76 The Irish Naturalist. 



Prof. Hull,' Prof. Sollas" and Dr. Hinde^ showed that sponge- 

 spicules were abundant in such specimens, as in the Vectian 

 cherts of England. The cherts must, indeed, be attributed to 

 the solution of the siliceous skeletons of organisms, such as 

 sponges, radiolarians, and diatoms, and the aggregation and 

 redeposition of the silica around their remains, and often as a 

 replacement of the limestone. While the fine limestone-mud 

 may be thus replaced b}^ chert, the • larger calcareous fossils 

 often escape ; and finally these become dissolved away, leaving 

 only hollow moulds in the hard chert. A pleasing instance 

 of this, with casts of stems of crinoids, occurs in a nearly 

 vertical bed in the shales north of Brook's End on the Lough- 

 shinny coast. Of the pseudomorphic replacement of chalk by 

 flint there can be no manner of doubt ; and the same process, 

 acting long subsequently to the consolidation of the rock, 

 has given us the frequently irregular chert nodules of the 

 Irish Carboniferous Limestone.'^ 



The second type of alteration of the limestone is its con 

 version into a dolomite by substitution of magnesia for lime. 

 The abrupt change of colour, from the blue-grey lime-stone to 

 the brown dolomite, is often startling; probably the chemical 

 substitution includes the introduction of iron also, the carbo- 

 nate of iron then readily oxidising and colouring the altered 

 patches. This irregular chemical change can be seen at St. 

 Doolagh's ; in a coarse degree on the shore south-east of Rush ; 

 and, nearer Dublin, in the low exposure under the drift at the 

 south-east end of Sutton strand.^ 



The highest Carboniferous beds in Co. Dublin are certain 

 shales in the north, which have been rather variously located 

 in the system. Mr. Baily^ preferred to retain them as Lower 

 Coal Measure Shales, to which they had been referred byjukes.7 

 They are seen, in striking contrast to the more resisting lime- 

 stones, caught in the sjmclinals of the latter south of the ham- 

 let of Loughshinn}^ The earth-movements which have so 

 beautifully folded the Carboniferous Limestones along that 

 coast, making it one of the finest studies that the physical 

 geologist can desire, have contorted and compressed the black 

 and orange shales until they imitate the reversed folds and 

 thrusts of a mountain-chain. During these post -Carboniferous 

 movements, denudation was activel}" at work, and some 2,000 

 feet of Coal Measures and other Upper Carboniferous strata, 

 formerl}" present, have been entirely removed from Co. Dublin. 



(TO BE CONTINUED.) 



* Sci. Trans. Roy. Dublin Soc, i. (new series), p. 80. 



^ " On Spon^e-spicules in Chert from Ireland," Ann. and Mag. Nat. Hist., 

 5th ser., vol. vii., p. 141. 



• 3 " On the Organic Origin of Chert in the Carboniferous Limestone of 

 Ireland," Gcol. Mag., 1887, p. 435. 



4 See, however, Hinde, Geol. Mag., 1887, p. 445; G. H. Kinahan, /^/V/., 

 p. 521; and Hull, ibid., p. 525. 



5 See Apjohn, G. S. D., i., 371 ; and Scouler, ibid., 382. 



« " Palaeontology of Co. Dublin," G. S. /., v., 94. 



7 G. S. £>., viii. (1859), 162. 



