184 New York State Museum 



are separated primarily by marked differences in the fos- 

 sils contained in the rocks, but also by crustal disturb- 

 ances. Periods are composed of epochs of time and sys- 

 tems of formations of series of strata. These divisions 

 are still more or less arbitrary, and are based either upon 

 the fossil content of the series of strata or the sedimen- 

 tary cycle which the strata are assumed to represent. The 

 further division of epochs into ages and series into stages 

 follows, but these divisions are not as yet as definitely 

 delimited. 



The formation is the unit of chronological or time 

 classification of rocks and in general use is the smallest 

 unit that can be plotted on a geological map. A forma- 

 tion may be either igneous or sedimentary. \i sedimen- 

 tary it may be a single thick bed of rock, such as the 

 Onondaga limestone, a succession of like beds or a suc- 

 cession of sediments that are unlike but have closely re- 

 lated faunas, such as the Hamilton beds. Where there 

 is more than one member the beds are conformable, that 

 is, showing no significant time breaks, and were deposited 

 during a limited time in the earth's history. Formations 

 bear local names based upon the town, river etc. with 

 which the formation is associated. Their horizontal ex- 

 tent is limited since the conditions under which they were 

 deposited were of local character and varied from place 

 to place. Two formations may be laid down, at the same 

 time in different places and be of a different lithological 

 character, that is, one may be a sandstone and another 

 a limestone. Sometimes, when first studied, the age re- 

 lationship is not recognized and the formations receive 

 different names though of the same age. If their identity 

 in age is recognized from the beginning they receive the 



