1869.] GEOLOGY AND MINERALOGY. 75 



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 wall of an apartment 

 somewhat 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 

 end of a strip ro of an inch. The ro 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. 

 Stratified formations 10,000 feet in thickness, for example, 

 may, under some conditions, have been formed in as many years, 

 while under other conditions it may have required as many 

 centuries. Nothing whatever can be safely inferred as to the 

 absolute length of a period from the thickness of the stratified 

 formations belonging to that period. Neither will this method 

 give us a trustworthy estimate of the relative lengths of geo- 

 logical periods. Suppose we find the average thickness of the 

 Cambrian rocks to be 26,000 feet, the Silurian to be 28,000 

 feet, the Devonian to be 6000 feet, and the Tertiary to be 

 10,000 feet, it would not be safe to assume, as is sometimes 

 done, that the relative duration of those periods must have cor- 

 responded to these numbers. Were we sure that we had got 

 the correct average thickness of all the rocks belonging to each of 

 those formations, we might probably be able to arrive at the 

 relative lengths of those periods; but we can never be sure of 



