338 M. p. L. COKNET ON THE UPPER CEETACEOTJS SERIES 



described, that the rich phosphate is the residue of a chemical altera- 

 tion undergone by the Brown Phosphatic Chalk after its deposition. 

 This alteration has caused the fossil shells preserved as carbonate of 

 lime to disappear, together with the greater part of the carbonate 

 of lime originally contained in the rock. It has not aifected the 

 flints or the siliceous sponges, or the bones of vertebrate animals, in 

 the composition of which there is a large proportion of phosphate 

 of lime. 



The agent which has dissolved the carbonate of lime could only 

 have been water charged with carbonic acid. Any other acid 

 would at the same time have acted on the phosphate as well as on 

 the carbonate. 



The water, rendered corrosive by carbonic acid, has not come 

 from the interior of the earth, that is to say it is not water from 

 springs ; for if such had been the case, it would have first attacked 

 the beds of "White Chalk which underlie the Brown Phosphatic Chalk. 

 One would only find the cavities produced by its action, and the 

 pockets of rich phosphate would be found not only at the places 

 where the Brown Phosphatic Chalk underlies the Tertiary or Quater- 

 nary deposits, but also where it is covered by the Tufaceous Chalk. 

 But up to the present time nothing of this sort has been found to 

 exist, notwithstanding the extent of the works in the neighbourhood 

 of Mons. A few pockets have been discovered under the Tufaceous 

 Chalk, but only where it was from two to three feet thick. In this 

 last instance, the Tufaceous Chalk was traversed by a pocket of 

 which the upper part was always found filled with Tertiary or 

 Quaternary deposits. Sometimes also pockets of rich phosphate 

 have been worked in the White Chalk, but it has also always 

 been proved that they were only the prolongation in depth of 

 those which traversed the Brown Phosphatic Chalk. The dissolving 

 action had not been arrested at the base of this latter, but it had 

 penetrated into the White Chalk, which is composed of nearly pure 

 carbonate of lime. * 



Since the water charged with carbonic acid has not risen from 

 below, it could only have come from above. We must look to the 

 atmosphere for its source. 



The study of the phosphatic beds of the neighbourhood of Mons 

 raises many scientific questions of the highest interest : but I shall 

 not now attempt to discuss them. I shall confine myself to saying 

 a few words on one point only. 



At those places where the succession of the beds of phosphatic 

 chalk, d} and d^, is complete, the total thickness is from 60 to 85 

 feet. This would give an average of 70 feet. 



The specific gravity of the phosphatic chalk, drained of the quarry 

 water, is 1*55. A cubic foot of this rock weighs therefore 31 lb. 

 12 oz. We may therefore take as a mean minimum a proportion 

 of tribasic phosphate of lime of 18 per cent. One cubic foot 

 therefore would contain 5 lb. 11 oz., and 70 cubic feet 355 lb.; 

 that is to say, that for every square foot measured on the surface of 



