286 



SCIENCE. 



[N. S. Vol. VIII. No. 192. 



20° from the tropics, rising above the trop- 

 ical belt, is the temperate plateau, and far- 

 ther up the subalpine snow-clad summits 

 of Popocatapetl, Orizaba and other lofty 

 peaks. So in the Appalachians of the 

 Paleozoic the cryptogamous forests and 

 their animal life may have been confined 

 to the coastal plains and lowlands, while 

 on the higher, cooler levels may have ex- 

 isted a different assemblage of life ; and it 

 is not beyond the reach of possibility that 

 a scanty subalpine flora peopled the cooler 

 summits. 



But the unceasing process of atmospheric 

 erosion and river action continued through 

 the Jurassic, which was, as stated by Scott, 

 in his Introduction to Geology, ' a time of 

 great denudation, when the high ranges of 

 the Appalachian mountains were much 

 wasted away, and the newly upheaved, 

 tilted and faulted beds of the Trias were 

 deeply eroded.' At about the time of the 

 opening of the Cretaceous the range was 

 reduced to a peneplain (the Cretaceous pene- 

 plain), with only vestiges of once lofty 

 mountains ; the scenic features roughly re- 

 calling those of North Carolina and New 

 England at present, although more subdued 

 and featureless, more like the Kittatinny 

 peneplain of the Piedmont district at the 

 eastern base of the Blue Ridge to-day as 

 contrasted with the present mountain re- 

 gion of Pennsylvania and New Jersey. 

 There were also extensive changes in the 

 interior. What was the Colorado island 

 was added to the mainland, and a great 

 Mediterranean sea extended from the Uinta 

 mountains of southeastern Wyoming to 

 New Mexico ■ and Arizona, and stretched 

 from the Colorado peninsula westward to 

 Utah. In the upper Jurassic as the result 

 of a depression a gulf was formed over 

 northern Utah, Wyoming and southern 

 Montana (Scott). 



The formation of this Cretaceous pene- 

 plain was succeeded by a re-elevation, and 



the surface which is now Virginia was 

 gradually raised to a height of 1,400 feet, 

 and again the sluggish rivers of the Creta- 

 ceous times were revivified, cutting through 

 the harder strata forming the walls of the 

 longitudinal valleys and, widening into 

 broad estuaries, emptied into the Atlantic. 



In the Eocene Tertiary, as Willis tells us : 

 " The swelling of the Appalachian dome 

 began again. It rose 200 feet in New 

 Jersey, 600 feet in Pennsylvania, 1,700 feet 

 in southern Virgijiia and thence southward 

 sloped to the Gulf of Mexico." In conse- 

 quence of the renewed elevation the streams 

 were revived ; and Willis adds : " Once 

 more falling swiftly they have sawed, and 

 are sawing, their channels down, and are 

 preparing for the development of a future 

 base-level."* 



We can in imagination see, as the result 

 of these widespread physical changes, in- 

 ducing as they must have done the for- 

 mation of separate basins or areas enclosed 

 by mountain ranges, with different climates 

 and zones on land, however uniform might 

 have been the general temperature of the 

 world at that time and the other physical 

 conditions of the sea — ^we can imagine the 

 profound and deep-seated influence thus 

 exerted on the life-forms peopling the un- 

 even surface of the land. 



The vegetation of the lowlands was rich 

 and luxuriant, as the Triassic (Newark), 

 coal deposits near Richmond testify, and, 

 while the uplands and hills were probably 

 clad with dense forests of conifers, on the 

 dryer desert areas of the peneplain the trees 

 may have been more scanty, like the scat- 

 tered pines of the dryer elevated region of 

 the Southwest and of the Great Basin at 

 the present day. The distribution of the 

 animal life must have corresponded ; one 

 assemblage, especially the amphibians, 

 characterizing the hot and humid lowlands ; 



■^Quoted from Scott's Introduction to Geology, p. 

 342. 



