Date of the Glacial and the Upper Miocene Period. 375 



a sensuous representation of time ; the mind thus becomes deeply 

 impressed with a sense of immense duration; and when one 

 under these feelings is called upon to put down in figures what 

 he believes will represent that duration, he is very apt to be de- 

 ceived. If, for example, a million of years as represented by 

 geological phenomena and a million of years as represented by 

 figures were placed before our eyes, we should certainly feel 

 startled. We should probably feel that a unit with six ciphers 

 after it was really something far more formidable than we had 

 hitherto supposed it to be. Could we stand upon the edge of a 

 gorge a mile and a half in depth that had been cut out of the 

 solid rock by a tiny stream, scarcely visible at the bottom of this 

 fearful abyss, and were we informed that this little streamlet was 

 able to wear off annually only ^ of an inch from its rocky bed, 

 what would our conceptions be of the prodigious length of time 

 that this stream must have taken to excavate the gorge ? We 

 should certainly feel startled when, on making the necessary cal- 

 culations, we found that the stream had performed this enor- 

 mous amount of work in something less than a million of years. 



If we could possibly form some adequate conception of a 

 period so prodigious as one hundred millions of years, we should 

 not then feel so dissatisfied at being told that the age of the 

 earth's crust is not greater than that. 



Here is one way of conveying to the mind some idea of what 

 a million of years really is. Take a narrow strip of paper an inch 

 broad, or more, and 83 feet 4 inches in length, and stretch it along 

 the wall of a large hall, or round the walls of an apartment some- 

 what over 20 feet square. Recall to memory the days of your 

 boyhood, so as to get some adequate conception of what a 

 period of a hundred years is. Then mark off from one of the 

 ends of the strip T V of an inch. The -jV of the inch will then 

 represent one hundred years, and the entire length of the strip 

 a million of years. It is well worth making the experiment, 

 just in order to feel the striking impression that it produces 

 on the mind. 



The methods which have been adopted in estimating geologi- 

 cal time not only fail to give us the positive length of geo- 

 logical periods, but some of them are actually calculated to mis- 

 lead. The method of calculating the length of a period from 

 the thickness of the stratified rocks belonging to that period can 

 give no reliable estimate ; for the thickness of the deposit will 

 depend upon a great many circumstances, such as whether the 

 deposition took place near to land or far away in the deep re- 

 cesses of the ocean, whether it took place at the mouth of a 

 great river or along the sea-shore, whether it took place when 

 the sea-bottom was rising, subsiding, or remaining stationary. 



2C2 



