DIVISIONS OF GEOLOGICAL TIME 381 



Werfenian = Scythian. 



Anthracolithic = Permian 4- Carboniferous. 



Avonian-Dinantian practically. 



Silurian is used in three senses: (i) as here = Gothlandian, the 

 usual English use; (2) as=Gothlandian + Ordovician, the 

 usual French and German and former ^English use : this use 

 may always be assumed when ' ' Lower " and " Upper Silurian ' ' 

 are spoken of ; (3) as = Older Palaeozoic, not a frequent use. 



The division of stratified rocks into zones i.e., divisions 

 less than stages based on palaeontological characters alone, 

 and extending, if not over the whole world, far beyond 

 the limits within which lithological distinctions remain 

 constant was initiated by Oppel for the Jurassic system 

 about sixty years ago. It has since been applied to 

 most other systems, but no other system has proved 

 susceptible to such detailed application of the method. 

 As Oppel's original zones proved capable of division to 

 a greater or less degree, Mr. S. S. Buckman proposed 

 the term hemera as the name of the smallest possible 

 time-division which palaeontological facts make possible 

 it is the period of acme of a particular species (or 

 occasionally of a genus). The term zone has since been 

 regarded by many as the rock-division corresponding 

 to a hemera ; but Dr. Lang and others consider that the 

 zones defined by Oppel must continue to stand, and 

 subdivisions made in them corresponding to hemerae 

 must be called sub-zones. Accordingly, in the table of 

 Jurassic zones, both Oppel's original zones and the 

 latest recognized hemerae are given. In quoting the 

 hemerae (or zones) it is usual to give only the trivial 

 name of the index fossil, thus : Uarmatum zone, obtusum 

 hemera (or hemera obtusi, using the genitive case). 



The fossils that have proved of the greatest value as 

 zone-fossils are Ammonoids and Graptolites ; but 

 Brachiopods, Trilobites, Foraminifera, and Lamelli- 

 branchs (in order of decreasing value) also serve ; and 

 isolated species of other groups, e.g., Crinoids, may often 

 be of great use. In freshwater Cainozoic deposits 

 Mammals are of the highest value. 



It should be unnecessary to add that these tables of 

 zones are given for reference only, and by no means as an 

 exercise in memorizing ! 



