ON A PIECE OF CHALK 67 



we are tracking. If we can learn 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 

 of aggregation which has taken place in the carbonate of 

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

 ulates the most delicate and elegantly arborescent foliage 

 — proving that the mere mineral water may, under certain 

 conditions, assume the outward form of organic bodies — 

 so this mineral substance, carbonate of lime, hidden away 

 in the bowels of the ea^rth, 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 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 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 expe- 

 rience tends to show^ that oyster-shells are formed by the 

 agency of oysters, and in no other way. And if there were 

 no better reasons, we should be justified, on like grounds, 

 in believing that Glohigerina is not the product of anything 

 but vital activity. 



Happily, however^ better evidence in proof of the organic 

 nature of the Globigerime than that of analogy is forthcoming. 

 It so happens that calcareous skeletons, exactly similar to 

 the GlohigerifUB of the chalk, are being formed, at the pres- 

 ent moment, by minute living creatures, which flourish in 



