PALEOZOIC ROCKS AND ERA. 3H 



clay full of roots and its roof-shale full of impressions of 

 leaves and flattened branches, etc. 



3. Coal has been accumulated at the mouths of 

 rivers, and therefore subject to overflows and deposits of 

 mud by the river, and to occasional incursions by the sea. 

 This is proved by the alternation of river-sand and clay 

 with marine deposits of limestone. 



It may be difficult to put these three propositions to- 

 gether and form a clear picture of the precise manner of 

 accumulation, and therefore there is still a large field for 

 the play of fancy. 



Estimate of Length of the Coal Period. 



If the sands and clays of a coal-field have been accu- 

 mulated by river-deposit, then we have a means of making 

 a rough approximate estimate of the time embraced by 

 the Coal period. It is true, agencies may have acted 

 then at a different rate from now, but our estimate will 

 be liberal. 



For this purpose we take the Nova Scotia coal-field, 

 because the evidence of river-deposit is very strong in 

 every part. It has been estimated that there were not 

 less than 54,000 cubic miles of river sediment in the 

 original field. Now, the Mississippi River at present ac- 

 cumulates one twentieth cubic mile per annum, and 

 would therefore take twenty years to accumulate one 

 cubic mile, and 1,080,000 years to accumulate 54,000 

 cubic miles. But, as already said (page 298), the Coal 

 period is but a small fraction, certainly not more than 

 one twentieth to one thirtieth, of the recorded history of 

 the earth. Therefore, this recorded history can not be 

 less than twenty to thirty millions of years. It is proba- 

 bly much more. We only give this estimate in order 

 to accustom the mind to the great periods of time with 

 which geology deals. 



