.^,. LIFE-ZONES IN LOWER PALEOZOIC ROCKS. 129 



of origin than were animals of other groups, but at present we cannot 

 say much upon this head, for we do not know how far other groups 

 will yield " exallogous " forms, after a further study of the vertical 

 distribution of the " species." 



Hitherto I have made no attempt to give a definition of a zone. 

 It is advisable, as in the case of other stratigraphical divisions, not to 

 insist on too rigid a definition. A zone may be defined as a stratum or 

 set of strata characterised by an organism, which may or may not be the 

 dominant form of that zone, but which is nearly or entirely confined to it, 

 and from which it takes its name ; it usually contains an assemblage of other 

 forms occurring in similar association in that zone only ; it may be of consider- 

 able thickness, and may contain material of different composition in different 

 parts of its extent. Such a definition permits the use of the term for 

 deposits characterised by genera, species, or even varieties. Some 

 authors speak of a zone when the form after which the zone is named 

 is absent. It is an open question how far this is desirable, but in 

 practice little harm can result, if the absence of the characteristic 

 fossil is noted, and the evidence for the identification of the zone is 

 fully given. It is clear that the assemblage of fossils other than the 

 characteristic one will not be constant in all areas in which the zone 

 is found, so that the stratigraphical geologist must necessarily 

 exercise his judgment in determining how far it is safe to subdivide a 

 group of deposits into zones, and to correlate these with one another 

 in different areas. The success of such correlation will depend upon 

 the cumulative evidence. 



I shall devote the latter part of this article to a discussion of the 

 value of the zonal method to the geologist. It is above all things 

 of value as furnishing a chronological index, by which we may fix 

 the relative dates of the different events, which taken together 

 constitute the earth's history. As Sir A. Geikie observes (6), strati- 

 graphical geology " gathers up the sum of all that is made known by 

 the other branches of the science, and makes it subservient to the 

 interpretation of the geological history of the earth." Now it is 

 impossible to read that history aright, unless we have a trustworthy 

 chronological index, and this consideration probably influenced those 

 who, alarmed by the jeremiads of eminent men of science, abandoned 

 the study of stratigraphical geology, to pursue the subject along what 

 seemed to be more promising lines. It is indeed difficult to see what 

 would be the ultimate value of geology if our stratigraphical chrono- 

 meter were untrustworthy ; no detailed study of the mineral com- 

 position, organisms, and physical changes of our globe could furnish 

 material for a connected history of that globe. But the institution of 

 the zonal method has given us a brighter outlook, and already we have 

 gained knowledge which has been turned to excellent use. I may refer, 

 in support of this remark, to the discovery of the " Secret of the High- 

 lands," and to the establishment of a horizon — the Olenellus zone — 

 which enables us to fix the upper limit of the pre-Cambrian Rocks. 



K 



