the corallian rocks of england. 261 



Introduction. 



Whatever may be the value of the grounds on which the ordinary- 

 line of separation between the " Middle " and " Upper " Oolites is 

 drawn, there can be no denying that from the Oxford to the Kim- 

 meridge Clays, in some parts of England, we have one continuous 

 deposit, which, from a physical point of view, has been aptly termed 

 a great "pelolithic" formation. During the period when this was 

 being quietly laid down, at a quicker or slower rate, in these areas, 

 other portions of the same great region were at various times subject 

 to alterations of the physical conditions, in consequence of which the 

 deposits were changed in character; they became Grits or Limestones, 

 and supported coral reefs, with their accompanying Oolites. The 

 very fact of these changes being local — a fact proved as much by 

 the discontinuous as by the variable nature of the deposits — pre- 

 vents our assuming that they were all contemporaneous, even in a 

 large sense, as has been done by those who have used the term 

 Corallian, as indicative of a period, for all or any of the coral deposits 

 that have been formed between the commencement of the Oxfordian 

 and the close of the Kimmeridgian epoch. 



In England, however, these rocks, originally called " Coral Rag " 

 by Smith, from their development in that form in the area studied 

 by him, have been divided into Upper Calcareous Grit, Coralline 

 Oolite, and Lower Calcareous Grit, the classification being originally 

 derived from Yorkshire ; and it has been too hastily assumed that 

 rocks of similar lithological character are the equivalents of each 

 other in different localities. To determine the true relations of the 

 various Corallian beds to each other, as developed in different parts 

 of the country, we have examined them consecutively, and are now 

 attempting to present a general history of their formation. Geolo- 

 gists, indeed, are already possessed of valuable information on several 

 isolated districts ; but their correlation has scarcely been at all 

 attempted, and several areas have never, or but very imperfectly, 

 been described. 



In the study of no other series of rocks, perhaps, is it more neces- 

 sary to remember the influence that physical conditions have upon 

 the contained fauna. The close resemblance, not to say identity, of 

 some of the species with those of similar beds belonging to the 

 Great or the Inferior Oolite proves that much of the peculiarity of 

 the Coralline fauna is due, not to the lapse of time, but to the con- 

 ditions of deposit, as we hope to indicate in the sequel. Neverthe- 

 less palaeontology does not fail us as a guide, and there are still a 

 sufficiency of forms, above the influence of physical conditions, which 

 indicate satisfactorily by their association the relative age of the 

 deposits. At the same time it is possible that if we were to elimina f e 

 all the influenced forms, the remainder might indicate but two well- 

 marked faunas, the Oxfordian and the Kimmeridgian — the one end- 

 ing and the other commencing during the period of the deposition 

 of the Corallian deposits with their additional fauna due only to 

 physical conditions. This may bo the ense, 1 hough wo are scarcely 



