CHANGES OF POSITION AND LEVEL. 721 



explanation of the facts mentioned. Elie de Beaumont and some 

 other geologists have attributed these effects, and especially the 

 elevation of mountains, to the contraction of a cooling globe, — 

 the last of the above-mentioned causes ; and this appears to be 

 the only one adequate for the results. 



The facts observed correspond precisely with the effects of the 

 cause mentioned ; and it is hardly necessary for those readers who 

 have in mind the structure of the Appalachians as it has been ex- 

 plained, to enter here into details. On page 410 it is shown that 

 the force in the case of the Appalachian region acted in a direction 

 from the Atlantic Ocean, — that is, at right angles to the axial direc- 

 tion of the folds. It follows, therefore, that the subsiding area 

 which determined the formation of the folds and the uplifts was 

 beneath the Atlantic Ocean. 



3. Flexibility of rocks. — It is a fact recently established that there 

 is scarcely any material so solid that when in broad tabular masses 

 it will not become flexed under a heavy pressure very gradually 

 applied. By " very gradually" should be understood that degree of 

 extreme slowness which has so often been exemplified in geological 

 history, and which is the most common of nature's methods of 

 progress. The rock or other solid, though apparently inflexible, 

 will undergo, under such conditions, a molecular movement, adapt- 

 ing it to its new condition. Even brittle ice, as stated on p. 673, 

 becomes flexed by its own weight if a slab be supported by only one 

 end. There is no doubt that if ice covered a lake to a thickness 

 of a dozen or more feet, and a slowly-accumulating pressure to a 

 sufficient amount could be brought to bear against one side of it, 

 the ice might be plicated over its surface as boldly and numerously 

 as the formations of the Appalachians. 



Fractures have usually been produced in the course of the flexing 

 of the earth's crust ; a violent exertion of pressure under such cir- 

 cumstances would naturally produce them on a grand scale ; but 

 they are not an inevitable result of the process of plication. If the 

 rocks were moist, — as has been the case during these upturnings, — 

 the plication would take place the more readily. If they were 

 heated also, and if by this means they were penetrated by super- 

 heated water or steam, the mobility of the particles would be still 

 greater, and they might even have, as observed on p. 708, a degree 

 of plasticity. 



In general, the arenaceous and argillaceous beds which have been 

 folded were not at the time firmly consolidated, but derived their 

 consolidation from the heat which escaped from below during the 

 progress of the movement, and which was the cause of metamor- 



47 



