CHRONOLOGICAL SUCCESSION OF AQUEOUS ROCKS. 37 







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 conditions. 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 fossils. 

 The most celebrated case of this subdivision of a formation 

 by means of its fossils is that afforded by the Lias. This 

 great and essentially argillaceous formation can be divided 

 into a number of zones, each of which is characterised by 

 possessing some special fossils, and particularly by some 

 special Ammonite. These zones are extremely constant, and 

 they are traceable wherever the formation is fully developed, 

 and has been fully examined, in Europe ; so that they enable 

 us to effect a division of the formation into special horizons, 

 which have no stratigraphical existence, and are not separ- 

 ated by any physical break, but which are of the utmost 

 palseontological importance, and which can be rendered 

 readily available in working out the stratigraphy of the 

 formation. Similar " zones " are recognisable in the other 

 Jurassic rocks and in the Cretaceous system ; and it is 

 tolerably certain that in time we shall be able to establish 

 a similar, if less perfect, series of palaeontological divisions in 

 all the great formations. 



The principal difficulty that we have to confront in deal- 

 ing with these " zones," is to produce any plausible explana- 

 tion 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 any physical dis- 

 turbance 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 recognised were deposited 

 with extreme slowness, whereas those which show an essen- 



