TRANSACTIONS OP THE SECTIONS. 183 



circulating around it, that the whole solar system is formed of the same descrip- 

 tions of matter, and subject to the same general physical laws. These conclu- 

 sions further support the supposition that the earth and other planets have been 

 formed by the aggi-egatiou of matter once diflhsed in space around the smi ; that 

 the first consequence of this aggi'egation was to develop intense heat in the con- 

 solidating masses; that the heat thus generated in the terrestrial sphere was 

 subsequently lost by radiation ; and that the surface cooled and became a solid 

 crust, leaving a central nucleus of much higher temperature within. The earth's 

 surface appears now to have reached a temperature which is virtually fixed, and 

 on which the gain of heat from the sun is, on the whole, just compensated by the 

 loss by radiation into suri'ouuding space. 



Such a conception of the earliest stage of the earth's existence is commonly 

 accepted, as in accordance with observed facts. It leads to the conclusion that the 

 hollows on the surface of the globe occupied by the ocean, and the great areas of 

 di'y land, were original irregularities of form caused by unequal contraction ; and 

 that the mountains were coi-rugations, 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 upwards generated in 

 the interior by gases or otherwise. It has recently been very ably ai'gued by Mr. 

 Mallet that the phenomena of volcanic heat are likewise consequences of extreme 

 pressm-es 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 produced the gigantic changes which have occmTed 

 in the distribution of land and water over immense areas, aud in the elevation 

 of the bottoms of former seas so that they now fomi 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 gi'eatest 

 heights of our moimtains and depths of om* seas may be, and enormous though 

 the masses which have been put into motion, when viewed according to a human 

 standard, they are insignificant in relation to the globe as a whole. Such heights 

 and depths (about 6 miles) on a sphere of 10 feet in diameter would 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 1000 feet) 

 above the mean level of the siu'face would hardly amount to the thickness of an ordi- 

 nary 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 sm'face in causing elevations, contortions, or disruptions of the strata, 

 cannot be commensurable with those produced by forces having the intensities, or 

 by strains in bodies of the dimensions, with which om- ordinary experience is con- 

 versant. 



The difficulty in respect to the vast extent of past time is perhaps less gi'eat, 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 occun-ed any appreciable modifi- 

 . cation of the main outlines of land and sea, or of the conditions of climate, or of 

 the general characters of living creatures ; and that the distance that separates us 

 from those days is as nothing when compared to the remoteness of past geological 

 ages. No useful approach has yet been made to a numerical estimate ofthe diu-a- 

 tion even of that portion of geological time which is nearest to us ; and we can say 

 little more than that the earth's past history extends over himdi'cds of thousand's 

 or millions of years. 



The solid nucleus of the earth with its atmosphere, as we now find them, may 

 thus be regarded as exhibiting the residual phenomena which have resulted on its 

 attaining a condition of practical equilibrium, the more active process ol 

 aggregation having ceased, and the combination of its elements into the various 

 solid, liquid, or gaseous matters found on or near the surface having been completed. 

 During its passage to its present state many wonderful changes must have taken 



