On a Piece of Chalk 79 



tifully-constructed calcareous fabric, made up of a num- 

 ber of chambers, communicating freely with one another. 

 The chambered bodies are of various forms. One of the 

 commonest is something like a badly-grown raspberry, 

 being formed of a number of nearly globular chambers 5 

 of different sizes congregated together. It is called 

 Globigerina, and some specimens of chalk consist of little 

 else than Globigerina and granules. 



Let us fix our attention upon the Globigerina. It is 

 the spoor of the game we are tracking. If we can learn 10 

 what it is and what are the conditions of its existence, 

 we shall see our way to the origin and past history of 

 the chalk. 



A suggestion which may naturally enough present itself 

 is, that these curious bodies are the result of some process 15 

 of aggregation which has taken place in the carbonate 

 of lime; that, just as in winter the rime on our windows 

 simulates the most delicate and elegantly arborescent 

 foliage proving that the mere mineral water may, under 

 certain conditions, assume the outward form of organic 20 

 bodies so this mineral substance, carbonate of lime, hid- 

 den away in the bowels of the earth, has taken the shape 

 of these chambered bodies. I am not raising a merely 

 fanciful and unreal objection. Very learned men, in 

 former days, have even entertained the notion that all 25 

 the formed things found in rocks are of this nature; and 

 if no such conception is at present held to be admissible, 

 it is because long and varied experience has now shown 

 that mineral matter never does assume the form and 

 structure we find in fossils. If any one were to try to 30 

 persuade you that an oyster-shell (which is also chiefly 

 composed of carbonate of lime) had crystallized out of 

 sea-water, I suppose you would laugh at the absurdity. 

 Your laughter would be justified by the fact that all ex- 



