ORIGIN OF CHALK 



137 



tinguished by a very remarkable purity, especially in its 

 uppermost flint-bearing beds, which are almost entirely 

 free from admixture with terrigenous sediment. This of 

 itself would suffice to show that it must have accu- 

 mulated in a clear sea, uncontaminated by muddy water, 

 and seldom visited by showers of volcanic ash. Its 

 nearest equivalent at the present day is the grey ooze 

 of the deep sea, on which so much light has been thrown 



FIG. 36. View of a Chalk Pit at Horstcad, near Norwich, showing 

 the position of the paramoudra. From a drawing by Mrs. 

 Gunn. Taken from Lyell's " Student's Elements of Geology," 

 with kind permission of Messrs. A. & J. Murray. 



by the dredgings of the Challenger. Both chalk and 

 grey ooze consist to a large extent of coccoliths (Fig. 37), 

 extremely minute discs or rings of carbonate of lime, 

 which were for a long time a standing puzzle to the 

 biologist. There was a general disposition at one time 

 to regard them as the skeleton of some simple form of 

 calcareous alga ; but quite recently they have been 

 definitely shown by Lohinann to belong to a lowly 



