126 NATURAL SCIENCE. ^p^.^. 



preserved in a perfect or nearly perfect condition ; but they cannot 

 always be obtained, and in such cases corals, brachiopods, trilobites, 

 and other forms will be found serviceable. The trilobites have already 

 been extensively utilised by the Scandinavian and Russian geologists, 

 and they will probably yield more valuable assistance than heretofore 

 in our own country when they have been more fully studied with 

 reference to their distribution in the field. Some of our "species," 

 such as Cheivnvus binmcronaUis, probably contain many " exallogous " 

 forms which are of value for stratigraphical purposes. 



The actual utility of different forms of fossils for determining 

 horizons can only be ascertained by working out their vertical ranges 

 in the strata. It is a well-known fact that some fossils have a longer 

 range than others which are closely allied to them ; for instance, in 

 the Llandovery Rocks of the Lake District, Monograptus gvegaviiis 

 ranges through more than thirty feet of rock, while Monograptus 

 aygentens is limited to eight inches of the same set of strata. 



When an organism has a long range it by no means follows 

 that it will be found equally distributed throughout all the strata 

 included in its range, and so we get re-appearances of a form. 

 Among the many cases of re-appearance noted by Barrande among 

 the rocks of the Bohemian Basin (3), two of the most noteworthy are 

 those of the trilobites ^glina rediviva and Dionide fovniosa, which have 

 been found in the bands d i, d 2,, and d 5, but have not been recorded 

 from the intermediate bands d 2 and d 4. It is not necessary to 

 suppose that these creatures migrated from the Bohemian area during 

 the deposition of the bands d 2 and d 4. If they lingered in diminished 

 numbers in the area during a period marked by conditions unfavourable 

 for their existence, they might well escape the notice of collectors.' 

 It is evident that recurring forms are not adapted for marking zones. 

 They do not, however, raise any real difficulty when zones are being 

 established, for they are not usually prominent forms, and, Vv^hat is 

 of more importance, the association of other organisms is different in 

 the beds in which a particular organism recurs, from that which 

 is found in the beds in which it first appears. From this considera- 

 tion, it follows that a successful establishment of zones will more 

 probably be made if the organisms associated with the form after 

 which the zone is named be also carefully noted, and this is constantly 

 done by those who use the " zonal method," 



Another possible cause of suspicion is the frequent thinness of 

 the deposit marking each zone in the area in which such zones were 

 first determined. It is, however, natural that such determinations 

 should be made in areas of exceptionally thin deposits, for there 

 many zones may be studied in the course of a single section ; and it is 

 not only easier to establish zones in such an area, but when a thick 



1 In illustration of this I may notice that during the summer of 1876 the butterfly 

 Erebia cassiope swarmed on many of the hills of the Lake District. Since that time 

 I have not seen one, though I have usually spent my summers in the district. 



