the Greensand compared with those of existing Species. 113 



Mr. Vicary and myself have thus taken out several hundred, 

 from which I have selected seventy-six out of the seventy- 

 nine illustrations in the plates. These, however, must not be 

 viewed as rare specimens, but rather as the more perfect ones 

 of myriads of the same kind in the deposit, which are all 

 more or less fractured, worn away by attrition, or otherwise 

 altered by petrifaction. 



When we consider that they are imbedded in quartz-sand, 

 and that therefore they must be the spicular remains of dead 

 and disintegrated sponges which, for some time previously, 

 had been drifting about at the bottom of the sea with the ma- 

 terial in which they are now found, we cannot wonder that, 

 under such circumstances, they should be chiefly the larger 

 spicules of the sponges to which they respectively belonged, 

 and that they should be more or less fragmental, and more or 

 less altered in shape by the trituration to which they have 

 been exposed — also that there should be almost an entire 

 absence among them of the delicate and more minute spicular 

 forms which in addition characterize most sponges. 



Nor should we wonder that the solvent influences which 

 have been affecting them for ages during and since their trans- 

 formation into chalcedony (for such is their present state) 

 have involved a certain amount of change in their form as 

 well as in their composition. Thus we And that their canals 

 are frequently distorted and enlarged, that they are more or 

 less filled with glauconite or brown oxide of iron, &c., or that 

 they are altogether obliterated, while their surfaces partake of 

 the botryoidal character, in miniature, of the mineral (chal- 

 cedony) into which they have been transformed. 



Still, uneven as their surface now is, and great as is the altera- 

 tion in other respects which they have thus undergone, the 

 greatest wonder of all is, how such delicate little objects could 

 survive the changing hand of time so long as to be presented 

 to us now, after an interval almost too oppressive in extent to 

 be conceived, in forms so unmistakable and so easily obtained 

 that they almost fall out of themselves from the sand in 

 which they are imbedded as distinctly and as separately as 

 if the deposit had been but of yesterday's formation. 



No less remarkable is the fact that, while the grains of 

 quartz-sand still retain their angles and smooth surfaces, the 

 surfaces of the spicules and those of every other organic par- 

 ticle amongst them present the dimpled or tubercled form of 

 chalcedony. Hence it becomes easy to determine at once what 

 has not been organized, from the beginning, however small the 

 particle may be. In short, the quartz-sand has yielded less 

 to the chalcedonizing influence than the organic remains. 



