42 THE POLAR GLACIERS. 



of coal, if there are that many divided off by marine deposits 

 of considerable thickness, would have consumed 1,250,000 years. 

 The average thickness of all the strata that lie above the Old 

 Red Sandstone is not far from two miles. But this formation is 

 itself, in many places, two miles thick. And the lower Primary 

 systems will add at least ten miles to the vertical measure of the 

 fossil-bearing rocks. It is estimated that " the f ossilif erous beds 

 in Great Britain, as a whole, are more than 70,000 feet in thick- 

 ness;" and many that are there wanting, or nearly so, elsewhere 

 expand into beds of immense depth. There are certainly fifteen 

 miles deep of strata to be accounted for the slow accretions of 

 the ages mainly ocean-sediment that has come down from the 

 wear and washings of the solid rocks. It would be by no means 

 a bold assumption to say that 20,000,000 years had elapsed since 

 the eozoon first built its reefs in the warm Laurentian seas. 



