256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



ing crystal was greater than it is now, and hence probably that a 

 metope of uranium then existed with possibly very different proper- 

 ties from the uranium now known to us. 



If Professor Joly's conclusions are sound, it is clear that the ura- 

 nium clock has not been keeping uniform time, and the change of rate 

 in the disintegration of uranium is as much in question as the age 

 of the earth. The problem is a physical one, and geologists must 

 leave it in the hands of the physicists while anxiously awaiting its 

 solution. 



It would not be fair to end here without admitting, what Prof. J. 

 W. Gregory's remarks will sufficiently reveal, that geologists are not 

 an undivided family. There are some who welcome the expansive 

 vistas now opened to their view, and Barrell has already attempted 

 to readjust the geological perspective. He pointed out how the cal- 

 culations of the earth's age, based on the thickness of deposits and 

 the existing rate of deposition, as well as those based on the amount 

 of sodium in the ocean, may be vitiated by a too servile interpretation 

 of the doctrine of uniformity. The rate of disintegration of uranium 

 may have changed, but so may the rate of denudation and deposition ; 

 so far from being constant, it may have increased with the progress 

 of time, so that a foot of sediment which in the Pleistocene epoch ac- 

 cumulated, according to Barrell, in the course of 375 years would 

 have required no less than 3,700 years for its formation in the early 

 days of the Paleozoic era. Thus at a period when the earth was more 

 highly charged with energy its activities were diminished. We must 

 no longer picture a time when the earth was " young and wantoned 

 in her prime," but must supposa that she has exchanged the passive 

 indolence of youth for the fiery activity of old age. 



In support of his views Barrell pointed out that the continents of 

 the present day are more elevated as a whole than they were during 

 a great part of geological time, and that their interior is not flooded 

 to so great an extent by continental seas. It is doubtful, however, 

 Avhether this would greatly affect those estimates which have been 

 based on the miximum thickness of sedimentary deposits, for this is 

 only to be found in the foredeeps which lay in front of mountainous 

 lands and lands now vanished from our sight. 



Barrell also laid great stress on the occurrence of gaps in the 

 stratified series, unconformities, disconformities, and still smaller 

 lacunae which he termed diastemata. Of the important bearing 

 which unconformities must have upon this discussion there can be 

 no doubt. They were not overlooked in arriving at an estimate of 

 100,000,000 years. The disconformities are only now beginning to 

 receive the attention to which their importance entitles them. In 

 our own country we are familiar with them in the Jurassic system. 



