202 REPORTS ON INVESTIGATIONS AND PROJECTS, 



yet from which, in the vicinity of Padua, a few isolated monadnocks 

 rise. Near Bologna I observed a peneplain, diversified with many 

 monadnocks, which, rising from beneath the plain of the Po, ascends 

 in a broad, flat arch to the summit of the mountains, whence it 

 descends toward the southwest to the valley of the Arno. Near 

 Florence the old topographic surface is depressed and buried beneath 

 lake sediments of late Pliocene age. The down warp is but about 8 

 miles across at the level of the plain, and near Sigua the surface 

 rises again in the upwarp, across which the Arno has cut its lower 

 valley. A study of the gorge below Signa showed all the features 

 of an antecedent valley of erosion, and the peneplain was observed 

 to sink beneath the coastal plain toward the west. 



The valley of the Serchio corresponds with a similar depression 

 between the two upwarps of the Carrara Mountains and the Apen- 

 nines, which here reach altitudes of about 6,000 feet. This down- 

 warp also contains sediments of late Pliocene age, while the tributary 

 valleys and mountain spurs are characterized by remnants of old 

 valley floors that mark stages in the progress of erosion during the 

 process of warping. 



For a nice analysis of these phenomena a further study of the 

 work of Italian geologists is needed, but the observed facts form a 

 consistent body of evidence, which leaves no doubt that, after the 

 Eocene and Miocene strata had been folded, they were extensively 

 eroded to a lowland, above which there still rose isolated hills or 

 groups of hills that maintained altitudes of several hundred feet. 

 This cycle of erosion was interrupted by pronounced warping, to 

 which the present mountains are due, and the geologic dates assign- 

 able for the several steps of development fall within the Pliocene 

 period, as do those of Istria. 



The Alps. — The districts to which my observations were otherwise 

 confined were non- glaciated, and consequently presented compara- 

 tively simple topographic features. The Alps are physiographically 

 and structurally among the most difficult regions of the world , and they 

 have been so carefully studied that exhaustive work alone is worth 

 while. Nevertheless, in passing through them, I made observations 

 which lead me to think that they are peculiar among the other ranges 

 having a similar structural history only in the intensity of the phe- 

 nomena. The special features which characterize mountains of the 

 Alpine class in Europe are folds and overthrusts, which have thrown 

 strata of the early and middle Tertiary into very complex relations with 

 older rocks. In interpreting these facts European geologists have 

 relied wholly upon assumptions in stratigraphy and structure and 



