84 



SCIENCE 



[Vol. LV, No. 1413 



even some hints about its birth and antece- 

 dents. 



The biologic phase of the subject over- 

 shadowed the physical in the minds of the his- 

 torical geologists of the nineteenth century so 

 largely that we now know more about the char- 

 acter and sequence of organic forms than about 

 other phases of the Earth's history. Coui-ses 

 in historical geology have generally been 

 taught by paleontologists, and the history of 

 animals and jDlants fills most of the pages de- 

 voted to historical geology in textbooks and 

 manuals. In the last few decades, however, 

 more and more attention has been given to the 

 nature and sequence of the climates, physio- 

 graphic changes, volcanic outbursts and other 

 physical events; and our knowledge of this 

 phase is now growing so rapidly that it bids 

 fair to overhaul and pass the earlier wave of 

 biological study. To appreciate the truth of 

 this statement, it is only necessary to scan 

 the list of papers that have been published in 

 recent years on such subject as unconformities, 

 sedimentation, erogenic epochs, paleogeog- 

 raphy, physiographic history, glacial epochs, 

 and the like. 



The study of the physical history of the 

 Earth assumes new aspects from decade to 

 decade, as a question here and there is an- 

 swered and other problems arise out of the 

 interaction of newer ideas. One of the com- 

 monest questions which geologists are asked is 

 — "How old is the earth?" and "How many 

 years have elapsed" since this or that event in 

 its history took place? The question has been 

 attacked from many angles, and although exact 

 calculations are not yet and perhaps never will 

 be possible, we may have increasing confidence 

 in the general character of the returns. 



Counting the growth-rings in the big trees 

 of California shows that some of those giant 

 gi-owths are more than 4,000 years old, and 

 j-et no marked change has taken place in the 

 surface of the land on which they stand since 

 they sprouted from the soil. 



Baron De Geer, after a most painstaking 

 study of the laminated glacial clays of north- 

 western Em-ope, seems to have compiled a 

 definite record of seasonal changes of deposi- 

 tioa covering some six or seven thousand 



years, dui'ing which certain post-glacial lakes 

 were being filled with deposits of silt. 



Less accurate estimates of geologic time 

 have been based on recessions of Niagara 

 Falls and other cataracts. These problems are 

 usually complex and the factors not all well 

 known. Hence, estimates of the number of 

 years since the ice sheet left Niagara vary all 

 the way from 7,000 to 50,000 yeai-s. Never- 

 theless, this gives a general order of magni- 

 tude which is of value. The mean of the two 

 extremes is probably not very far from the 

 truth. 



The most complete part of oui- geologic rec- 

 ord is the composite pile of strata of the sedi- 

 mentary rocks. Many attempts have been 

 made to translate the thickness of these beds 

 into terms of years, but our knowledge of the 

 rate at which sediments accumulate under va- 

 rious conditions is still far too inaccurate. We 

 know rates in certain isolated instances, but 

 tliey vary v/ithin wide limits, and we do not 

 yet understand the factors that govern the 

 variations. There is good reason to believe, 

 however, that the rate of deposition generally 

 has been overestimated, and hence the periods 

 of time have been made too short. 



Within the last fifty years, the various esti- 

 mates which have been made of the time rep- 

 resented by the Paleozoic, Mesozoic and Ceno- 

 zoic eras have varied between three million 

 years for the lowest and six thousand million 

 years for the highest. The average of twenty 

 such estimates is about 250 million years, but 

 most of the individual figures are about 100 

 million or less. 



On the whole, from decade to decade there 

 has been a disposition to lengthen the esti- 

 mates, as the testimony of the stratigraphic 

 column became better rmderstood. In 1893, 

 Walcott allowed 45 to 70 million years for 

 the three eras. In 1899 Geikie estimated 100 

 million years. In 1913, Holmes calculated an 

 average of about 300 million years for the 

 same group of periods, and, in 1916, Barrell 

 suggested 360 to 540 million years as the best 

 estimate based on the sedimentary deposits. 

 This tendency to recognize more clearly the 

 immense duration of the various geologic pe- 

 riods has been due largelj' to a growing un- 



