38 SCIENCE PROGRESS 



nor can we assign any date to it. Possibly its formation was 

 an event so gradual that the beginning was spread over 

 immense periods. We can only trace the history back to 

 certain events which may with considerable certainty be 

 regarded as ushering in our geological era. 



Notwithstanding our limitations the date of the birth-time 

 of our geological era is the most important date in Science. 

 For in taking into our minds the spacious history of the 

 universe, it must play the part of time-unit upon which all our 

 conceptions depend. If we date the geological history of the 

 earth by thousands of years, as did our forerunners, we must 

 shape our ideas of planetary time accordingly; and the duration 

 of our solar system, and of the heavens, becomes comparable 

 with that of the dynasties of ancient nations. If in millions 

 of years the sun and stars are proportionately venerable. If 

 in hundreds or thousands of millions of years the human mind 

 must consent to correspondingly vast epochs for the duration 

 of material changes. The geological age plays the same part 

 in our views of the duration of the universe as the earth's 

 orbital radius does in our views of the immensity of space. 

 Lucretius knew nothing of our time-unit : his unit was the 

 life of a man. So also he knew nothing of our space-unit, and 

 he marvels that so small a body as the sun can shed so much 

 heat and light upon the earth. 



A study of the rocks shows us that the world was not 

 always what it now is and long has been. We live in an epoch 

 of denudation. The rains and frosts disintegrate the hills ; and 

 the rivers roll to the sea the finely divided particles into which 

 they have been resolved ; as well as the salts which have been 

 leached from them. The sediments collect near the coasts of 

 the continents ; the dissolved matter mingles with the general 

 ocean. The geologist has measured and mapped these de- 

 posits and traced them back into the past, layer by layer. He 

 finds them ever the same : sandstones, slates, limestones, etc. 

 But one thing is not the same. Life grows ever less diversified 

 in character as the sediments are traced downwards. Mammals 

 and birds, reptiles, amphibians, fishes, die out successively in 

 the past ; and barren sediments ultimately succeed, leaving the 

 first beginnings of life undecipherable by him. Beneath these 

 barren sediments lie rocks collectively differing in character 

 from those above : mainly volcanic or poured out from fissures 



