ORIGIN AND AGE OF THE SUNS HEAT. 267 



It is true that the views which formerly prevailed 

 amongst geologists, in regard to the almost unlimited 

 extent of geological time, have of late undergone very 

 considerable modifications ; but there are few geologists, 

 I presume, who would be willing to admit that the 

 above period is sufficient to comprehend the entire 

 history of stratified rocks. 



It is the facts of denudation which most forcibly 

 impress the mind with a sense of immense duration, 

 and show most convincingly the great antiquity of 

 the earth. 



We know unquestionably that many of the greatest 

 changes undergone by the earth's crust were produced, 

 not by convulsions and cataclysms of nature, but by 

 those ordinary agencies that we see at work every 

 day around us, such as rain, snow, frost, ice, chemical 

 action, &c. Valleys have not been produced by violent 

 dislocations, nor the hills by upheavals, but both have 

 been carved out of the solid rock by the silent and 

 gentle agency of chemical action, frost, rain, ice, and 

 running water. In short, the rocky face of our globe 

 has been moulded into hill and dale, and ultimately 

 worn down to the sea-level by means of these apparently 

 trifling agents, not merely once or twice, but probably 

 dozens of times over during past ages. Now, when we 

 reflect that with such extreme slowness do these agents 

 perform their work that Ave might, if we could, watch 

 their operations from year to year, and from century 

 to century, without being able to perceive that they 

 make any sensible impression, we are necessitated to 

 conclude that geological periods must be enormous. 

 The utter inadequacy of a period of 20 million years 

 for the age of our earth is demonstrable from the 

 enormous thickness of rock which is known to have 

 been removed off certain areas by denudation. I shall 



