THE PAST AND FUTURE OF GEOLOGY. 177 



dence so often brought forward, and wliicli liad in vain sought examina- 

 tion, respectiug the antiquity of man, was confirmed and admitted by 

 geologists. 



Such was the aspect of our science at the time this chair was estab- 

 lished ; and I propose in this address briefly to notice some of the larger 

 features, whether on questions of theory or on questions of fact, by 

 which its progress has been marked, and which, while they may serve 

 to show how much has been done, will yet indicate how much still 

 remains to be accomplished. 



The geologist commences where the astronomer ends. We have to 

 adapt the large and broad generalizations of cosmical phenomena to 

 the minuter details of terrestrial structure and constitution which it is 

 our business to study. The common origin of the solar system has been 

 long inferred from the spheroidal figure of the earth and the relations 

 of the planets to one another, and explained by evolution from an 

 original nebulous mass ; and geologists have had to consider how far 

 such a hypothesis is in accordance with geological facts. The ques- 

 tions connected with the earliest stages of the earth's history are on the 

 very boundary-line of our science, but they have too important a bear- 

 ing on its subsequent stages not to comumnd our serious attention ; 

 and thongii obscure and theoretical, they serve to guide us to firmer 

 ground. This nebular hypothesis has recently received from ])hysicists 

 corroboration of a most novel and striking character, equally interesting 

 to geologists and astronomers. 



The wonderful discoveries with respect to the solar atmosphere, made 

 by means of the spectroscope, have now presented us with an entirely 

 new class of evidence, which, taken in conjunction with the argument 

 derived from figure and plan, gives irresistible weight to the theory of 

 a common origin of the sun and its planets ; and while serving to con- 

 nect our earth with distant worlds, indicates as a corollary what of 

 necessity must have been its early condition and probable constitu- 

 tion. 



The whole number of known elements composing the crust and atmos- 

 phere of the earth amounts only to sixty-four, and their relative distribution 

 is vastly disj)roportionate. It has been estimated that oxygen in com- 

 bination forms by weight one-half of the earth's crust, silicon enters for a 

 quarter, then follow aluminium, calcium, magnesium, potassium, sodium, 

 iron, and carbon. These nine together have been estimated to consti- 

 tute -^JJjj of the earth's crust. The other j^j?_ consist of the remain- 

 ing fifty-five non-metallic and metallic elements. 



The researches of Kirchhotl", Angstroni, Thalen, and Lockyer have 

 made known that of these sixty-four terrestrial elements there are 

 twenty i)resent in tliose parts of the solar atmosphere called the 

 "chromosphere" and "reversing layer,'' as the stratum which sur- 

 rounds the photosphere is called from certain optical properties. 

 S. Mis. 115 12 ^ 



