14 



of Chalcedonic silica is very irregular, the one form apparently 

 gradually merges into the other. It would seem that the 

 silica in these examples, to a certain extent retained the cha- 

 racters of the originally colloidal silica of the spicules, whilst 

 in the majority of the spicules in this material the metamor- 

 phosis of the silica had been extended further , until , with 

 the exception of a few minute spots, the whole of the spicule 

 had become changed into chalcedony. I may remark here, 

 that so distinctly are 'the sponge spicules in this deposit shown 

 by polarized light, that, in a small quantity of the flint-meal 

 mounted in balsam , the minutest particle of spicule at once 

 reveals itself by the prismatic tints which it exhibits, and I 

 availed myself of this fact to determine the characters of some 

 delicate lamina, which otherwise had the appearance of small 

 portions of shell. 



Not only has chalcedonic silica thus replaced the original 

 colloidal silica in these spicules, but it has also, in the major- 

 ity of instances, completely filled their interior canals, so that 

 the spicules now appear solid throughout. Here and there 

 however, spicules are met with , in which the canals are either 

 wholly or in part preserved , but even in these instances they 

 do not present the form of regular even tubes as in recent 

 sponge spicules, but are abnormally and irregularly enlarged. 

 Sometimes the canal yet extends throughout the length of the 

 spicule, in other instances only a small part in the upper and 

 thicker part of the spicule is to be seen. The canal, where 

 retained , appears to be hollow , or infiltrated more or 

 less with ochreous particles, and an example of this latter 

 in which the canal is shown as a thin black tube extending 

 along the central axis of the spicule is given on Plate V. 



% 30- 



The present condition of the sponge spicules in the interior 

 of this Horstead flint, thus shows a great alteration from their 

 original condition. The smooth surfaces have become rough 

 and eroded, the delicate spines have been dissolved away, and 



