140 LECTURES 



least rendered extremely probable, that neither of these theorists is 

 in the right ; that, in fact, while the igneous agencies have been de- 

 creasing in activity, the aqueous have been constantly increasing in 

 the same proportion. As I believe I differ from all other geologists in 

 my views on this pointy I deem it important to go a little more fully 

 into this subject. 



It is generally admitted by geologists, and indeed there is good and 

 substantial evidence of the fact, that the earth has been gradually cool- 

 ing throughout all geological history from an original very high tem- 

 perature. We have also, as I believe most geologists will admit, good 

 and substantial evidence that the land has constantly increased both 

 in extent and in elevation with the course of time, while the ocean has 

 as constantly decreased in extent in the same proportion. In other 

 words, these two elements, land and water, have been, as it were, 

 gradually differentiated. Admit these two points and all the rest 

 logically follow. 



The activity of igneous agencies depends upon the internal temper- 

 ature of the earth. As this has constantly decreased the igneous 

 agencies have also decreased in energy in the same proportion. The 

 aqueous agencies, on the other hand, are the result of currents of air 

 and water upon the surface of the earth ; and the rapidity of these 

 currents depends, not upon the mean surface temperature, but upon 

 the difference of temperature in different parts of the surface ; *. e. , 

 between pole and equator or between land and water. It only remains 

 to prove, then, that this difference of temperature has been constantly 

 increasing with the course of time. 



Land, as is well known, is both a better absorber and a better radi- 

 ator of heat than water ; i. e. , will both heat faster and cool faster 

 under given circumstances than water. A globe of land would be both 

 hotter at the equator and colder at the poles than a globe covered with 

 water and exposed to the same influences. Although the mean tem- 

 perature would be nearly the same in the two cases, the difference of 

 temperature would be much greater in the former than in the latter. 

 It follows, therefore, that as the extent of land increased and that of 

 the ocean decreased with the course of time the difference of tempera- 

 ture between pole and equator must have increased in the same proportion. 

 The gradual decrease of the mean temperature would evidently contri- 

 bute to the same result; for it is evident that with a higher mean temper- 

 ature a larger portion of water would exist in the form of vapor. This 

 excessive vapor would rise into the atmosphere and become condensed 

 into universal clouds, mist or fogs, but seldom, and to a very limited ex- 

 tent, in the earlier periods of the earth's history, into rat'w, because, as 

 yet, there were neither extensive high land nor cool currents sufficient 

 for extensive precipitation. Thus would result a thick, murky atmo- 

 sphere, enveloping the whole earth. The necessary effect of this would 

 be still further to prevent absorption of heat at the equator and radiation 

 at the poles, and thus to produce still greater uniformity of climate. 

 In the earliest geological periods, therefore, when the surface temper- 

 ature^ iv ova internal causes, v/as very great, and the ocean almost uni- 

 versal, the difference of temperature between pole and equator was 

 reduced to a minimum. In such a condition of things it is evident 



