582 HENRY SHALER WILLIAMS 



New York was in large measure synchronous with the sedimen- 

 tation of gray rocks with abundant marine fossils, including 

 Spirifer disjimctus, in western New York and Pennsylvania. 



If the endurance of the Spirifer disjimctus fauna be made the 

 mark of the Spirifer disjunctus biochron, it becomes quite possible 

 to state and discuss the facts as they are, and to understand the 

 natural explanation of the facts as a gradual transgression of 

 the shore from which the red sediments were deposited west- 

 ward with the progress of time. The geochron of red beds 

 began earlier in the scale of biochrons, and as we expand the 

 idea we will discover that it is also earlier in actual geological 

 time, and that the biochron is the measure upon which we must 

 depend in constructing the standard geological time- scale. 



THE CONSTRUCTION OF A BIONIC TIME SCALE. 



The next question arises, how shall this time-scale be con- 

 structed ? First, it must be constructed, primarily, on the basis 

 of the bionic values of the fossils. Secondly, the names of the 

 time divisions must be distinct from those used in designating 

 the formational divisions of the rocks. Thirdly, biological names 

 should be chosen for the divisions, and so far as practicable, 

 some abundant or characteristic fossil should furnish the name ; 

 as in naming formations the locality in which the formation 

 appears in full force and with full characteristics is used. 

 Fourthly, it cannot be expected that the divisions of an accu- 

 rate time-scale, based upon many different sources of measure- 

 ment of the time intervals, will present a continuous series 

 without breaks and without overlapping of the divisions. If a 

 single means of measuring the lapse of time were used, such 

 breaks and lapping might be avoided ; only in case the begin- 

 ning of one species or fauna was everywhere dependent upon 

 the cessation of the preceding one, could the scale be made 

 without liability of failure to meet at the limits of the time 

 intervals indicated. Fifthly, the classification and definition of 

 geological formations should be made, so far as practicable, 

 independent of the fossils or the time relations indicated by 



