148 THE JOURNAL OF GEOLOGY. 



Potsdam as names of geological divisions of the scale of which 

 there are a definite number. These arranged in a definite order, 

 according to English sequence of systems : Cambrian, Silurian, 

 Devonian, and Carboniferous, etc., constitute the standard geo- 

 ■logical scale. 



These formations have also been called Epochs and Periods, 

 and because the succession of time is continuous, the strict appli- 

 cation of the method has required the filling of the whole inter- 

 val, from the Cambrian to the Carboniferous, by these formations, 

 except when unconformity gives marked evidence of break in 

 the record. The old system requires that in all the sections, 

 the lines separating the formations from one another shall coin- 

 cide with the stratigraphical divisions, and progress has been 

 checked by the authoritative rebuke, from those who are sup- 

 posed to know, of any timid suggestion that the geology of a 

 newly discovered section does not conform to the standard. 

 We are already familiar with the proposition that there are such 

 systems, groups and formations, on the one hand, and Ages, 

 Periods and Epochs, on the other, but our whole nomenclature 

 and classification is applied and used as if the divisions indicated 

 by the two categories were strictly synonymous ; in fact, the 

 nomenclature of the International Congress went no farther 

 than to propose that the names of the divisions of the one cate- 

 gory, viz.: group, system, series, stage, should be applied to the 

 same concrete geological facts as the corresponding names of 

 the other category, era, period, epoch, age, and that these names 

 of the categories should be universally used ; as if, a century 

 ago, zoologists had proposed that class, order, family, genus, be 

 used in a uniform manner by all naturalists. It is the essential 

 idea contained in this differentiation of nomenclature, in two 

 directions, which I would here emphasize and elaborate. 



When geologists consider the two scales, the time-scale and 

 the formation-scale, it is found that the divisions are not synony- 

 mous, but that there are two distinct sets of facts confused in 

 our present nomenclature and classification. There is a geolog- 

 ical time-scale, and, however we subdivide it, or however we 



