GEOGRAPHY AND EVOLUTION. 197 



commonly accepted, as in accordance with observed facts. It leads 

 to the conclusion that the hollows on the surface of the globe occu- 

 pied by the ocean, and the great areas of dry land, were original 

 irregularities of form caused by unequal contraction ; and that the 

 mountains were corrugations, often accompanied by ruptures, caused 

 by the strains developed in the external crust by the force of central 

 attraction exerted during cooling, and were not due to forces directly 

 acting upward generated in the interior by gases or otherwise. It 

 has recently been very ably argued by Mr. Mallet that the phenomena 

 of volcanic heat are likewise consequences of extreme pressures in the 

 external crust, set up in a similar manner, and are not derived from 

 the central heated nucleus. 



There may be some difficulty in conceiving how forces can have 

 been thus developed sufficient to have j^roduced the gigantic changes 

 which have occurred in the distribution of land and water over im- 

 mense areas, and in the elevation of the bottoms of former seas so 

 that they now form the summits of the highest mountains, and to 

 have effected such changes within the very latest geological epoch. 

 These difficulties in great measure arise from not employing correct 

 standards of space and time in relation to the phenomena. Vast 

 though the greatest heights of our mountains and depths of our seas 

 may be, and enormous though the masses which have been put into 

 motion, when viewed according to a human standard, they are insig- 

 nificant in relation to the globe as a whole. Such heights and depths 

 (about six miles) on a sphere of ten feet in diameter ^vould-be repre- 

 sented on a true scale by elevations and depressions of less than the 

 tenth part of an inch, and the average elevation of the whole of the 

 dry land (about one thousand feet) above the main level of the surface 

 would hardly amoxxnt to the thickness of an ordinary sheet of paper. 

 The forces developed by the changes of the temperature of the earth 

 as a whole must be proportionate to its dimensions ; and the results 

 of their action on the surface in causing elevations, contortions, or dis- 

 ruptions of the strata, cannot be commensurable with those produced 

 by forces having the intensities, or by strains in bodies of the dimen- 

 sions, with which our ordinary experience is conversant. 



The difficulty in respect to the vast extent of past time is perhaps 

 less great, the conception being one with which most persons are now 

 more or less familiar. But I would remind you that, great though 

 the changes in human affairs have been since the most remote epochs 

 of which we have records in monuments or history, there is nothing 

 to indicate that within this period has occurred any appreciable modi- 

 fication of the main outlines of land and sea, or of the condition of 

 climate, or of the general characters of living creatures ; and that the 

 distance that separates us from those days is as nothing when com- 

 pared with the remoteness of past geological ages. No useful ap- 

 proach has yet beeu made to a numerical estimate of the duration 



