SOME ASPECTS OF GEOLOGIC TIME 25/ 



put forward. We have several historical instances of advance 

 and recession of the sea. Winchelsea was a port in Norman 

 times. Hudson Bay is disappearing at a measurable rate. 

 Estimates of geologic periods, on lines like this, are, at any rate, 

 based on events that actually occurred. They may vary, but 

 they can only do so within reasonable limits. When we have 

 no idea, or a false idea, and can only be guided by the maximum 

 thickness of sediment, estimates may vary to any degree. 



I mention coal beds for two reasons. In the first place they 

 represent the most important of the few strata, which are, for 

 commercial purposes, actually bored. Borings for purely 

 scientific investigation are far too costly to be undertaken on 

 a large scale. Consequently, in the mapping of most strata, the 

 geologist must confine himself to the outcrops. Such a method 

 does quite well for the tracing of the strata of the larger epochs, 

 but it is very doubtful how far it would suffice for mapping out 

 small beds. The borings in the coal fields are already made, 

 and a suggestion such as this will not present insuperable diffi- 

 culties. The second reason is to put a doubtful or disputed 

 point, in one specific instance, beyond the range of controversy. 

 If we have two successive coal beds of known large area, with a 

 layer of shale in between, there can be no possible doubt, grant- 

 ing that the beds were formed in situ, of an advance and a 

 recession of the sea. That such events have continually taken 

 place in the ordinary strata, I thoroughly believe. That even 

 the maximum thicknesses were formed intermittently with con- 

 siderable intervals of emergence from the sea masking the great 

 epochal submergence, is a fixed opinion of my own. But proof, 

 as a general rule, is not easy. Fortunately, the structure and 

 arrangement of coal beds make the speculation, for certain times 

 and conditions, a certainty. 



As the science of geology progresses, and as more and more 

 detailed facts are discovered, new methods will come to light, 

 and such suggestions as these will be trite and obvious. There 

 is, in the study of the rocks, a wealth of material which requires 

 only careful and intelligent study to solve many problems now 

 obscure. But such careful study will not be the work of 

 a day. 



Until the science of geology attains greater clearness and 

 exactness, some other lines of investigation may assist in giving 

 a clue to the order of the result. One of these is found in the 



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