300 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



a short passage towards the close of this treatise. Brief 

 although they are, the remarks on the influence of the slow 

 cooling of the earth on surface conformation and the origin of 

 furrows and fissures, are at once recognised by a reader of the 

 present day as the starting-point of our modern views on 

 mountain-structure. Favourable reviews by Brongniart and 

 Arago helped to spread the fame of the young geologist, and 

 to win rapid recognition for his work. 



It was not until 1852 that Elie de Beaumont discussed the 

 details in full, and gave expression to his conceptions in his 

 three-volume work On Mountain- systems. He points out that 

 in virtue of the continued cooling of our planet the radius is 

 shortened and the crust is affected by a general centripetal 

 movement. Delesse had calculated 1,340 metres as the amount 

 by which the earth's radius had already been shortened; in 

 other words, the earth's crust in the course of the geological 

 epochs had approached the earth's centre by a distance about 

 equal to the height of Chimborazo or the Himalayas above 

 sea-level. As the more rigid crust tried to subside and accom- 

 modate itself to the contracting molten mass of the nucleus, 

 inequalities and excrescences formed ; or if the tension became 

 too great a sudden rupture of the crust ensued, and the lateral 

 compression gave origin to mountain-folding. The rock-masses 

 in seeking relief from the crust-strains were pressed upward, 

 and might under certain circumstances pierce the surface as 

 a finger might pass through a button-hole. This, in Elie de 

 Beaumont's opinion, was the explanation of the fact that granite 

 masses so often form the summits and ridges of mountain- 

 chains, whose flanks consist of uplifted sedimentary rocks. 

 The latter, he said, were covered towards the base of the 

 mountain for the most part by gently-inclined or horizontal 

 strata, which spread over the neighbouring plains. The 

 inclined strata often strike sharply against the horizontal 

 layers, any marked contrast in the position of the neighbour- 

 ing series of deposits indicating that after the deposition 

 of the uplifted strata, and before the deposition of the undis- 

 turbed series, a convulsion of the earth's crust had taken 

 place in that region and had culminated in the uplift of the 

 mountain-chain. The exact geological period of the crust- 

 paroxysm could be determined from a comparison of the ages 

 of the inclined strata and the horizontal layers reposing upon 

 them. 



