GEOLOGY — WILLIS. 20I 



they are long, narrow upwarps. The trend of these flexures of the 

 surface is parallel in a general way to the trend of folds in the Eocene 

 and Miocene strata, across which the surface is eroded. The rela- 

 tions suggest a renewal of compression after an interval of erosion, 

 and should be closely investigated to determine just what may be the 

 validity of the suggestion. 



Although in many instances the old topographic surface may be 

 observed sinking into and rising from the sea, there are other slopes, 

 such as that of Monte Maggiore at Abbazia and of the island of 

 Cherso, opposite, which exhibit features of erosion characteristic 

 of a river valley. These may be provisionally interpreted as the 

 valleys of antecedent streams which originated on the old peneplain 

 and maintained their courses across a rising upwarp until, in the 

 course of later movements, the valleys were submerged in conse- 

 quence of the general subsidence. 



There is another phenomenon of the Croatian district which should 

 be discussed in connection with warping. It is the prevailing occur- 

 rence of the ' ' karsts ' ' or extensive sinks in the limestone district 

 and the disappearance of large rivers in subterranean channels. A 

 typical case is that of the Gac-ko, east of Zengg. There is here a 

 large basin, probably 15 miles or more across, which appears to be 

 a downwarp encompassed on all sides by upwarps. The upwarp 

 which lies between it and the coast was observed in the ascent to the 

 Vratnik Pass, and it was inferred that warping had proceeded with 

 such energy as to subject the river to diversion to underground 

 channels that originated in the fractured limestones of the district 

 with the sinking of the ground- water level. 



In general this extremely interesting region may be said to show 

 the nearest approach yet observed to a genetic relationship between 

 highlands which originated with the folding of the strata and moun- 

 tains produced by warping ; yet even here there are evidences of an 

 epoch of erosion, which intervened between the two episodes of 

 deformation and in which the cycle reached advanced maturity or 

 even old age. If we attempt in a general way to place these events 

 in the geologic time scale, we may assign the folding to the late 

 Miocene or early Pliocene, the erosion to the greater part of the 

 Pliocene, and the warping to the latest episodes of that period. 



The Northern Apennines. — Opportunities to observe the Apennines 

 were had between Bologna and Florence, in the vicinity of Florence, 

 and in the valley of the Serchio between the Apennines and the 

 Carrara Mountains. The large features are simple. On the north 

 is the great downwarp of the Po Valley, which is deeply aggraded, 



