GEOPHYSICAL RESEAECH — DAY. 361 



In a word, the field has been given a tliorougli general examination, 

 but the manifold problems which this examination has developed, 

 although early recognized, and often the subject of philosophical 

 speculation and discussion, still await an opportunity for quanti- 

 tative study. They are often problems for the laboratory and not 

 for the field, problems for exact measurement rather than for infer- 

 ence, problems for the physicist and chemist rather than for the 

 geologist. This is not a result of oversight; it is a stage in the 

 development of the science — first the location and classification of 

 the material, then the laboratory study of why and how much. 



Certain indications have led us to believe, for example, that the 

 earth was once completely gaseous and in appearance much like 

 our sun. Indeed, it possibly formed a part of the sun, but through 

 some instability in the system became split off — a great gaseous 

 ball which has cooled to its present condition. The cooling probably 

 went on rapidly at first until a protecting crust formed about the 

 ball, then more and more slowly, until now, when our loss of heat 

 by radiation into space is more than compensated by heat received 

 from the sun. Obviously, the earliest portions of tliis history are 

 and must remain dependent upon inference, but the formation of a 

 soUd crust can not advance far before portions of it become fixed 

 in a form such that further disturbance does not destroy their 

 identity. From this point on, the history of the earth is a matter 

 of record and can be interpreted if only we have sufficient knowledge 

 of the mineral relations through all the stages of their development. 



It must have been a very turbulent sea, the molten surface of 

 our earth upon which the rocky crust began to form. The first 

 patches of crust were probably shattered over and over again by 

 escaping gases and violent explosions of wliich our waning volcanic 

 activity is but a feeble echo. If the eartli was first gaseous, and the 

 outer surface gradually condensed to a licjuid, its outer portions at 

 least must have been whirled and tumbled about sufficiently, even 

 in a few thousand years — which is a very small interval in the forma- 

 tion of an earth — to mix its various ingredients pretty thorougldy. 

 It has accordingly been hard to see just how it came to separate into 

 individual rocks of such widely different appearance and character. 

 Of course the number of its ingredients was large. We have already 

 discovered 80 or more different elementary substances in the earth, 

 and there is an almost endless number of more or less stable com- 

 pounds of these. The freezing of an earth is therefore different from 

 the freezing of pure water, but the freezing of salt water offers a clue 

 to the explanation of the way in which the earth solidified as we find 

 it. Wlien salt water freezes, the salt is practically all left behind. 

 The ice contains much less salt and the remaming water relatively 

 more salt than before freezing began. Applying this familiar obser- 



