Rev. T. G. Bonney — Lithodomous Perforations. 315 



tinued, the whole is dissolved usually without leaving any trace of 

 siliceous spicula. 



The sand and gravel in which these fossils are embedded are 

 loosely cemented by carbonate of lime, and it is by no means certain 

 at what period after the sponges were entombed this coating of 

 crystals was deposited upon them. But it is quite possible that 

 something similar to what has taken place in these Farringdon fossils 

 may occasionally occur in other cases as a preliminary to fossiliza- 

 tion, in consequence of the sponge, deprived of its sarcode, having 

 long soaked in water charged with carbonate of lime. 



The ordinary mode in which fossilization takes place, however, is 

 in one of the two following ways : either the sponge, after the de- 

 struction of its sarcode, becomes infiltrated by fine sediment which 

 completely fills up the interstices, and forms, as it were, a mould of 

 the sponge skeleton in which the fossilizing process takes place ; or 

 the sponge is simply buried in the deposit, which forms a nidus 

 about it, filling perhaps the tubules and oscular passages, or even 

 the superficial parts of the tissue, but leaving the latter for the most 

 part open and pervious, into which mineral matter is carried in solu- 

 tion, and there deposited. The result of this may be, either to fill 

 up the iaterspaces entirely, or merely to encrust and consolidate the 

 fibres as in the sponges of the gravels of Farringdon ; but in either 

 case there is formed around the fibre or twig, a mould, in which the 

 fossilizing process takes place, which is the same precisely as that 

 which is known to take place in fossils generally, viz., the removal 

 of the original material of the skeleton, and its replacement by 

 another. These changes are greater and more complete in proportion 

 to the antiquity of the deposit in which the fossil occurs. 

 ( To be concluded in our next number.) 



V. — On Certain Lithodomous Peefokations in Derbyshire.^ 

 By the Rev. T. G. Bonnet, M.A., F.G.S. 



IN the Geological Magazine for 1870 (Vol. VII., p. 267), I 

 published a brief account of some burrows in Derbyshire ; 

 one group of which, at the bottom of Miller's Dale, and on a scarp 

 of rock which was probably artificial, appeared to me wholly irre- 

 concilable with the theory which attributes them to the action of 

 Pliolades. This conclusion was questioned by Mr. E. Brown (Geol. 

 Mag., Vol. VII., p. 585), who had visited the spot, and, failing to 

 find the burrows in question, was of opinion that I had mistaken 

 a bed of toadstone -in the valley, for " limestone, and the vesicular 

 cavities therein for the borings of animals." Fortunately, in expec- 

 tation of some controversy, I had carried away a specimen, which 

 sufficed to demonstrate (Geol. Mag., Vol. VIII.. 13. 40) that I really 

 knew limestone from basalt. Wishing, however, to see whether by 

 some inadvertence I had not quite clearly described the locality, or 

 if the rock had been defaced after my visit, I returned thither, in the 

 Easter vacation of the present year, with the following result : 

 ' Eead before the Cambridge Philosophical Society, April 29, 1872. 



