60 INTRODUCTION. 



known example of the same phenomenon is afforded by the Jurassic 

 deposits. These have been shown to contain a number of well- 

 marked zones, each of which is characterised 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 palaeonto- 

 logical importance, and can be rendered readily available in working 

 out the stratigraphy of any given area. 



Certain life-zones appear to have nothing more than a local devel- 

 opment and importance, but in other cases they have proved to be 

 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 subdivision 

 of the Ordovician system), which have been recognised 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 distinct and limited 

 life-zones can be recognised 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 stand-still, but which were long enough to allow of 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 comparatively 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. 



" Colonies." 



It only remains in this connection to consider very briefly the 

 doctrine of " colonies," laid down by M. Barrande, the eminent 



