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- Boor VI.] GENERAL PRINCIPLES. 635 
the rocks of other and far distant regions. This application has 
brought out the fact that some of the most marked geological breaks 
in Kurope do not exist elsewhere, and, on the other hand, that some 
portions of the record are much more complete there than in other 
regions. Hence, while the general similarity of suceession may 
remain, different subdivisions and nomenclature are required as we 
pass from continent to continent. . 
A bed, or limited number of beds, characterized by one or more 
distinctive fossils, is termed a zone or horizon, and, as already 
mentioned, is often known by the name of a typical fossil, as the 
different zones in the Lias are by their special species of ammenite. 
Two or more such zones, united by the occurrence in them of a 
number of the same characteristic species or genera, may be called 
beds or an assise, as in the “ Micraster beds or assise” of the 
Cretaceous system, which include the zones of M. cor-testudinarium 
and M. cor-anguinum. Two or more sets of such connected beds or 
assises may be termed a group or stage (étage). A number of 
eroups or stages similarly related constitute a series, section 
(Abtheilung), or formation, and a number of series, sections, or 
formations may be united into a system.’ 
The nomenclature adopted for these subdivisions bears witness to 
the rapid growth of geology. It is a patch-work in which no 
uniform system nor language has been adhered to, but where the 
influences by which the progress of the science has been moulded 
may be distinctly traced. Some of the earliest names are lithologi- 
eal, and remind us of the fact that mineralogy and petrography 
1 The unification of geological nomenclature throughout the world is one of the 
objects aimed at by the recently instituted “ International Geological Congress,” which 
at its late meeting at Bologna recommended the adoption of the following terms, the 
most comprehensive being placed first : 
Divisions of sedimentary formations. Corresponding chronological terms. 
Group. Era. 
System. Period. 
Series. Epoch. 
Stage. Age. 
As equivalents of Series, the terms Section or Abthetlung may be used; as a subdivision 
of stage, the words Beds or Assise. 
“ According to this scheme,” Mr. Topley; one of the secretaries, remarks, “ we would 
speak of the Paleozoic Group or Era, the Silurian System or Period, the Ludlow Series 
or Epoch, and the Aymestry Stage or Age. The term ‘formation’ raises a difficulty, 
because this word is used by English geologists in a sense unknown abroad. To bring 
our nomenclature into conformity with that of other nations it will be necessary to use 
the word only as descriptive of the mode of formation, or of the material composing the 
rock. We may speak of the ‘ Carboniferous Formation’ as a group of beds containing 
coal: but not as a name for a set of rocks apart from the mineral contents. In like 
manner, we may speak of the ‘ Chalk Formation’ but not of the ‘Cretaceous Forma- 
tion.’ ” (Geol. Mag. 1881, p. 557.) It may be doubted whether the recommendations 
of any congress, international or other, will be powerful enough to alter the established 
usages of a language. The term group has been so universally employed in English 
literature for a division subordinate in value to series and system that the attempt to 
alter its significance would introduce far more confusion than can possibly arise from 
its retention in the accustomed sense. 
