86 



SCIENCE 



[Vol. LV, No. 1413 



of the Paleocene Period. The deformation 

 "vras again renewed in the Miocene period and 

 in a milder form still more recently. From 

 the btdged and wrinkled tract thus prepared, 

 the Rocky Mountains, as we know them to- 

 day, have' been carved by the combined agencies 

 of erosion. 



By contrast, on the eastern slope of North 

 America, periods of relative stability in the 

 crust alternated with these compressive dis- 

 turbances, from a remote pre-Cambrian time 

 down to about the close of the Paleozoic era. 

 Since then the region has been comparatively 

 quiet, and its history is chiefly the story of the 

 erosion of the surface, modified from time to 

 time by slight and gentle uplifts and subsi- 

 dences of broad areas. 



These are examples of a great geologic prin- 

 ciple. All regions of the globe have had sim- 

 ilar periods of disturbance alternating with 

 those of quiet, but as to time and intensity they 

 have differed among themselves. In the earlier 

 days of geology these crumpling distm'bances 

 were usually ascribed to the cooling and con- 

 sequent shrinkage of the globe. Other 

 hypotheses are now in the field, and the eon- 

 test is stUl in full swing. Whatever their 

 cause, we do not find much evidence of a gen- 

 eral decrease in the activity of such internal 

 earth forces. On the contrary, the middle 

 Tertiary revolution was one of the most wide- 

 spread and intense of which we have any rec- 

 ord. There have been many fluctuations but 

 no general trend. , 



Volcanic action likewise varies in intensity, 

 and transfers its activity from one region to 

 another from period to period; generallj^ ac- 

 companying but locally ignoring the diastro- 

 phic disturbances. Yet, on the whole, the vol- 

 canic activity of the last two geologic periods 

 is equal in intensity and widespread distribu- 

 tion to anything known to us in earlier 

 periods. 



The demolishing of the hills and mountains 

 by streams, winds, glaciers, and the other ero- 

 sive agencies proceeds most actively during the 

 periods of crumpling. At such times broad, 

 flat-bottomed valleys are planed off only on 

 the outcropts of the softest rocks. The wear- 

 ing away of the harder masses proceeds at a 



much slower pace. Before such plains can be 

 extended over the more resistent outcrops, 

 the heaving and sagging of the uneasy earth 

 interrupt the process and rejuvenate the ei*o- 

 sion cycle. 



Eventually the internal unrest subsides; the 

 body movements vii'tually cease for a long pe- 

 riod until renewed in the oncoming of the next 

 orogeny. In this period of relative quiet, the 

 erosive agencies are able to reduce to compara- 

 tive flatness not only the outcrops of the soft, 

 but also the harder rocks, until peneplains of 

 broad extent have been produced. Only the 

 hardest masses and those farthest removed 

 from the main drainage channels continue to 

 stand out as mountains. The early Cambrian 

 and the Jurassic periods saw peneplains ex- 

 tended over most parts of North America of 

 which there is sufficient record. Meanwhile, 

 the adjacent seas were gradually being filled 

 with the detritus swept from the land, and 

 were in consequence creeping out upon the 

 continental platforms in the form of shallow 

 seas. 



To-daj- we evidently are living in the midst, 

 or perhaps near the close, of one of the les- 

 ser periods of internal disturbance. For this 

 reason our Inroad plains of erosion, such as the 

 Mississippi Basin, Central Russia and north- 

 west Siberia are limited for the most part to 

 the outcrops of soft rocks. Nevertheless, we 

 appear to have, even to-day, in central Can- 

 ada and eastern Siberia ancient peneplains, on 

 hard rocks, which liave been so little dusturbed 

 and dissected that they still retain in large 

 measure their distinctive character. 



For a hundred years or more paleontologists 

 have been gathering and assembling informa- 

 tion regarding the kinds of animals and plants 

 represented by the fossils in the rocks. From 

 them we learn the distribution of these plant, 

 and animal societies both in space and time. 

 When enough information has been gathered 

 regarding the distribution of the marine sedi- 

 ments and fatmas of a particular age, we are 

 in a position to map, at least roughly, the sea 

 and land areas of the time. Such maps have 

 been made within the last few years in con- 

 siderable variety and for more than one of the 

 continents. They are admittedly crude approx- 



