ON A PIECE OF CHALK 9 



Happily, however, better evidence in proof of 

 the organic nature of the Globigerince than that 

 of analogy is forthcoming. It so happens that 

 calcareous skeletons, exactly similar to the 

 Gloligerince 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^. 

 Glcbigerince, and of the part which they play in 

 rock building, is singular enough. It is a 

 discovery which, like others of no less scientific 

 importance, has arisen, incidentally, 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 imperatively necessary it became for 

 sailors to ascertain with precision the depth 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 re- 

 cording of the form of coasts and of the depth 

 of the sea, as ascertained by the sounding-lead, 

 upon charts. 



At the same time, it became desirable to ascer- 

 tain and to indicate the nature of the sea-bottom, 

 since this circumstance greatly affects its goodness 



