Date of the Glacial and the Upper Miocene Period. 377 



It is far too short to be used with safety in determining the dis- 

 tance of periods so remote as those which concern the geologist. 

 But even supposing the palaeontologist had a period of sufficient 

 length measured off correctly to begin with, his results would 

 still be unsatisfactory ; for it is perfectly obvious, that unless 

 the climatic conditions of the globe during the various periods 

 were nearly the same, the rate at which the species change 

 would certainly not be uniform. But we have evidence, geolo- 

 gical as well as cosmical, that the climate of our globe has at 

 various periods undergone changes of the most excessive cha- 

 racter. 



The palseontological method, as we have already seen, will 

 give 60 millions of years or 240 millions of years as the period 

 that has elapsed since the commencement of the Cambrian pe- 

 riod, just as we choose to adopt 250,000 years ago or 1,000,000 

 years ago as the commencement of the glacial epoch. 



It is the modern and philosophic doctrine of uniformity that 

 has chiefly led geologists to overestimate the length of geological 

 periods. This philosophic school teaches, and that truly, that 

 the great changes undergone by the earth's crust must have been 

 produced not by great 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, and chemical action, 

 &c. It teaches that the valleys were not produced by violent 

 dislocations, nor the hills by sudden upheavals, but that they 

 were actually carved out of the solid rock by the silent and 

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

 water. It teaches, in short, that the rocky face of our globe 

 has been carved into hill and dale, and ultimately worn down to 

 the sea-level, by means of these apparently trifling agents, not 

 only once or twice, but probably dozens of times over during 

 past ages. Now, when we reflect that with such extreme slow- 

 ness do these agents perform their work, that we might watch 

 their operations from year to year, and from century to century, 

 if we could, without being able to perceive that they make any 

 very sensible advance, we are necessitated to conclude that 

 geological periods must be enormous. And the conclusion at 

 which we thus arrive is undoubtedly correct. It is, in fact, im- 

 possible to form an adequate conception of the length of geolo- 

 gical time. It is something too vast to be fully grasped by our 

 conceptions. What those to whom we have been alluding err in 

 is not in forming too great a conception of the extent of geolo- 

 gical periods, but in the way in which they represent the length 

 of these periods in numbers. When we speak of units, tens, 

 hundreds, thousands, we can form some notion of what these 

 quantities represent ; but when we come to millions, tens of mil- 



