DYNAMICAL GEOLOGY. 2O3 



atmosphere. The fresh surfaces in turn decompose, and the 

 cycle of chemical transformation and denudation goes on until 

 a land area acquires the particular aspect of erosion which the 

 eye has learned to associate with certain characters of rock, 

 and conditions of altitude, of meteorology, and of drainage. 

 The final phases of the work of denudation would be to reduce 

 a land surface to sea-level unless other circumstances con- 

 spired to prevent complete degradation of the land. 



Highly characteristic forms of weathering may be produced 

 in cases where certain portions of a sheet of rock are more 

 soluble than others, and become a more easy prey to the pro- 

 cesses of disintegration. Heirn has described the scenic effects 

 due to the weathering of the different kinds of rock-material 

 exposed in many of the mountain plateaux of the Alps. 

 Irregular, boldly-hewn outlines and sharp aiguilles are char- 

 acteristic forms in the crystalline masses composed of coarse- 

 grained granitoid rocks at the higher altitudes of the Alps; the 

 finely-serrated ridges with steep slopes and grassy hollows are 

 characteristic of the softer shales and clays, while the lime- 

 stone and dolomite mountains present alternating terraces and 

 prominent escarpments capped by picturesque summit forms; 

 in some cases, wide summit-plateaux have been rendered almost 

 j impassable by the innumerable petty pinnacles and ravines 

 I into which the rock has been weathered. Such summit- 

 | plateaux are known as " Karrenfelder." 



The precise origin of the "Karrenfelder" has long been 

 a matter of discussion. Among the earlier Alpine authors, 

 Scheuchzer and De Saussure attributed these limestone wastes 

 to the erosive action of occasional floods. Hirzel, who in 

 1829 introduced the term of " Karren," attributed them to 

 combined mechanical and chemical weathering acting upon 

 perpendicular limestone strata at a certain height above sea- 

 level. Among recent authors, Von Richthofen, Heim, Mojsi- 

 sovics, and many others explained the jagged and channeled 

 character of these high plateaux as in the main a chemical 

 effect, due to the action of rain-water containing carbonic 

 acid gas in solution, upon the lime carbonate of the rock. 

 Favre, on the other hand, associated the particular effect with 

 the mechanical operations of glacial water. 



Mojsisovics has described characteristic " Karrenfelder " in 

 Carniola like those in other limestone groups of the Alps, "and 

 has also observed funnel-shaped depressions on the surface of 



