206 R. D. Irving — Is there a Huronian Group f 



the great fossiliferous rock complexes are classed in three orders 

 of magnitude, namely: the System, the Group, and the Forma- 

 tion. Of these classes the most comprehensive is the System, 

 which term is applied to those great divisions of the geological 

 column which are defined by paleontology, and are recogniza- 

 ble the world over. Such are the Cenozoic and the Mesozoic 

 Systems. Next in order comes the Group, which term is 

 designed to cover those great sub-divisions of the Systems 

 which are "defined above all by paleontology, and subordi- 

 nate^ by petrography, or are admitted by all geologists, for 

 motives in part arbitrary. These are the groups. Those ad- 

 mitted by the Survey have been discovered in various coun- 

 tries; they have probably a universal distribution,* compris- 

 ing ail the formations of known clastic origin, and appear to be 

 approximately comparable among themselves as to volume. It 

 is not maintained that their limits are clearly marked, nor that 

 they can be strictly traced along equivalent stratigraphical 

 levels. Nor yet is it held that the testimonies of the different 

 classes of fossils — vertebrates, invertebrates and plants — are 

 always consistent with each other, or with that of petrography, 

 as to their limits. These divisions are, then, to a certain ex- 

 tent arbitrary. We may increase or diminish their extent 

 from time to time by adding or withdrawing formations in the 

 neighborhood of their respective boundaries, just as the orders 

 of biology are perpetually modified. . . . "f Such are the 

 Cambrian, Silurian, Devonian and Carboniferous Groups. 

 These generally recognized subdivisions of the geological 

 column are groups, especially in the sense that they include 

 each a number of subordinate divisions distinguishable from 

 one another petrographically, genetically, and often even 

 palaaontologically. These subordinate members finally are 

 the Formations of this system of nomenclature, though the use 

 of the term formation is allowed also in a vaguer sense to cover 

 any rock mass whose distinction from the surrounding masses 

 is desirable on one ground or another. 



In the paper referred to, this classification is proposed only 

 for what are in it termed the clastic groups — clastic, in the 

 sense that they are formed mainly of debris, whether it be of 

 preexistent rocks or of the hard parts of organisms. The 

 rocks composing the groups have been deposited by the ordi- 

 nary processes of sedimentation. Outside of these so-called 

 clastic groups are left to be separately provided for the various 

 rocks which have been termed collectively the crystalline 

 schists, and which, in the paper referred to, are embraced under 



* This is the signification of the French; but in the original English MS., from 

 which the French translation was made for publication, this clause reads, " are 

 presumptively world-wide in distribution." 



f Translated from the French of the original article, op. cit., p. 15. 



