GEOLOGY — WILLIS. 199 



controlled by the fault which occasioned a diversion of the river from 

 its pre- Pliocene course southeastward beyond Milanovac. 



The gorge of the Iskar exhibits features of an antecedent valley 

 cut across an upwarp, features closely resembling those of the cor- 

 responding part of the Danube Canyon ; and the mountains about the 

 basin of Sophia descend to the plain by sloping surfaces produced by 

 warping that are distinguishable from sharply cut surfaces produced 

 by erosion under existing conditions. 



In general, observations throughout the journey across Hungary, 

 Servia, and Bulgaria led to the opinion that upwarps and downwarps 

 may be traced out on a large scale ; and in their relations will be 

 found the key to the later Tertiary and Quaternary history. The 

 structural history of the mountains is earlier Tertiary or older. 



The Bosphonis and Dardayiellcs. — The vicinity of Constantinople 

 is characterized by an extensive plain, a peneplain, which has been 

 recognized in its true character by Cvijic, Phillipson, and Davis. 

 An appropriate name for it is the Pera Plain. It is eroded across 

 folded Palezoic and Eocene strata and coincides with the surface of 

 deposition of late Tertiary limestones. Rising gently from the Sea 

 of Marmora, beneath which it is submerged, it reaches an altitude of 

 200 meters above Therapia and Bojukdere, whence it descends more 

 steeply into the Black Sea. The upwarp which the Pera Plain thus 

 presents is crossed by the Bosphorus in a valley which exhibits all 

 the features of a valley of erosion. Inspection of the slopes shows 

 that the valley developed as the antecedent stream cut down into the 

 rising upwarp. The present submergence is a later event. 



East of the Bosphorus, in Asia Minor, the surface corresponding 

 to the Pera Plain bears numerous and extensive monadnocks. The 

 hills ro to 20 kilometers southeast of Haida Pasha appear to rise 300 

 meters above the plain, and the Princes Islands are submerged features 

 of similar magnitude. We traced this Pera surface as far east as 

 Ismid and southwest to the Dardanelles. Our observations were dis- 

 connected, but the physiographic features are consistent through- 

 out the region. The Sea of Marmora is a downwarp in which the 

 monadnocks form islands. Its eastern bays, the Gulfs of Ismid and 

 Gemlik, are extensions of the general depression between which lies a 

 very strongly developed upwarp, forming a high mountain range, of 

 which the higher summits are probably upraised monadnocks. The 

 Dardanelles, like the Bosphorus, have the character of an antecedent 

 valley, eroded by a flowing stream and now submerged ; but the 

 course of the stream was less directly across the upwarp than was 

 that of the Bosphorus river. The phenomena are complicated by 



