Fraf. E. Hull— Origin of Chert. 525 



now to admit, what T stated in Manchester, that in examining the 

 slides of chert under the microscope, I had, at least in many instances, 

 mistaken forins of sponge-spicules for those of Crinoid stems. Nor 

 was this, I submit, at all surprising when we recollect that these 

 organisms are often very vague, and that the bands of chert occur 

 intercalated with beds of limestone abounding in stems, ossicles, 

 and plates of Crinoids. It was not unnatural, therefore, that I should 

 have supposed the little discs seen in the chert-sections under the 

 microscope to be minute cross-sections of these stems or ossicles.' 

 Dr. Hinde's great experience in the examination of the sponge- 

 structures of the Cretaceous beds has given him an advantage 

 in this line of investigation which certainly has not fallen to my lot ; 

 consequently, when on examining my slides in this office, he stated 

 that the forms were those of sponge-structures, I accepted his state- 

 ment without question. 



Dr. Hinde's recent investigations imdoubtedly show that siliceous 

 sponge-structures enter far more largely into the composition of 

 Carboniferous chert than has hitherto been suspected, and even, 

 that they exceed in numbers other organic forms ; all this I now 

 willingly concede. But I am not prepared to go to the full length 

 of Dr. Hinde's demands, as I understand them, nor to abandon as 

 untenable the proposition that much of the silica of Carboniferous 

 chert has been derived by a transmutation process from the waters 

 of the ancient seas. Not only are there to be found forms, such as 

 those of Corals, Brachiopods and Polyzoa and ossicles of Crinoids, 

 originally calcareous, now occurring silicified in the chert, but the 

 amorphous cementing material of the organic structure which may 

 be supposed to have originally been calcareous has now been trans- 

 muted, or may have been directly deposited from the waters under 

 such favourable conditions as those supposed by Mr. Hardman and 

 myself in our original memoir.^ A similar conclusion has been 

 arrived at by Prof. Renard by a process of investigation analogous 

 to, though quite independent of, that pursued by ourselves. Whatever 

 doubt I might have entertained in regard to my own conclusions, I 

 cannot extend to those of so competent an observer as Prof. Renard. 

 Let me ask Dr. Hinde, does he deny the possibility of siliceous bodies 

 or masses having been formed by the transformation process ? If so, 

 he is confronted by the evidence of a large number of the ablest 

 observers, both British and Foreign, amongst whom may be mentioned 

 Bowerbank. Rupert Jones, Sullivan, Sterry Hunt, and Bischof.^ If 

 so, it is Hinde contra miindum ; and let me say that these observers 

 appeal to Nature as well as does Dr. Hinde himself. 



I feel obliged to Dr. Hinde for having called my attention to the 

 fact that Professor Sollas had previously identified siliceous sponge- 

 spicules in the chert-sections which I had forwarded to him for 



^ From what Dr. Hinde has stated in his original paper (Phil. Trans. 1885, note, 

 p. 433), it would appear that a more competent observer than myself has fallen into 

 a similar error ; from which it will be inferred that it is a matter of much difficiiltv, 

 and requiring great experience to distinguish the one from the other. 



^ Scientific Transactions, Eoy. Dublin Society, vol. i. new ser. (1878). 



3 Bischof : " Chemical and Physical Geology," Cavendish edit. vol. ii. p. 486, eiseq. 



