EARTH SCIENCES IN NINETEENTH CENTURY 673 



have taken in the past century are those by which the necessity and 

 the value of theorizing have gained frank acceptance among investi- 

 gators, and by which many of the resuks of theorizing have gained 

 an order of verity that compares well with that of facts of mere 

 observation. 



An illustration of this phase of our progress is to be found in two 

 definitions, each of which has a certain currency. By some writers, 

 geology is defined as the study of the earth's crust, thus emphasizing 

 the observational side of the subject; by others, geology is defined as 

 the study of the earth's history, thus giving fuller recognition to the 

 growth of inference upon observation. The second definition does 

 not lessen the essential importance of observation as the foundation 

 of knowledge, but it accords a proper value to inferences, and in this 

 way is characteristic of what seems to me sound scientific progress. 

 The earth's crust contains the incomplete, partly concealed, partly 

 undecipherable records on which we are to construct the science of 

 geology; just as human monuments and writings are the records on 

 which we are to construct human history; but in neither case are the 

 records and the history identical, for the history in both cases includes 

 a great body of inferences as well as of more directly recorded or 

 observed facts. 



The wholesome appeal to observation in the search for visible facts 

 has loosened the control of supposed authority and has given us much 

 of the freedom necessary for progress ; but the assistance of the trained 

 imagination in the search for invisible facts has in a far greater 

 degree corrected the assumptions of an earlier stage of inquiry; it 

 has even revised the dicta of philosophy and remodeled the dogmas 

 of religion. 



The inferential element of our progress has worked most benefi- 

 cently. It is largely through our inferences that we have come to 

 recognize the interdependence of the different parts of earth science. 

 The climatologist may remain as provincial as he wishes; or he may 

 enter through the gateway of present conditions the vast domain of 

 past time, and on the way make friends with all the world; for he 

 will then join hands with the petrographer, who has evidence of ancient 

 desert conditions in the form of the grains in certain sandstones; 

 and with the paleontologist, who infers the existence of ancient ocean 



