~74 .J. BARRELL MEASrRE.\rENTS OF GEOLOGIC TIME 



era, marked by the absence of marine sediments on the lands, eonstitutiiv-r 

 the Lipalian era of Waleott. Even in the Lower Cambrian deeply warpe I 

 geusynclines retained tlie waste, in [)art as continental deposits, and not 

 until the Tipper Cambria ii were tlie broad expanses of tbe continents c:n- 

 ered by the shallow seas. 



Although the seas' came and went during the Paleozoic era in periodic 

 oscillation, the thickeiiing anrl spreading mantle of sediments marks the 

 whole Paleozoic in contrast to the Lipalian as an era of higher mean sea- 

 level. Over the greater part of the continents erosion of a crystalline 

 I^asement had ceased, and over even the xVrchean shields it was very slight. 

 Erosion was periodically rejuvenated in mountain regions and doubtless, 

 occasionally and locally, attained rates as high as the present. The aggre- 

 gate rate for the whole world, however, must have l)eeu very low as com- 

 pared to the present. 



The long ("-rescendo of orogenic movements which ended in the Permian 

 revolution gave rise to great mountain systems, but did not materially 

 (devate the continents, as 'sho\Ani by the preservation of broad mantles of 

 Pennsylvanian and Permian sediments. Beginning with the Epi-Paleo- 

 zoic interval, however, epeiric seas played a less conspicuous part and 

 show by this restriction a somewhat lowered mean sealevel. Triassic ma- 

 I'ine deposits are scanty; the Jurassic seas acquired a Avider spread, but in 

 the Cretaceous the comlitions resembled again those of the Paleozoic. 



Ai Uw close of the Cretaceous another great revolution was inaugu- 

 rated — the Laramide — and the average elevation of the lands may have 

 resembled that of the close of the Paleozoic. But with the opening of the 

 Xeocene the earth^s intei'ual forces reawoke to great activity and tlie pres- 

 ent grander mountain systems of the earth began to take their form. Tlie 

 ^^Teocene revolution is so close to the Laramide, compared to the length of 

 the older eras, that the latter revolution may be regarded as a preliminary 

 stage leading up to a crescendo, much as the Pennsylvanian movements 

 preceded and led up to the greater Permian movements. The magnitude 

 of the Neocene revolution is seen, however, not only in the breadth and 

 height of the mountain , systems, but in the pronounced warping of the 

 continents, giving a steeper hypsographic curve, and the drawing down of 

 the sea, probably through continental fragmentation, gi^'ing a more elc- 

 \ated hypsographic cur\e. This great height of the continents is a mos^ 

 impol'tant feature bearing on thfe aggregate rate of denudation. There 

 appears to be nothing analogotis this side of the Precambrian. The Neo- 

 cene revolution has continued into the Pleistocene and there is no indica- 

 I ion that the culnunation has yet been passed. 



liookiii'.'r iX^ond tlic coming nnd o'oiuu' of seas winch mark ofp the 



