28H A CENTURY OF GEOLOGY. 



periods are iisiuilly times of prevalence of limestones or organic sedi- 

 ments, and the fossils are very a))undant. The provincial periods are 

 usually characterized by sandstones and shales or mechanical sedi- 

 ments, and are comparatively poor in fossils. Moreover, it is believed 

 that the cosmopolitan limestone perjods are oceanic periods — i, e., 

 periods of wide oceans and lower and smaller continents and little 

 erosive activity, while the sandstone periods, characterized b}" provin- 

 cial faunas, are periods of higher and larger continents, and therefore 

 of great erosion and abundant mechanical sedimentation. 



Now. according to Chamberlin, these remarkalile alternations are due 

 to oscillations of the crust, in which the continents are alternately 

 lifted and depressed. It nuist be remem})ered that abyssal faunas are 

 almost unknown among fossils. This is the necessary result of sub- 

 stantial permanenc}" of oceanic basins. The whole geological record 

 is in shallow- water faunas. These shallow^ waters are along continental 

 shon^ lines andin interior continental seas. According to Chamberlin 

 again, during a period of continental depressit)n all the flat continental 

 margins ai'e submerged, forming broad submarine platforms, and the 

 lower interior portions of the continents are also sn})merge(l, forming 

 widt' and shallow interior seas. Under these conditions continental 

 waste, and therefore sand and clay sediments, are reduced to a mini- 

 mum. Life, animal and vegetal, abounds, and therefore nuich lime- 

 stone is formed. The oceans are wideh' connected with one another, 

 and therefore the faunas are widespread or cosmopolitan. During the 

 period of elevation, on the contrary, the continents are extended to 

 the margin of the deep oceanic l)asins, the broad, shallow, submarine 

 platforms are abolished, the interior seas are also abolished, the 

 shallow-water areas are reduced to isolated bays, and their faunas are 

 peculiar or provincial. Also, elevated and enlarged continents give 

 rise to maximum erosion, and therefore abundant sediments of sand- 

 stone and clay, and comparative poverty of life and therefore of lime- 

 stone. Chamberlin also gives reasons why the oceanic periods should 

 be warm, humid, equable in temperature, and the atmosphere highly 

 charged with CO^, and therefore highly favorable to abundant life, 

 both vegetal and animal, while land periods would ])e drier and cooler, 

 the atmosphere deficient in CO.^. and therefore cold from that cause 

 and in many ways unfavorable to abundant life. 



These extremeh' ijiteresting views, however, must be regarded as 

 still on trial, as a provisional hypothesis to be sifted, continued, or 

 rejected, or in any case moditied, in the next century. 



Lastly, it is interesting to note the ever-increasing part taken by 

 American geologists in the advance of this science. There has been 

 through the century a gradual movement of what might })e called the 

 center of gravity of geological research westward, until now, at its 

 end, the most productive activity is here in America. This is not due 



