50 ON A PIECE OF CHALK 



ture we find in fossils. If any one were to try to per- 

 suade 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 

 experience 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 Globigerina is not the 

 product of anything but vital activity. 



Happily, however, better evidence in proof of the 

 organic nature of the Globigerince than that of analogy 

 is forthcoming. It so happens that calcareous skele- 

 tons, exactly similar to the Globigerince of the chalk, 

 are being formed, at the present moment, by minute 

 living creatures, which flourish in multitudes, literally 

 more numerous than the sands of the sea-shore, over 

 a large extent of that part of the earth's surface which 

 is covered by the ocean. 



The history of the discovery of these living Globi- 

 gerince, and of the part which they play in rock build- 

 ing, is singular enough. It is a discovery which, like 

 others of no less scientific importance, has arisen, in- 

 cidentally, out of work devoted to very different and 

 exceedingly practical interests. 



When men first took to the sea, they speedily learned 

 to look out for shoals and rocks; and the more the 

 burthen of their ships increased, the more impera- 

 tively necessary it became for sailors to ascertain with 

 precision the depths of the waters they traversed. Out 

 of this necessity grew the use of the lead and sounding 

 line; and, ultimately, marine-surveying, which is the 

 recording of the form of coasts and of the depth of the 

 sea, as ascertained by the sounding-lead, upon charts. 



