12 



Eozoic and the three succeeding eras, — Paleozoic, Mesozoic, 

 and Cenozoic, — for which, taken collectively, geologists have 

 failed to provide a name. 



Several divisions have been established among the crystallines 

 of Eastern Massachusetts, based, for the most part, on litho- 

 logic and stratigraphic characters. The groups thus formed are 

 believed to have chronologic value, to be chronologically dis- 

 tinct. It is not without hesitation that, in describing these 

 series as both lithological and chronological, I raise the vexed 

 question of the value of purely physical characters as tests of 

 age among crystalline rocks ; for I began my study of the 

 crystallines of this State with a wholesome distrust of the 

 value of lithological data in establishing chronological divisions, 

 and am not yet wholly prepared to apply this principle to wider 

 regions. Within my limited field of observation, however, I 

 am satisfied that its application is safe. It adapts itself so per- 

 fectly to all the facts, so far as I know them, that I do not hesitate 

 to assert that the lithological characters of the divisions which 

 have been worked out among the crystallines of Eastern Mas- 

 sachusetts — the chronological and geographical distinctness of 

 which can scarcely be doubted — are as unlike as the faunae of 

 any two successive geological formations. 



