320 II . F. REID ISOSTACY AND EARTH MOVEMENTS 



reduced to a peneplain, and in the Tertiary they were raised into a broad, 

 low arch, and were carved by erosion to their present topography. A 

 simple consideration of the strength of rock shows that such an arch, tens 

 of miles in span, could not be supported by its ends, and therefore could 

 not have been raised by simple tangential pressure against its ends, but 

 must have been raised by vertical forces. The Rocky Mountains were 

 reduced to a peneplain after the Cretaceous revolution. Tbe distance 

 from the sea makes it probable that this peneplain was originally a few 

 thousand feet above sealevel ; but its present elevation is eight to nine 

 thousand feet, an elevation evidently due to vertical forces. The great 

 monocline and the steep faults limiting the range along the Colorado 

 front show the action of vertical forces. 



The Andes were elevated long after the strata were folded. The major 

 folding was apparently pre-Cretaceous ; although some folding occurred 

 later, it was not important. From early Tertiary time until well into 

 the Pliocene the area stood at only moderate altitudes and suffered a 

 general baseleveling. Transgressions of the sea occurred in several parts 

 of South America in Eocene times and reached the flanks of the Andes 

 in Peru. Transgressions also occurred in Lower and Middle Miocene 

 times. The Lower Miocene flora on parts of the coast of Peru is iden- 

 tical with that of the Amazon basin, indicating free communication be- 

 tween the regions, with no intervening mountains more than about 6,000 

 feet high. There may have been some elevation in the later Miocene, but 

 fossil Pliocene floras found at altitudes of 11,500 to 13,500 feet, and 

 which could not have flourished at greater altitudes than 5,000 to 6,000 

 feet, show moderate altitudes in that period. The late Tertiary pene- 

 plain, standing now at an altitude of about 15,000 feet, must have been 

 brought to maturity at not more than a few thousand feet above sealevel. 

 The very slight difference between the present fauna and flora of the 

 humid tropical western coast of Peru and the Amazon basin shows that 

 an effective barrier has existed between these regions for only a short 

 time, geologically speaking. There is a great scarp, still 4,000 feet high, 

 in eastern Peru which marks a normal fault limiting the range, and enor- 

 mous quantities of Pliocene and Pleistocene detritus cover both flanks of 

 the eastern Andes in Bolivia. 



All the facts prove that the elevation of the chain has been very recent, 

 whereas we have seen that the important folding was pre-Cretaceous. 9 



The Catskills have a base of intensely folded Silurian strata, on which 

 lie unconformably only slightly disturbed marine Devonian, and probably 



Personal communication from Prof. E. W. Berry. Seo also Bowman : The Andes of 

 southern Peru. 



