June 1, 1899.] 



KNOWLEDGE 



129 



from extraneous sources, or was it once, as D'Aubuisson 

 said, uniformly diffused throughout the mass ? 



It is necessary to lay some stress on the infiltration 

 theory, because it is so commonly true when applied to 

 lavas, and may thus be true also of certain sedimentary 

 rocks. The waters of modern geyser-regions frequently 

 impregnate the trunks of trees before they have had time 

 to decay under ordinary influences. Step by step, the 

 organic matter passes away in combination with the alkalies 

 of the solution, and the silica is deposited in its place. 

 Here we have an example of the rapid silicification of 

 non-volcanic material. 



Yet we can hardly go as far as a recent writer, M. de 

 Ctossigny," and argue, from the occurrence of true flint in 

 fissures, that the nodular flints result from the filling 

 up of cavities in the chalk. Many of these nodules are 

 hollow, and so far remind one of the gradual choking of 

 cavities in lavas, by the deposit of chalcedony on their 

 walls. Such hollows, however, in the flints have long ago 

 received another explanation ; and their original contents, 

 which are sometimes retained, have impressed observers 

 as far back as the beginning of the century. 



The fact is that, again and again, the fracture of a 

 nodular flint discloses a fossil sponge. The hard parts, 

 the spicular meshwork, of the sponge may be in very 

 various conditions. Sometimes the general form is 

 preserved, but the spicules are lost in a deposit of extraneous 

 silica, which has bound the whole together into a white 

 and knotty mass. Sometimes this mass crumbles between 

 the fingers, sometimes it is almost as resisting as the 

 nodule round it. In the latter case, dark flint will generally 

 be seen filling its interstices, and the spicular structure is 

 revealed in microscopic sections. 



At other times, a pseudomorph or replacement of the 

 sponge may occur in limonite, which is mere hydrated 

 iron oxide, or iron rust. This probably results from the 

 decay of a previous pseudomorph in marcasite, an iron 

 sulphide which is highly susceptible to decay. Visitors to 

 the south-east of England will notice how frequently, in 

 the grey Lower Chalk, sponges have thus been replaced by 

 brassy marcasite. 



At other times, a mere powder remains in the hollow of 

 the flint, and is found, on microscopic examination, to 

 contain a few spicules, and silicitied foraminiferal shells. 

 Such powders ofler a fertOe field to the student of 

 microzoa.f 



Seeing that the enclosed sponges possessed, originally 

 at any rate, siliceous skeletons, it seemed natural to con- 

 clude that additional silica had collected round them. 

 Zirker quotes Parkinson, Guettard, de Luc, Faujas St. 

 Fond, Dolomieu, and Huot, as early observers in this field. 

 But the source of the additional silica still remained in 

 doubt. 



The association of flint and chert with sponge-remains 

 became more and more evident as geological surveys 

 spread. The grey flints of the Upper Greensand in the 

 Isle of Wight are shot through, as it were, with minute 

 rods, visible to the naked eye. Similar opaque rods abound 

 in the brown cherts (cemented sands) of the Hythe Beds 

 in the hills of Surrey. These are sponge-spicules in the 

 form of casts, or even the hollows left by them ; and their 

 forms show that they also belonged to siliceous genera. 

 The fact that these spicules are so often dissolved, like 



* " L'origine des silex de la craie," Sail. Soc. , Q4ol. de France, 

 3me. Ser., Tome IX. (1880J, p. 47. 



t See Jos. Wriglit, '■ Cretaceous Foraminifeva of Keady Hill," 

 Proc. Belfast Nat. Field Club, 188.5-6, p. 328. 



X " Lehrbuch der Petrograplue," Second Edition, Bd. III., p. 552. 



those of the sponges enclosed in nodular flints, is an im- 

 portant clue to the origin of the flint itself. 



Similarly, Giimbel " reports that the siliceous limestones 

 or marls of the Flysch system, in all the localities visited 

 by him in the Alps, consist almost exclusively of sponge 

 spicules. He does not comment on the actual condition 

 of the spicules ; but Dr. G. .J. Hindef has remarked that 

 such bodies seldom remain in their original condition. 

 Even when they still consist of silica, this is rarely the 

 colloid form, easily soluble in hot solutions of caustic 

 potash, but a replacement, in whole or in part, by chalce- 

 donic silica. The axial canals of the spicules are, more- 

 over, often filled by green glauconite, the curious sOicate 

 which accumulates from sea-water in so many recent 

 deposits, and which colours our fossil '■ greensands " and 

 glauconitic chalks. On complete solution of the spicule, 

 the resisting cast of glauconite remains, often with the 

 diverging rays that indicate, by their grouping, the 

 division to which the original sponge belonged. 



The ease with which the silica of sponge-spicules may 

 go into solution is emphasised in the U. Jurassic beds 

 of Bohemia and Wiirtemberg, where the sponges preserve 

 their form, but have entirely, or almost entirely, decayed. 

 Calcite has, however, crystallised in the hollows left by the 

 removal of the spicular mesh, and the appearance is that 

 of a complete calcareous sponge. Relics of the original 

 silica may occur ; but the character and arrangement of 

 the spicules, and comparison with forms from other 

 localities, serve more fully to convince the palsontologist 

 that a thorough pseudomorphosis has occurred. 



A few calcareous sponges, on the other hand, have 

 locally been converted into siliceous pseudomorphs,! which 

 adds to the complication. The microscopic study and col- 

 lation of sponge-spicules has thus become essential, and 

 geologists have reaped the results of the patient labours 

 of the specialists. 



In England, Prof. W. J. Sollas and Dr. G. J. Hinde 

 have laid great stress upon the readiness of spicules to 

 dissolve. The solution of such bodies may even be traced 

 in the depths of existing seas. The ends of detached 

 spicules become rounded, the axial canals become enlarged, 

 and it is easy to note all stages of the process of destruction. 



Hence the above-named authors have been foremost in 

 urging that, not only did flint accumulate round siliceous 

 sponges, but that the solution of the spicules of these and 

 of other sponges provided the silica which thus accumulated. 

 They see no need for mysterious springs and siliceous 

 infiltrations, the occurrence of which, in association 

 with beds containing sponge-remains, would be in itself 

 a remarkable coincidence. The very absence of spicules 

 from a rock, which otherwise appears to be an oceanic 

 ooze — a rock, in fact, in which spicules should reasonably 

 occur — and the presence of flint instead, is, for Prof. 

 Sollas,? clear evidence of the origin of the flint. 



In 1878, Prof. Sollas | wrote : " In compact strata, such 

 as chalk or limestone, it may be taken as an almost 

 invariable rule that the replacement of organic silica by 

 calcite is always accompanied by a subsequent deposition 

 of the silica in some form or other ; and thus, if one finds 



* " Au9 den Alpen " ; Letter in Neues Jahrbuch fiir Min., i^e., 

 1880, Bd. II., p. 287. 



t "On Beds of Sponge-remains in the South of England," P/iil. 

 Trans., 1885, Part. II., p. 427. 



:;: Zittel, "HandbuchderPalseontologie," Bd. I. 



§ On the Flint Nodules of the TrimiuiDgham Chalk," Ann. and 

 Mag. Nat. Hut., Ser. 5, Vol. TI. (18S0), p. 460. 



\\ ■' On the Structure of the genus Catagma," Ann. and Mag. Nat. 

 Sisf., Ser. 5, Vol. II., p. 361. 



