walcott. ] CORRELATION. 425 



rocks of Europe, in which paleontologists, following Professor Lapworth, have recog- 

 nized numerous well marked " life-zones," characterized 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 known example of the same phenome- 

 non is afforded by the Jurassic deposits. These have been shown to contain a num- 

 ber of well marked zones, each of which is characterized by the possession of some 

 special fossils, and particularly by some special Ammonite. These zones are extremely 

 constant in any particular region, and they enable the observer to effect a division of 

 the formation into special horizons, which have no stratigraphical existence and are 

 not separated by any physical break, but are of the utmost paleontological impor- 

 tance andean be rendered readily available in working out the stratigraphy of any 

 given area. 



Certain life-zones appear to have nothing more than a local development and im- 

 portance, but in other cases they have proved to bo astonishingly constant even over 

 very large areas. Perhaps the most remarkable known instance of the extension of 

 particular life-forms over a vast area is that afforded by the Arenig rocks (a subdivi- 

 sion of the Ordovician system), which have been recognized as occurring in Europe, 

 in Canada, and in Australia, and contain in all these widely remote areas the same 

 peculiar types of Graptolites. 



The principal difficulty that we have to confront in dealing with these " zones" is 

 to produce any plausible explanation accounting for the destruction of the special 

 life-forms of the one zone and the appearance of those of the next zone. For the most 

 part, these zones are of very limited vertical extent, and they succeed each other in 

 such a manner as totally to preclude the idea that the dying out of the old forms can 

 have been in any way caused by a physical disturbance of the area. Perhaps the 

 most probable view to adopt in the meanwhile is that the formations in which dis- 

 tinct and limited life-zones can be recognized were deposited with extreme slowness, 

 whereas those which show an essentially compact and homogeneous fauna from base 

 to summit were deposited with comparative rapidity. Upon this view, a formation 

 like the Lias is one formed by a process of very slow and intermittent sedimentation, 

 the life-zones being separated by intervals during which sedimentation must have 

 been at a standstill, but which were long enough to allow more or less considerable 

 biological changes, some forms dying out or becoming modified while other new ones 

 came in. Upon this view, further, a formation like the Lias, though of compara- 

 tively small vertical extent, may represent as long a period of time as the whole of 

 such a great formation as the Lower Carboniferous, which appears to have been 

 formed under conditions of comparatively rapid sedimentation. 1 



Life-zones have been determined in the Middle Cambrian or Para- 

 doxides terrane of Sweden, Wales, New Brunswick, and Newfoundland 

 or the Atlantic Basin Province. In a broader range the genera Ole- 

 nellus, Paradoxides, and Dikelocephalus are considered typical of the 

 three primary zones or divisions of the Cambrian group. They are of 

 unequal value and decrease in precision from the older to the later 

 faunas. The genus Olenellus has a* wide distribution in western Eu- 

 rope and North America, while Paradoxides is more limited in Amer- 

 ica, and the true Dikelocephalus probably has a still smaller geo- 

 graphic range. 



Stage of evolution. — The evolution of organic life is a factor in corre- 

 lation that has been assuming some importance within the last few 

 years. To be of practical use the paleontologist must have a thorough, 



•General Introduction, Manual of Paleontology, by H. A. Nicholson and R. Lydekker, vol. 1, 1889, 

 pp. 58-60. 



