the Chalk, Greensands and Oolites. 193 



grains of chalk from the centre of a paramoudra, where we might with the greatest 

 degree of reason expect to have found a more than ordinary quantity of silex inter- 

 mixed, yielded but four per cent, of matter insoluble in muriatic acid, and of this 

 probably one or more per cent, of the weight was owing to the impurity of the acid 

 employed. If the immediate cause of the attraction of the silex had been due only 

 to the presence of the spicula, it is probable that the animal matter would have 

 undergone decomposition with a much greater degree of rapidity than appears to 

 have been the case, and that the spicula would have been disengaged and preci- 

 pitated to the bottom of the cavity caused by the imbedment of the sponge. But 

 if we consider the attractive principle to have been the animal substances present, 

 it will readily account for all the horny parts of the sponge, with the enclosed 

 extraneous bodies, having retained the positions which they occupied during life ; 

 as the immediate and progressive deposition of the silex w^ould probably afford a 

 greater degree of strength and support to the tissues than that abstracted by their 

 decomposition, and thus the whole would be sustained in its pristine form. The 

 silicified shells of the greensand of Blackdown confirm this view of the cause 

 of silicification, for in these testacea there are no siliceous spicula to serve as 

 nuclei ; the carbonate of lime having been completely dissolved and carried off" 

 by percolation, while the remaining animal matter, it would appear, has influenced 

 the deposit of a solid and compact mass of pure siliceous material, unmixed with 

 sand and unimpregnated by the silicate of iron immediately surrounding it ; and 

 the ligamentous hinge of the bivalves is frequently replaced in the same manner 

 as the other parts of the shell. 



In the corals of the mountain-limestone, and in those of the Tisbury limestone 

 also, the same attraction appears to have been exerted, as in both instances we 

 find a considerable portion of the carbonate of lime has been removed and its place 

 supplied by silex ; and in neither case can we suppose the previous existence of 

 siliceous matter as an attractive agent. 



From the foregoing facts, therefore, it appears probable, that these abundant 

 deposits of silex are in a great measure due to the presence of either animal or 

 vegetable matter, and especially to the former. 



Similar phsenomena have taken place, but with a different material, in the London, 

 Kimmeridge, and Oxford clays ; the fossilizing substance in such cases being sul- 

 phuret of iron in the place of silex. In nearly all these animal and vegetable 

 remains, the tissues are literally replaced by sulphuret of iron. In the numerous 

 pyritical remains of stems of trees and of fruits in the London clay we find 

 the cells and vessels often completely hollow ; while the delicate tissues form- 

 ing their walls have been replaced by bright, thin films of pyrites. In the animal 

 remains in the same formation this attraction has been exerted in a very striking 



VOL. VI. SECOND SERIES. 2 C 



