156 THE JOURNAL OF GEOLOGY. 



The table on page i 55 presents the facts regarding these forma- 

 tions, and what I conceive to be the chief difficulties in attempt- 

 ing to use a single nomenclature and classification. The question 

 at once arises how will a dual nomenclature help us over these 

 difificulties. 



On the left I have placed a part of the time-scale adjusting 

 the formation-scales for the several sections, approximately as I 

 think the fossils indicate their time relations to have been. 



Formations maybe described for a particular geological section 

 with precision as to kind of rock, stratigraphical sequence and 

 thickness, and, as we compare one section with another, we are 

 obliged to become more indefinite in our description and speak in 

 generalities, or averages ; and therefore the wider we extend the 

 use of a formation name, the less accurate and the more indefinite 

 does it become. 



On the contrary, in the distinguishing of time-relations or 

 position in the time-scale, the individual facts, the local handful 

 of fossils are the more indefinite, indicating only, it may be, 

 Paleozoic or Mesozoic age. The extension of the comparison to 

 the study of fossils above and below, and to the comparison of 

 fossils in adjoining sections, and to their geographical distribution 

 for a wide extent, increases the precision in locating position in 

 the geological time scale. 



It happens, therefore, that, while formation definitions are best 

 constructed in the field in the presence of the actual rock section, 

 the time definition becomes more accurate the more thorough the 

 investigation of the fossils is made. Hence in constructing a 

 time-scale, it begins properly with the grander divisions, and it 

 is built up and perfected by subdivision, whereas the forma- 

 tion-scales begin with the lesser strata and are elaborated and 

 completed by adding together successive strata. 



The grander divisions of the time-scale are already in use. 

 These are universally known as the Paleozoic, the Mesozoic, and 

 the Cenozoic. The primary sub-divisions of these geological 

 times are also very widely understood and used. They may be 

 called eras, and are named Cambrian, Ordovician, Silurian, Devo- 



