58o HENRY SHALER WILLIAMS 



order of magnitude, while the latter is of tertiary order of mag- 

 nitude. So far the magnitudes are relative, and they become 

 concrete only when the bionic value of the characters of some 

 particular species or genus is considered. 



Paleontologists are familiar with the very long range of the 

 species Atrypa reticularis ; R]iy?ichonella cuboides, on the other hand, 

 has a very short range. In the nomenclature proposed (so long 

 as both are considered to be species), the fact would be expressed 

 by saying that the Atrypa reticularis biochron is longer than the 

 Rhynchonella cuboides biochron. Nevertheless, in the familiar 

 categories used by the zoologist and botanist is found a means of 

 expressing definiteness in, at least, the order of magnitude of 

 biochrons ; whereas there is no way of distinguishing the order of 

 magnitude of geochrons, except in feet thickness. In the case 

 of the biochron it is only necessary to indicate the name of the 

 species or genus in order to fix a definite value to the biochron. 

 Such values are already definitely expressed when we speak of 

 the "reptilian age," the " age of fishes," the "olenellus zone." 

 The definiteness is indicated by the name of the particular group 

 of organisms made use of, or adopted as the measure of the 

 biochron. 



THE DUAL NOMENCLATURE. 



In proposing a dual nomenclature it is essential to indicate 

 this basis of measurement of the chron, and to distingruish the 

 geochron (expressed in terms of feet thickness of stratified sedi- 

 ments of uniform lithologic constitution) from the biochron 

 (expressed in terms of presence in the sediments of fossils of 

 the same species, genus, or family). 



Thus the time value of the Hamilton formation would be 

 spoken of as the Hamilton geochron; while the time value of the 

 species Tropidoleptus cari7iatus would be the Tropidoleptus biochron. 

 The same kind of difference in values is observed to pertain to 

 both biochrons and geochrons. As it is impossible to fix any 

 standard length for the endurance of a species, so it is impossible 

 to fix any standard of thickness for geological formations ; they 

 may vary from a few inches to many hundreds of feet in thickness. 



