*x.] ON A PIECE OF CHALK. 1S3 



the road would lie down-hill for about 200 miles to the 

 point at which the bottom is now covered by 1,700 

 fathoms of sea-water. Then would come the central 

 plain, more than a thousand miles wide, the inequalities 

 of the surface of which would be hardly perceptible, 

 though the depth of water upon it now varies from 

 10,000 to 15,000 feet; and there are places in which 

 Mont Blanc might be sunk without showing its peak 

 above water. Beyond this, the ascent on the American 

 side commences, and gradually leads, for about 300 

 miles, to the Newfoundland shore. 



Almost the whole of the bottom of this central plain 

 (which extends for many hundred miles in a north and 

 south direction) is covered by a fine mud, which, when 

 brought to the surface, dries into a greyish-white friable 

 substance. You can write with this on a blackboard, if 

 you are so inclined ; and, to the eye, it is quite like very 

 soft, greyish chalk. Examined chemically, it proves to 

 be composed almost wholly of carbonate of lime ; and if 

 you make a section of it, in the same way as that of the 

 piece of chalk was made, and view it with the micro- 

 scope, it presents innumerable Globigerince embedded 

 in a granular matrix. 



Thus this deep-sea mud is substantially chalk. I say 

 substantially, because there are a good many minor dif- 

 ferences ; but as these have no bearing on the question 

 immediately before us, — which is the nature of the 

 Globigerince of the chalk, — it is unnecessary to speak 

 of them. 



Globigerince of every size, from the smallest to the 

 largest, are associated together in the Atlantic mud, and 

 the chambers of many are filled by a soft animal matter. 

 This soft substance is, in fact, the remains of the creature 

 to which the Globicjeriiia shell, or rather skeleton, owes 

 its existence — and which is an animal of the simplest 



