Ixvi. PRESIDENT'S ADDRESS. 



chalk-mad of the present Atlantic. The white-chalk is very 

 homogeneods, more so perhaps than any other sedimentary rock, 

 and may be said to be almost pure carbonate of lime. Although 

 the white-chalk is almost always associated with chert and flints, 

 the chalk itself does not contain a particle of silica. The chalk-mud 

 of the Atlantic, on the other hand, contains from 20 to 30 per cent. 

 Although the chalk of our cliffs is of the purest form, it assumes a 

 very different character in various parts of the world. Sir Joseph 

 Prestwich instances a bed of white chalk from 23 to 30 feet 

 (Terrain Senonien), of Touraine, in which carbonate of lime is 

 entirely absent. A considerable proportion of the silica of the 

 chalk-mud consists of the spicules of sponges, of the spicules and 

 shields of Radiolarians, and the frustules of Diatoms. The 

 layers of flinty masses of nearly pure silica show frequently the 

 external form of more or less regularly-shaped sponges, fre- 

 quently filling up the cavities of Echini and Bivalves. We often 

 see the flint filling up the cavities of Galerites albogalerus or 

 Anancliytes ovatus, showing at the oral opening of the shell a 

 projecting knob. We have no escape from the conclusion that 

 after the death of the urchin the silica percolated into the shell 

 in solution, or in a gelatinous condition, and must have existed 

 in the form of organic silica distributed in the shape of sponge 

 spicules, and other siliceous organisms in the chalk, which have 

 been reduced or dissolved to a colloid state and accumulated in 

 moulds formed by the shells of embedded animals. 



Among the different forms of silica there are two which occur 

 everywhere in great abundance namely, crystallized silica as 

 quartz, and colloid silica known as opal. The former of these is 

 only acted upon by ordinary solvents, the latter is much more 

 easily affected and passes into solution with comparative ease, 

 especially in the presence of alkaline carbonates, and is capable also 

 of passing into crystallized quartz. That the silica forming flints 

 was originally dissolved in the waters of the sea there cannot be 

 any doubt, and that this dissolved silica was derived from the 

 rocks of the earth's crust, It has been calculated that the 



