130 



KNOWLEDGE 



[June 1, 1899. 



FlS. 1. — Microscopic section of riint (silici- 

 fied clialk) from gravel of Ardtun, Isle of 

 Mull. X 100. Traces of sponge-spieules, 

 represented by glauconitic casts of their axial 

 canals; " ghosts*' of foramiuiferal shells, the 

 interior being filled by clear chalcedony, and 

 the perforations in the shell replaced by 

 tiny rods of glauconite. The glaiiconitic 

 change doubtless set in in the original clialk, 

 and the siliceous replacement of tlie calcium 

 carbonate followed. 



flints, chalcedonized shells, or minute quartz crystals in 

 such strata, one will naturally look for the remains of the 

 siliceous organisms -which supplied them, and the search 

 will seldom be unsuccessful." 



The same author held that the absence of obvious 

 deposits of silica indicated that the rock originally con- 

 tained no siliceous 

 fossils ; but this 

 point has recently 

 been contested by 

 M. Lucien Cayeux. 

 Dr. G . J . 

 Hinde's* work on 

 the cherts of the 

 Lower Cretaceous 

 strata proved that 

 the spicules of 

 t e tracti n ellid 

 sponges supplied 

 the silica that has 

 cemented certain 

 layers of the sand- 

 stones. Hence the 

 abundance of 

 these humble 

 creatures at cer- 

 tain times on the 

 floor of the Cre- 

 taceous seas has 

 strengthened the 

 Hythe Beds with 

 sheets of silica, 

 and has been, in 

 all probability, responsible for their enduring qualities. 

 Sponges have thus given us the Leith HOI scarp, the 

 boldest feature in south-eastern England. 



The fact that Mr. .J. T. Youngt found that even the 

 freshwater flint of the Purbeck limestone contained spicules 

 of Spovfiilla. a sihceous sponge, placed the sponges, as 

 makers of flint, well in the ascendant. Dr. Hinde ' 

 showed later that the Carboniferous chert of Ireland gave 

 rise, on its weathered surfaces, to a porous crust consisting 

 of innumerable rod-like sponge-spieules, felted together in 

 the plane of bedding of the rock. All his microscopic 

 sections of these cherts exhibited spicules. 



The wide-spread layers of tabular chert and flint, ex- 

 cluding those formed in joints, may be explained by Sollas's ; 

 argument, that many sponges are known to extrude then- 

 spicules during life. These become spread out thickly on 

 the sea-floor, and thus each individual sponge does much 

 more in promoting the growth of flint than it could by 

 merely depositing its final skeleton. 



In the same paper, Prof. Sollas points out that diatoms 

 and radiolarians must not be forgotten as makers of flint. 

 The warning was, indeed, timely. Continental geologists, 

 notably Dr. Eiist, had begun to describe the beautiful 

 sUiceous skeletons of radiolaria from Jurassic formations ; 

 and obscurer indications were known to occur even in the 

 Trias. But most of this work dates only from 1870. The 

 well-kaown Tertiary deposit of Barbados, which supplies 



* " Catalogue of Fossil Sponges," British Museum (1883) p. 28. 



t " On the Ocaurrence of a Freshwater Sponge in the Purbeck 

 Limestone," Geol. Mag., 1878, p. 220. 



X "On the Organic Origin of the Chert in the Carboniferous 

 Limestone Series," Geol. Mag., 1887, p. 443. 



§ " A Contribution to the History of Flints," Sci. Froc. S. Dublin 

 Soc, 1888, p. 4. See also Ann. and Mag. X'at. SUt., Ser. 5, Vol. VI., 

 p. 450. 



the material for dealers in microscopic slides, is loose and 

 incoherent, and has no apparent relationship to flint. 



Similarly, the diatomaceous deposits of lakes — the 

 " tripoli '' and " kieselguhr" of commerce — were familiar 

 as Tertiary materials, soft and powdery enough. In 

 modern seas, also, especially in the Antarctic Ocean, 

 siliceous oozes, formed of these humble plant-remains, 

 were revealed by successive surveys. 



Where, however, were the diatom-beds, freshwater or 

 marine, of older periods ? Why, moreover, were radiolaria, 

 practically nowhere beside the host of calcareous fossil 

 protozoa ? 



Dr. G. J. Hinde," in 1890, described well preserved 

 radiolaria from chert, occurring among Ordovician rocks in 

 Lanarkshire and other counties in the south of Scotland. 

 This was only the first of a series of papers, and " radio- 

 larian cherts" at present hold the field of interest. Messrs. 

 Hill and Jukes-Browne, t in describing calcite casts of 

 radiolaria from the English chalk, have discussed the in- 

 stability of the radiolarian skeleton. In fact, vast numbers 

 of the more ancient radiolaria are for ever lost to us. 

 Their remains have been, like those of so many sponges, 

 dissolved and converted into flint. 



Similarly, the still more delicate frustules of the diatoms 

 have practically disappeared. Even those described by 

 Mr. W. H. Shrubsole," from the London clay of Sheppey, 

 are converted into discs of iron pyrites. Yet how many 

 flints, particularly those of freshwater formations, may in 

 future be traceable to diatoms V 



These three humble groups of organisms, two of them 

 animal, one vegetable, may be regarded, then, as the 

 essential makers of flint. They extract the silica from 

 the sea-water, which derives it mainly in solution from the 

 laud ; they deposit this as colloid silica in their skeletons ; 

 on their death, or, in the case ofsponge3,on the ejection of 

 their spicules, the silica is slowly picked up again by the 

 water, and ultimately comes out, in the consolidated rock, 

 in the minutely crystalline form, chalcedony or flint. In 

 this form it is proof against long attacks, though its white 

 exterior, and sometimes its crumbling character through- 

 out, show that some solution takes place, and that the 

 unhappy organic skeletons are destined to no certain 

 rest. 



Dr. Hinde; seems to have gone too far in asserting that 

 the Carboniferous chert of Ireland was in no sense a 

 pseudomorph of the limestone. Unless it has actually 

 replaced the limestone, particle by particle, it is hard to 

 account for the occasional ramifications of the chert-masses, 

 and for the crossing of a mass from one bed of limestone 

 to another. Prof. Sollas properly insisted that flint may 

 replace all the structures of ordinary chalk — as, indeed, 

 section after section will assure the enquirer with 

 the microscope. The foraminifera, coccoliths, and so 

 forth, and, more rarely, the larger fossils, become 

 completely replaced by the silica as it is redeposited. 

 Similarly, the oolitic structure of some limestones becomes 

 preserved in flint, and may even remain in this form when 

 mineral changes have destroyed it in the actual limestone.^ 

 The whole position is clearly summarised by Prof. Judd;*'- 



* Ann. and Mag. Kat. Sxst, July, 1890, p. 40. 

 + " On tlie Occurrence of Radiolaria in Chalk," Quart. Journ. 

 Geol. Soc, Vol. LI. (1895), p 600. 

 t Nature, Vol. XXI. (1879), p. 132. 

 § Geol. Mag., 1887, p. 445. 



II Ann. and Mag. Nat. Mist., Ser. 5, Vol. VI. (1880), p. 447. 

 ^ See " Aids in Practical Geology," ed. 3, p. 204. 

 ** " The Student's LyeU " (1896), p. 255. 



