l60 THE JOURNAL OF GEOLOGY. 



Marine invertebrates for the Paleozoic, and later marine verte- 

 brates constitute the most satisfactory means for discerning geo- 

 logic age, because the oceans, in which they live, are of world- 

 wide extent and inter-communication. They are distributed 

 around the globe, and from pole to pole, so far as the adaptation 

 to conditions of environment will permit. Hence, we find it to 

 be a fact that, at every period of geological time, there are repre- 

 sentatives in all regions of the globe of the same species, or of 

 closely allied species such as to mark the close correlation of time 

 of their living. While this is true, it must be borne in mind that 

 correlation cannot be made by fossils to a finer degree of dis- 

 crimination than the facts of specific integrity allow. The life- 

 period of species is not uniform, and the wider the distribution, 

 the longer is, as a rule, the range ; so we find, in practice, that 

 with the present knowledge of fossils it is rarely possible to dis- 

 criminate age to a finer degree than that indicated by the periods, 

 Eocambrian, Mesocambrian, and Neocambrian, or similar periods 

 for the other eras. Within such general limits, time relations can 

 be discriminated by the fossils alone, and the progress of science 

 will enable us to recognize more minute divisions of time as we 

 proceed. 



On the other hand, formations, being local phenomena, are dis- 

 criminated with greater precision locally, and the indefiniteness 

 in application of formation names increases with the wideness of 

 the territory covered. No classification of terranes can be of 

 universal application, for no formation, or formation character, or 

 criterion of determination, is of world-wide extent. The time 

 divisions are more precisely defined the larger they are ; the for- 

 mation divisions are more precisely defined the smaller they are, 

 and progress of knowledge will extend the precision in opposite 

 ways for the two scales. 



Henry Shaler Williams. 

 Yale University, 



New Haven, Conn. 



