DYNAMICAL GEOLOGY 303 



said they were separated from one another by somewhat 

 crooked fault-lines which ran in approximately parallel 

 directions, or diverged at various angles of bifurcation from a 

 main chain. In the case of the principal chains, the highest 

 fault-blocks were those on the western side of fault-lines, and 

 the mountain-curves were convex towards the west. Speaking 

 generally, Thurmann distinguished in the Jura mountains a 

 zone of the highest chains, a central zone of uplift, and a 

 slightly-folded plateau zone. From the whole structure of the 

 Jura, he finally concluded, in opposition to his earlier views, 

 that the chains had not taken origin as vertical uplifts, but 

 that lateral forces had acted from the Swiss side and had 

 compressed the strata along parallel folds. 



One of Thurmann's chief tenets was the long continuation 

 of the plastic state in sedimentary deposits. He held that 

 sediments remained plastic long after their deposition and 

 during the processes of mountain-formation, and he therefore 

 differentiated sharply between faulting, bending, crushing, 

 and shearing movements effected while the sediments were 

 still fairly plastic, and movements of adjustment accomplished 

 after the mountains had been formed. He contested the 

 hypothesis that rock already consolidated was reduced to a 

 molten or plastic condition by the processes of mountain- 

 making. 



While Elie de Beaumont and Thurmann were building up 

 their theories of mountain-upheaval upon field observations, 

 the English physicist, Hopkins, was trying to solve the 

 problem upon theoretical grounds, and one of his doctrines 

 is specially worthy of note. From his consideration of the 

 pressures exerted by explosive gases, vapours, and other 

 subterranean forces upon the crust, he concluded that in 

 almost all cases of crust-fracture two systems of faults must 

 take origin at right angles to each other, and must then be 

 fundamental directive lines during the formation of continents 

 and mountain-systems. 



Constant Prevost, in his report on the Island Julia 

 (ante, p. 264), contested the theory of Elevation-Craters, and 

 in opposition to Elie de Beaumont regarded the origin of 

 mountain-systems and continents only as results of slow 

 sagging of the crust, or of occasional inthrows when one side 

 of the fissure was pressed outward and the rock-material was 

 stemmed against it. Much later, similar ideas were enter- 



