244 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



§ 2. The equivalents of the Palceozoic groups of the Boulonnais. 



The long series of oscillations liere indicated, by wliich the same 

 ai'ea was placed successively under such different conditions as to 

 depth, had also this consequence — that at each change it became the 

 ground of a fresh assemblage of animal forms. At intenals, which 

 are wide apart in time, such as are represented by the limestones of 

 the lower division (Ferques limestone down to that of La Cedule), and 

 when somewhat like conditions were repeated, we meet with a recur- 

 rence of identical forms, which then prove the continuity of the same 

 general Fauna ; the differences which the intervals present, as mineral 

 groups, are marked by corresponding changes in the animal series ; in 

 this, as in all such cases, the occupancy of an area, under any new condi- 

 tions, must necessarily therefore have been by a process of immigration. 



This consideration is of some importance with reference to all 

 attempts at systematic classification : like mineral groups, tvifh iden- 

 tical faunas, are not necessarily synchronous ; lohereas many groups 

 icith nothing loh at ever in common, whether in respect of mineral com- 

 position or included fossil forms, must of necessity he of the same 

 age ; yet the basis on which geological reasoning has usually pro- 

 ceeded for determining the equivalents of the component parts of 

 great groups from one district to another, has been this — that Uke 

 assemblages imply a perfect coincidence in time. 



It is possible that the systematic arrangements of geologists maj- 

 have been altogether premature ; it is certain that they have been 

 based on considerations which require much collateral support : the 

 whole physical history of every portion of the area over which each 

 perfect group extended, must be first made out, and even then we shall 

 require to know much more than we now do, as to the habits of very 

 many extinct forms, ere a " System" can be much more than a vague 

 speculation ; it is with extreme caution, therefore, that I propose the 

 few following comparisons for the Palaeozoic groups of the Boulonnais. 



Geographically the Boulogne series is exposed mid-way between 

 Devonshire and the Cotentin on the west, and a Belgic-Rhenish area 

 on the east : as far back as 1 839 I suggested a comparison between 

 the South Devon and Eifelian calcareous groups. With reference 

 to the Belgic area, it will be shown in the sequel, that the series of 

 oscillations there was in strict correspondence along the whole line, 

 extending from the east of the Ardennes to the Boulonnais ; m other 

 words, that a remarkable parallelism extends throughout the whole 

 vertical thickness of the Anthraciferous series, up to the completion 

 of the productive coal-measures. Such considerations cannot be 

 extended to the West of England series, so that other terms of cojn- 

 parison are required. The great Anthraciferous period of Northern 

 and Middle Europe, though it presents no abrupt geological break, 

 yet readily admits of a twofold division, each of which has a facies of 

 its own. Amongst the numerous forms which compose this fauna, 

 there are very many of great range and endurance, and which may 

 be altogether disregarded in comparisons of this sort : there are others 

 ap]3arently of very limited duration, such for instance, m the lower 

 anthraciferous group, as Posidonia, Strigocephalus, Calceola, Spirifer 

 ostiolatus, Pleurodictyum. The species of Strigocephalus are found 



