ON A PIECE OF CHALK 53 



hundred miles from Valentia, I am not quite sure that 

 it would even be necessary to put the skid on, so gentle 

 are the ascents and descents upon that long route. 

 From Valentia the road would lie down-hill for about 

 200 miles to the point at which the bottom is now 

 covered by 1700 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 as- 

 cent 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 black- 

 board, if you are so inclined; and, to the eye, it is 

 quite like very soft, greyish chalk. Examined chemi- 

 cally, it proves to be composed almost wholly of car- 

 bonate 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 microscope, 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 

 differences ; but as these have no bearing on the ques- 

 tion immediately before us, which is the nature of 

 the Globigerince of the chalk, it is unnecessary to 

 speak of them. 



GlobigerincB of every size, from the smallest to the 

 largest, are associated together in the Atlantic mud, 



