432 G, K. Gilbert — Congress of Geologists. 



portance of the possible work of the congress that I shall devote 

 the present hour also to its consideration. 



The first thing the congress did was to select names for a set 

 of categories to express the taxonomic rank of stratigraphic 

 divisions, on the one hand, and of chronologic divisions on the 

 other. In the terminology of zoology and botany the words 

 kingdom, class, order, family, genus, species, etc., however diffi- 

 cult of definition they may severally be, nevertheless are used 

 always in the same order of inclusion. No systematist in those 

 sciences would think of grouping orders together and calling 

 them a family, or of styling a group of families a genus. But 

 in geology there is no such uniformity of usage. With some 

 writers a group is larger than a series, with others it is smaller. 

 With some an age includes several periods, with others a period 

 includes several ages. There are even writers who ignore the 

 distinction between stratigraphy and chronology; and among 

 the classifications submitted to the congress is one in which an 

 age is subdivided into systems. There is- a manifest advantage 

 in bringing order out of this chaos, and so great is the utility of 

 uniformity and perspicuity that the decisions of the congress in 

 this regard will unquestionably be followed by future authors. 

 The terms and the order adopted by the congress are as follows : 

 Of stratigraphic divisions that with the highest rank is group, 

 then system, series and stage. The corresponding chronologic 

 divisions are era, period, epoch and age. This order of rank is 

 strange to most English readers and writers, and so is one of 

 the terms — stage — but the strangeness is only a temporary dis- 

 advantage and will not seriously retard the adoption of the con- 

 vention. The fact that we have previously used the words in a 

 different sense, or that their etymology might warrant a different 

 meaning, need not deter us, for we know from frequent expe- 

 rience that the connotations of a word transferred from one use to 

 another quickly disappear from consciousness, leaving it purely 

 denotative. The introduction of the word stage, which can 

 hardly be said to have had an English status heretofore, or at 

 least the introduction of some new word for that part of the 

 column, was necessitated by the restriction of the word forma- 

 tion to a special meaning, — the designation of mineral masses 

 with reference to their origin. 



The same restriction vacated another office that had been 

 filled by formation, and to this office no appointment was made. 

 I refer to the use of the word to denote indefinitely an aggre- 

 gate of strata — as in saying, This formation should be called a 

 series rather than a system. This is an important function, for 

 which some provision must be made. I suggest that we may 

 advantageously enrich our language by the permanent adop- 



