LIFE-ZONES. 59 



a series of beds perhaps two or three thousand feet in thickness, is 

 due simply to the fact that the organic remains met with in the for- 

 mation have not been sufficiently investigated, and that the exact 

 horizon at which each occurs in the series has not been accurately 

 determined. The determination of the horizons of particular life- 

 forms is a work of time, and demands both great stratigraphical 

 knowledge and also a wide and accurate acquaintance with the char- 

 acters of the fossils themselves — two requirements rarely fulfilled in 

 the same individual. 



In a considerable number of cases, however, it has now been 

 shown that the fossils of a given formation may be divided into 

 two principal groups. In the one group is comprised a series of 

 common forms of life which may be regarded as characterising 

 the formation as a whole. In the other group are included cer- 

 tain special fossils confined to particular parts of the formation, 

 and characteristic of certain definite horizons or zones within the 

 limits of the formation. All the great formations are to some extent 

 capable of being broken up into minor rock-groups, characterised by 

 special life-forms. Some of the differences in the kinds of fossils 

 found in different parts of the same formation must, of course, be 

 simply set down to the fact that different kinds of sediment imply 

 changed conditions in the sea, and hence changes in the marine 

 fauna. If, for example, part of a formation consisted of limestone 

 and part of sandstone, we should expect, beforehand, to find that 

 each of these rock-groups would have some fossils not found in the 

 other, since the two would have been formed under different condi- 

 tions. Apart, however, from differences arising from causes of this 

 nature, we meet with cases in which a formation, even if essentially 

 homogeneous in its mineral nature, can be divided into zones, each 

 of which is characterised by the possession of special groups of fos- 

 sils. Organisms belonging to any class of animals may serve in this 

 way as test-forms (" Leit-fossilien ") for special horizons in a series of 

 stratified formations, but there are particular groups of fossils which 

 have been found to be pre-eminently available for this purpose. 

 Among the older rocks of the earth's crust, the forms which have 

 proved specially valuable for the determination of particular " zones " 

 are the Graptolites, the Trilobites, and the Brachiopods, while the 

 Cephalopods have been found to afford the most satisfactory tests in 

 the case of the Secondary rocks. A well-known instance of this 

 subdivision of a system of strata by means of special types of fossils 

 is that afforded by the Ordovician and Silurian rocks of Europe, in 

 which palaeontologists, following Professor Lapworth, have recognised 

 numerous well-marked " life-zones," characterised for the most part 

 by the possession of particular types of Graptolites, though in some 

 cases the distinctive fossils belong to other groups. Another well- 



