Vs,^'.'- LIFE-ZONES IN LOWER PALEOZOIC ROCKS. 125 



which that author has called the zonal (as opposed to the regional), was 

 first applied to the Lower Palaeozoic Strata in a detailed manner. 



The use of two words has probably done much to retard the 

 general acceptance of this " zonal method ; " these words are zone and 

 species. The physical geologist is naturally alarmed by the use of the 

 term zone, and the biologist by that of species. We may pause for a 

 moment to consider these words. 



The adoption of the word zone — a girdle — is unfortunate. Small 

 wonder if there are some who have looked askance at the rapidity 

 with which thin zones were identified in distant regions, and have 

 feared that the task of the modern stratigrapher was akin to that of 

 Puck when he said : — 



" I'll put a girdle round about the earth 

 In forty minutes." 



It will presently be shown that the fear is groundless, for the zones 

 are in reality far from being universally distributed. They are in no 

 sense girdles. 



The biologist justly asks how we can tell that a form is a distinct 

 species, when the soft parts are not preserved. May it not be merely 

 a variety ? Certainly it may be, but it is of value to the stratigrapher 

 for all that. There are two kinds of variety. Firstly, the departure 

 from the normal form, which co-exists with that form ; the number 

 of variations of form presented by the shells of Terebratiila biplicata in 

 the Cambridge Greensand illustrates this ; such variations, though 

 interesting to the biologist, are useless to the stratigraphical geologist 

 for the purpose of defining zones. The other variations are found, 

 by experience in the field, to mark successive periods of time. A 

 palaeontologist whose sphere of work was limited to the museum, 

 would see nothing by which he could separate these varieties from 

 the former, but they are all-important to the stratigrapher, and it 

 matters not to him whether they are true species or merely varieties 

 of an earlier species, for they mark definite horizons. Dr. W. T. 

 Blanford some time ago remarked to me that a distinctive term was 

 wanted for such a variety, and I would suggest the term Exallage 

 {iiaWayri, a variety " from what has previously been "). 



We may now proceed to consider the manner in which life-zones 

 have been established amongst the strata, and to defend their utility. 

 Though many sets of life-zones have been established, as for instance 

 in the Upper Cretaceous Rocks by the use of echinoids and other forms 

 of life, and in the Jurassic Rocks mainly by the use of Ammonites, I 

 shall refer chiefly to the graptolite-zones of the Lower Palaeozoic 

 Rocks, as my practical knowledge of zones is chiefly confined to those 

 of the last-named strata. 



Let us consider first the relative value of different groups of 

 organisms in working out zones. Hitherto special use has been made 

 of the graptolites, and they are, when found, extremely valuable on 

 account of their abundance and the frequency with which they are 



