3 12 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



form and the course of the folds into which the intervening 

 more yielding portions of the earth's surface have been thrown 

 by the tangential strains of contraction. While the first cause 

 of mountain-making is the secular cooling of the crust, the 

 precise form of a mountain-chain is subject to the modifying 

 conditions introduced by these ancient and resistant crust- 

 blocks or "archiboles." 



The above are some of the leading conceptions in the 

 remarkable work on mountain-structure published by Suess 

 in 1875, a d its great influence may be judged from the flood 

 of literature upon this subject which has poured forth since 

 that year. It is impossible to refer here to more than the 

 most important of these publications. 



After a long series of researches in a complicated district of 

 Switzerland, Professor Heim, in Zurich, published in 1878 

 his famous work, Untersuchungen iiber den Mechanismus der 

 Gebirgsbildung. The two geological maps and fifteen illus- 

 trative plates accompanying the text were lithographed by the 

 author himself. The scientific insight and technical skill 

 possessed by Professor Heim form a rare combination, and 

 have brought his views on mountain-structure wide popularity 

 and acceptance. 



Heim concentrated his attention on the tectonical pheno- 

 mena of folding. He depicted in the "Glarus Double- Fold " 

 an appearance which seemed contradictory to any doctrine of 

 mountain-movement, since on the north side of the central 

 Alps, where, according either to the conception of symmetry or 

 asymmetry of the chain the folds should have been towards 

 the north, Heim's observations showed that the major folds on 

 the north and south of the Glarus area had been overthrown 

 towards one another, and the upper portions had continued to 

 travel as "thrust-masses" advancing from opposite directions 

 towards one another. This was clearly inexplicable on the 

 assumption of a uniform direction of the horizontal movement 

 of the crust, and Heim concluded that the inclination of over- 

 cast folds depended upon local inequalities of resistance, upon 

 the presence of older folds as well as upon the relative height 

 of the two bases of origin on the opposite sides of any individual 

 fold. The second volume of Heim's work treats the general 

 problem of Mountain Architecture. Using his own field 

 observations as the ground-work of his discussion, he describes 

 the phenomena of rock-deformation during crust-movement 



