Correspondence — Messrs. Blake and Hudleston. 91 



The second portion of Prof, Bonney's paper relates to another 

 matter ; and here let us at once express our regret that any phrase- 

 ology of ours should even seem to imply that the Coral Eag of 

 Upware had been " imperfectly " treated in his " Geology of Cam- 

 bridgeshire." Having said this, we will proceed to discuss the 

 substance of his complaint. The chief points in which our account 

 differs from that of Prof. Bonney are the assignment of the Upware 

 rock of the south pit to the Coral Rag in a restricted instead of a 

 general sense ; and the separation of the north pit rock as belong- 

 ing, not to a mere variation of development, but to a different 

 horizon. That Prof. Bonney called the Upware rock " true Coral 

 Rag, as the word was then understood," was clearly acknowledged 

 in the sentence following the one he quotes, viz. "It has always 

 been called Coral Rag, except by Mr. Seeley." But Prof. Bonney 

 went further. He attempted to show to loliat part of the " Coral 

 Rag as then understood " the Upware rock belongs. He assigned it 

 to the lower part, and therefore to an horizon beneath our restricted 

 " Coral Rag " ; ' and it is on this point that we differ from him. He 

 did this on account of its containing Cidaris florigemma, which he 

 says extends down to the Lower Calcareous Grit. We ask, where ? 

 Not, we venture to say, in England. 



To prove a negative is of course a difficult task, and the experience 

 of years may be upset by the discovery of a moment. But the fact 

 is that Cidaris florigemma hardly ever occurs even in the Coralline 

 Oolite, which in nearly every locality where a sequence can be 

 traced is interposed between the Coral Rag and Lower Calcareous 

 Grit. So far, then, from making the position of this urchin less 

 constant than it was supposed to be, as is alleged by Prof. Bonney 

 in the second paragraph of his letter, our intentions were certainly 

 in a contrary direction, only we find it constant in the upper and not 

 in the lower part of the series ; or, to speak more accurately, it is 

 most plentiful in the lower portion of the upper division, i.e. the 

 restricted Coral Rag. If, however, English geologists are content 

 to derive their impressions of the distribution of the Mesozoic strata of 

 their own island from the detailed accounts of their supposed foreign 

 equivalents by continental authors, we can never get any real data 

 for comparison. In the present instance it would appear that the 

 Cidaris florigemma was contemporary with the earlier part of the 

 Coral growths in Eastern France. In the Boulonnais it was pretty 

 uniformly distributed throughout, as M. Rigaux informs us, and 

 he therefore wonders that we consider its position so constant. 

 In England again, except in the Weymouth area, which shows more 

 intermediate conditions, it is almost entirely confined to the upper- 

 most Coral growths. The lower reefs at Highworth and Hackness, 

 though presumably formed under similar physical conditions, are 



1 It may be necessary here to remind the readers of the Geol. Mag. that the 

 Coralliau of England admits of four primary subdivisions, as follows in descending 

 order: 1. SupracoraUine, or equiA'alents of the Upper Calcareous Grit; 2. Coral 

 Rag, or zone of Cidaris fiorigenmia \ 3. Coralline Oolite, or zone of Am. pHcatilis ; 

 4. Lower Calcareous Grit, or zone of Am. perarmatus. 



