Woochvard — Address to the Geologists' Association. 539 



ation. Taking an ideal cube of tbe original unsqueezed mass, the 

 average ratio (according to the observations of Sorby and Haughton) 

 of the greatest diameter {i.e. in the plane of cleavage) to the least 

 diameter (or at right-angles to the cleavage) becomes, after squeezing, 

 as six to one. Suppose then a mass of sediments 10,000 feet thick 

 subjected to horizontal pressure and crushing sufficient to develope 

 well-marked cleavage-structure: a breadth of 2|- miles would be 

 crushed into one mile, and 10,000 feet thickness would be swelled to 

 25,000 feet, making an actual elevation of the surface of 15,000 feet. 



Thus •' the phenomena of plication and of slaty cleavage demon- 

 strate a crushing together horizontally, and an upswelling of the whole 

 mass of sediments ; slaty cleavage proves, in addition, that the up- 

 swelling produced by this cause alone is sufficient to account for the 

 elevation of the greatest mountain-chains in the world." 



Le Conte's essay has not only added many original and well- 

 directed ideas upon this important subject, but it also embraces the 

 earlier observations of Scrope, Hersehel, Dana, Sedgwick, Lyell, 

 Sterry Hunt, Studer, Hall, etc. His article deserves the careful con- 

 sideration of geologists.^ 



But there is another phenomenon presented by mountain-ranges 

 which it is necessary to account for, — namely, the evidence which 

 many of them present of repeated oscillations of elevation and 

 depression going on for immense periods of time, and extending over 

 areas of many thousands of square miles. 



Such earth-movements which cannot be referred to the secular 

 cooling and contraction of the outer belt must be, says Mr. Poulett- 

 Scrope, attributed to the irregular transmission of heat by conduction 

 from one part of the subterranean matter to another, its isothermal 

 planes, and consequently its expansion and contraction, varying with 

 the changes in the conductivity of the overlying rocks. 



Before I leave this subject, I think it may be well to suggest a 

 caution as regards the reception of one proposition which has been 

 most ably advocated, both in this country^ and also in America,^ in 

 reference to areas of deposition. It is that the " slowly progressing 

 subsidence over areas of accumulation loas occasioned hij tJie weiglit 

 of the slowly and successively accumulated sediments." There must 

 be a fundamental error in this proposition; for if 500 feet thickness 

 of moist sediment have power by gravitation to depress the sub- 

 jacent crust of the ocean-bed, how much more must the compact and 

 solid mass of the Himalayas — covering a length of 1550 miles and a 

 breadth of 620 miles, with its 380 peaks upwards of 20,000 feet 

 above the sea and two nearly 30,000 feet high — sink down into the 

 yielding crust beneath ! 



In a paper on " Volcanos,"* communicated by me to this Society, 



1 See Dana's and Silliman's American Journal of Science, 1872, vol. iv. no. 23, 

 p. 345. no. 24, p. 460 ; 1873, vol. v. no. 30, p. 448. 



2 " On Subsidence the Effect of Accumulation," by Dr. Charles Eicketts, P.G.S. 

 Geol. Mag. 1872. Vol. IX. p. 119. 



3 Prof. James Hall, Palaeontology of New York. 



* " On Volcanos," by H. "Woodward, Proc. Geol. Assoc. 1871, vol. ii. p. 6, fig. 1 ; 

 and Geol. Mag. 1871, Vol. YIII. p. 338. 



