CONTRIBUTION OF THE SCIENCES 237 



astronomic history of the earth antecedent to the record presented by the 

 lowest or earliest known strata. 



Tlie astronomic period of our earth's history is a subject for investiga- 

 tion by astronomer, physicist, and geologist. As yet the results of studies 

 in this region are in large part of a speculative nature. The field fur- 

 nishes one of the most attractive opportunities in science for further 

 investigation. Although this phase of the problem has in it very much 

 of fascination, the contributions are as yet of such a nature as to con- 

 tribute little toward the objects of the present discussion. I shall there- 

 fore refer to geologic history only in terms of the distinct record extend- 

 ing to the lowest known strata in the second chapter of the account. 



The length of the period which remains after elimination of the earlier, 

 or astronomic, period may be very short, measured against the total ago 

 of the earth. We know that the lowest strata, wherever we find them, 

 rest upon rocks which have been molten and in their molten state have 

 destroyed the basement upon which the oldest known stratified rocks now 

 rest. We admit, therefore, that not only have we lost the record before 

 the earliest strata were formed, but that the earliest strata themselves 

 have disappeared. The record remaining is, however, by no means brief 

 in terms of human understanding. Few recent estimates have suggested 

 that the section comprises less than two hundred thousand feet of strata, 

 or that the time involved measures less than one hundred million years. 

 This time may not be long compared to the entire age of the earth, and 

 may not be more than a moment compared with the age of our solar 

 system, but it furnishes all that we require for purposes of interpretation 

 of human history. 



Eeduced to their simple terms, the geological data of the stratified 

 record give us a history of the accumulation of sediments, of movements 

 of the earth's crust, of the making of continents and ocean basins, of 

 volcanic activities, and of climatic changes. In this evidence is included 

 a history of the forming of mountains, and the wearing down of moun- 

 tains and continents by the processes of erosion to form the sediments 

 spread over great basins of accumulation. 



This history presents, as its first significant lesson, the fact of the 

 instability of the earth's crust and the evidence that throughout geologic 

 time, as we know it, the surface has shown diversity of form dependent 

 upon vertical movements of large magnitude. By offering the opportunity 

 for erosion forces to act, the movements which have produced continents 

 and mountain ranges have also been responsible for accumulation of the 

 sediments washed into the deeps to form the strata from which our record 

 is read. Also intimatelv related to tlie succession of crustal movements 



