and other Siliceous Bodies, 89 



form those which are found imbedded in the chalk flints ; and 

 some of the species so closely resemble those found in the 

 Grignon sand of the calcaire grossier, as to render it very diffi- 

 cult, if not impossible, to decide whether they are or are not 

 absolutely the same species. These organic remains are fre- 

 quently found in much greater quantities in some of the strata 

 of the pebbles than in others, which would seem to indicate 

 that they had been accidentally cast in and there imbedded ; 

 and it often occurs that in the very next stratum to the one in 

 which they abound few or none are to be seen. 



I have examined a considerable number of specimens of 

 Mocha stones, but have in no case observed any indication of 

 organic remains in them ; the moss-like appearance in many 

 of them being evidently of dendritical origin. Occasionally 

 there are appearances, as if beautiful thin organic tissues, some- 

 what similar to the reticulated cuticles of plants, had been im- 

 bedded in the mass. Sometimes they assume a nearly regu- 

 larly reticulated form, while at others they present a series of 

 irregularly shaped rings and spots much like those in the skin 

 of the leopard ; but in almost all such cases these appearances 

 are accompanied by an evident fracture in the mass ; and upon 

 a careful examination of many specimens of this description, 

 I am convinced that these appearances of organic structure 

 are but some of the many curious results that arise from the 

 infiltration between two closely approximating surfaces of 

 fluids containing solutions of metallic substances. A mass of 

 Hertfordshire pudding-stone which I examined appeared to 

 consist entirely of large and small rolled pieces and frag- 

 ments of chalk flints cemented together by crystalline quartz. 

 The larger of the imbedded masses, especially, presented all 

 the characteristic spongeous structure, spicula, and Forami- 

 niferce which are to be observed in almost every true chalk 

 flint. 



In all the specimens of agates and jaspers which I have ex- 

 amined, there are very frequently considerable spaces in which 

 no remains of spongeous texture is to be seen ; and these are 

 filled up with silex, which in some cases assumes the form of 

 chalcedony, while in others it has the banded appearance of 

 the Scotch pebbles or agates, being arranged in a series of 

 layers, which are more or less conformable to the shape of the 

 surface presented by the surrounding spongeous mass. When 

 the silex has a predisposition to the latter mode of arrange- 

 ment, it frequently happens that we find the decomposed and 

 free particles of the animal matter have assumed a form in 

 accordance with the law which affects the disposition of the 

 silex ; but when the arrangement assumed is that of chalce- 



