131. 



of the so-called Malm or FirexUme Hock. -Si^ 



covered by Dr. Eiist^ as low down as Jurassic strata in Germany ; 

 lastly, it may now be fairly considered as certain that the flints 

 in the Chalk, the cherts, and siliceous rocks of the Upper and 

 Lower Greensands (including the malm and firestones), are due 

 to the siliceous skeletons of sponges. It is probable also that 

 the cherty rocks of still earlier geological periods are mamly 

 derived from these organisms ; this has been proved as regards 

 Liassic strata in Germany,^ and I have also been able to determine, 

 from a recent examination, that the massive beds of chert (in 

 some cases nearly 20 ft. in thiclmess) in the Carboniferous strata 

 of Yorkshire are as distinctly derived from sponge -spicules as 

 the Malm rock referred to in this paper. 



This Malm rock is certainly not at all attractive in outward 

 appearance, and even by many geologists would be passed by as 

 a very uninviting material for study ; but the facts shown by a 

 microscopic investigation of its structure render it of some 

 mterest, and they certainly furnish another exemplification of 

 the truth of the poet's remark, 



" The dust we tread upon was once alive." 



[Appendix. — May, 1887. — The derivation of the Carboniferom 

 chert from the spicular remains of siHceous sponges has been 

 further confirmed by an examination which I have lately made 

 of the series of chert beds exposed in and near Holywell and 

 other places in Flintshire. These beds, which reach the unusual 

 thickness of over 300 ft., are in places filled with spicules, and 

 there can be no doubt that the silica of the chert, like that in 

 the Cretaceous strata, is derived from these organisms. This 

 series of beds has hitherto been regarded by Mr. A. Strahan, 

 F.G.S., and Mr. G. H. Morton, who have specially described 

 them, as of sedimentary (i. e., inorganic) origin.] 



* " Beitrage zur Kenntniss der fossilen Radiolarien aus Gesteinen des Jura," 

 'Palffiontographica,' vol. xxxi. (1885). 



^5 Dunikowski, ' Die Spongien, Kadiolarien, und Foraminiferen der Unter- 

 liassischen Schichten vom Schafberg, bei Salzburg.' Bd.xlv. Denkschriften 

 Kais. Akad. Wiss. Wien. 1882. 



t8 JUN igpg 



*' %^ 



