GEOLOGIC TIME. 303 



the doctrine of geologic catastrophes and rendered possible a rough 

 computation of the age of the eartliou the principle that geologic proc- 

 esses were the same daring geologic time as at the present. Before 

 this effort the scientist and theologian (with tlie exception of Huttou 

 and his followers) vied with each other in their attempts to harmonize 

 the Mosaic record with that of nature; they exjianded the seventeenth 

 century views of the former and contracted the inductive reasoning 

 from geologic phenomena, aiid called in the aid of special creations, 

 great catastrophes and other unusual phenomena. This was cleared 

 away among geologists by Lyell's work, supplemented later by the 

 general law of evolution which, since the appearance of Darwin's Origin 

 of Species, has with a few and rare exceptions controlled and directed 

 scientific work and thought in this direction. 



Lyell based an argument for the age of the sedimentary rocks as 

 known to him on the rate of modification of the species of mollusca 

 since the beginning of the " Cambrian i)eriod."' He divided the geo- 

 logic series into twelve periods and estimated that 20,000,000 years 

 were demanded for a complete change in the species of each period, or 

 240.000,000 years in all. This estimate excluded the Primoidial of Bar- 

 raudeand the ''antecedent Laurentian formations.*'* 



Darwin. — In the chapter summing up the imperfections of the geo- 

 logical record Darwin concludes that, if his theory of the origin of 

 species is true, '■'■ it is indisputable that before the lowest Cambrian 

 stratum was deposited, long periods elapsed, as long as, or probably 

 far longer than, the whole interval from the Cambrian age to the i)res- 

 entday; and that during these vast periods the world swarmed with 

 living creatures." When mentioning the opinions of various authors 

 on the duration of geologic time, he indirectly gives his own views, as 

 follows : 



"Mr. Croll estimates that about 60,000,000 years have elapsed since 

 the Cambrian period, but this, judging from the small amount of organic 

 change since the commencement of the Glacial epoch, appears a very 

 short time for the many and great mutations of life which have cer- 

 tainly occurred since the Cambrian formation; and the previous 

 140,000,000 years can hardly be considered as sufficient for the devel- 

 opment of the varied forms of life which already existed during the 

 Cambrian period. It is however probable, as Sir William Thompson 

 insists, that the world at a very early period was subjected to more 

 rapid and violent changes in its ])liysical conditions than those now 

 occurring; and such changes would have tended to induce clianges at 

 a corresjjonding rate in the organisms which then existed."! 



Houghton. — Eev. Samuel Houghton, in commenting on the geological 

 calculus, states that he believes that the time during which organic life 

 has existed on the earth is practically infinite. On the basis of the time 

 of cooling he assigns an age of 1,280,000,000 years for the Azoic period 

 and remarks that the globe was habitable, in part at least, for a longer 



* Principles of Geology, 10th ed., 1867, vol. i, p. 301. 



\Orig'm of Species, American cil., from 6tli Eug. ed., 1882, p. 286. 



