DYNAMICAL GEOLOGY. 317 



resolved into tangential and radial tensions, which give effect 

 to horizontal and vertical displacements. Under horizontal 

 displacements Suess describes folds, anticlinal domes, over- 

 thrusts, and lateral shifts effected by dip-faults. The vertical 

 displacements are evidenced by subsidence or inthrows, and 

 they are accompanied by numerous fissures and faults, which 

 may again be sub-divided into peripheral, radial, diagonal, and 

 transversal faults. The nature of the subsidence in dislocated 

 segments of the earth's crust determines the arrangement of 

 the faults as limiting-lines of crust-basins, crust-troughs, 

 flexures, or table-lands. The combination of a subsiding 

 and tangential movement gives origin to specially complicated 

 tectonical appearances, such as the development of fore-folds 

 and back-folds. 



Suess regards volcanoes only as slight and superficial 

 indications of important phenomena in the nuclear mass of 

 the earth. He describes a number of examples showing the 

 gradual denudation and partial disturbance of volcanoes, and 

 establishes a " series of denudation forms " intended to prove 

 that there is no fundamental difference between the volcanic 

 explosions and ejections of the present time, the massive flows 

 of earlier periods, and the laccolites and deep intrusions of the 

 oldest periods. The fissures and dykes of active and extinct 

 volcanoes are carefully discussed, also the dislocations caused 

 by earthquakes. 



After these preliminary chapters, Suess makes a compara- 

 tive investigation of the mountain-systems of the earth, and an 

 attempt to discover their geological history from their tec- 

 tonical structure. To the geologist the subject is opened out 

 with unflagging interest. Beginning with the Northern Sub- 

 Alpine area, Suess emphasises the obstructive influence which 

 had been exerted by the mountain-ranges of Central Europe, 

 the Sudeten mountains, and the Russian plateau. These 

 resisting crust-blocks had for the most part successfully 

 stemmed back the advancing Alpine folds, or in the case of 

 the Sudeten and a part of the Russian plateau, the northward 

 crust-creep had carried the Carpathian folds partially over the 

 ancient mountain-masses. 



Suess elucidates the direction of strike of the dominant 

 folds in the Alpine system, and his description of the cur- 

 vature and whirl-shaped arrangement of the leading lines 

 of strike has thrown an entirely new light upon Alpine geology. 



