GEOLOGY OF MOUNT DESERT. 63 



were ascribed. The truth as it now appears lies between 

 these extremes. 



The geological dates of the intrusions and deformations 

 that the Island has suffered are even more indefinite than 

 the times of its denudation. Accepting the provisional 

 date of Cambrian for the lower members of the bedded 

 series, and late Cretaceous for the end of the denudation 

 of the peneplain, the deformations and intrusions must be 

 placed somewhere within the long intermediate interval. 

 This is like saying that a certain battle occurred some- 

 where between the time of the founding of Rome and the 

 invention of printing; but if its date had been previously 

 still less determinate than this, we should be glad enough 

 to have even so wide a limitation of its occurrence. It 

 is probable that comparisons of the structure of Mount 

 Desert with similar structures in other parts of New Eng- 

 land will in future suffice to set narrower limits to the 

 dates of several of the events whose time of occurrence is 

 now so loosely circumscribed. 



The Glacial Invasion. 



It was over a country thus made and unmade, over an 

 uplifted peneplain surmounted by isolated and clustered 

 Monadnocks, and dissected by newly etched valleys and 

 valley lowlands, that the ice sheet crept down from the 

 north. I shall not enter on the cause of its coming, or 

 on its source ; suffice it to call attention to the manifest 

 marks of its presence. Wherever the rocky floor of the 

 Island, or of almost any part of New England, is freshly 

 uncovered, it is found to be more or less smoothed or 

 rounded and distinctly striated or grooved, as if it had 

 been severely rubbed down by some gritty burnisher. Such 

 surfaces may be seen at innumerable points along the rocky 

 shore, of Mount Desert, where the drift has lately been 

 stripped off and where the waves have not yet made 



