OF LAKE-BASINS. 653 



which science could suggest was that, within a comparatively modern 

 period, a period closely trenching upon the time when man made his 

 appearance upon the face of the earth, the Himalayahs had been thrown 

 up by an increment closely approaching 8,000 or 10,000 feet. 1 



II. — Extracts from Letters by Dr. Falconer in the 'Reader' for 

 February and March, 1864. 



Terrestrial mechanical phenomena are susceptible of mathematical 

 analysis on mechanical principles. A very eminent mathematician, 

 and one of the most distinguished physical geologists of our time, 

 Mr. Hopkins, has undertaken the mathematical investigation of the 

 problem of mountain elevations, viz., ' to determine the nature of the 

 effects produced by a general elevatory force acting at any assigned 

 depth on extended portions of the superficial crust of the earth, and 

 with sufficient intensity to produce in it dislocations and sensible 

 elevations.' Suppose a given area, bounded by parallel lines, and in- 

 definitely long as compared with the width, to be subjected to an 

 expansive subterraneous force, acting simultaneously upon every point, 

 and sufficient to bear up the superincumbent mass, the strata will at 

 first rise in the form of an arch, limited by their extensibility, and at 

 length give way to the strain. The resultant fractures will take place 

 simultaneously in two directions ; those of the first class, parallel to the 

 line of greatest extension (the strike) ; the second, at right angles to the 

 first, i.e. across the strike. Local and partial causes — such as ' irre- 

 gularity in the intensity of the elevatory forces, and in the constitution 

 of the masses on which they are supposed to act' — -will cause the effects 

 to deviate from strict geometrical laws. 



Such, broadly and very imperfectly put, is the mechanical theory of 

 Mr. Hopkins, which, so far as it has gone, has been regarded by mathe- 

 maticians as a great step in the dynamics of geology, resembling that 

 which affected astronomy when mathematical reasoning and calculation 

 were directed to it ' at the turning point of its splendid career.' (Dr. 

 Whewell, 'Anniversary Address,' G.S., 1838, p. 27.) Practically ap- 

 plied, it has been accepted by geologists as remarkably in unison with 

 the phenomena of mineral veins in a definite known area ; and the 

 special application of it to the phenomena of the Weald and the Bas 

 Boulonnais by Mr. Hopkins has been regarded as a very successful 

 illustration of the truth of the theory. ' The remarkable breaks in the 

 bounding chalk-ranges which give passage to the rivers flowing from 

 the Wealden northward and southward are shown to correspond in 

 situation with cross fractures, indicated by the theory, and sometimes 

 rendered probable, and occasionally proved, by observation.' (Professor 

 Phillips's ' Manual,' 1855, p. 598.) Mr. Hopkins has not yet given to 

 the world the extension of the mechanical theory to the phenomena of 

 great mountain-chains like the Himalayahs, where angular elevation, so 

 to speak, is what we now see; but he distinctly admits: 1. That 

 ' along the flanks of elevated ranges longitudinal valleys are not unfre- 



1 See vol. i. p. 177 to 185. The views 

 above expressed led to an animated dis- 

 cussion in the ' Reader.' The extracts 



which follow aro taken from Dr. Fal- 

 coner's letters which appeared on that 

 occasion. — [Ed.].. 



