1869.] GEOLOGY AND MINERALOGY. 77 



that has elapsed since the commencement of the Cambrian 

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

 years ago as the commencment 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 dis- 

 locations, nor the hills by sudden upheavals, but that they were 

 actually carved out of the solid rock by the sileiit and goutle 

 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 tlie 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 slowness 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, impossible to form an 

 adecjuate conception of the length of geological time. It is some- 

 thing 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 geological 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 millions, hundreds of 

 millions, thousands of millions, the mind is then totally unable to 

 follow, and we can only use these numbers as representations 

 of quantities that turn up in calculation. We know, from the 

 way in which they do turn up in our process of calculation, 

 whether they are correct representations of things in actual 

 nature or not; but we could not, from a mere comparison of 

 these quantities which the thing represented by them, say 

 whether they were actually too small or too great. It is hero 



