468 ME. A. STBAHAN OX A PHOSPHATIC CHALK [Aug. 1 896, 



"Without, perhaps, inferring actual re-elevation on this evidence, I 

 should so far agree with Dr. Hume that the rock has originated in 

 a decided, though temporary, change in the conditions under which 

 :tne Chalk was being deposited. To suppose a change to have taken 

 place in the strength or direction of the local currents seems less 

 ^violent than to assume a reversal of the movement of the earth's 

 ■crust. 



The glauconite and phosphate of lime occur in the form and 

 position in which they were deposited, for a large proportion of both 

 nils the interior of more or less perfect foraminifera. In this respect 

 the glauconite differs from that found in the Greensands, where 

 it occurs as loose grains seldom enclosed in their original moulds 

 and generally with an appearance of having been corroded. Both 

 minerals, as shown in the Challenger Reports (' Deep Sea Deposits/ 

 pp. 382, 383), are being formed on the sea-bottom at the present 

 day, and in constant association. Glauconite appears in its most 

 typical form and greatest abundance along high and bold coasts 

 where no rivers enter the sea and where accumulation is not rapid. 

 Between 200 and 300 fathoms it is more abundant than in deeper 

 water, and though glauconitic casts have been met with at depths 

 of over 2000 fathoms, they are stated never to occur in truly pelagic 

 deposits (op. cit. p. 396). Its occurrence, therefore, in the phosphatic 

 .chalk indicates a comparatively shallow-water origin for the rock. 

 The foraminifera, though, as shown by Mr. Chapman in the appendix, 

 indicating a deeper-water origin for the Southerham than for the 

 Taplow phosphatic chalk, support this inference, while the mode of 

 occurrence of the phosphate points in the same direction, for the 

 source of the phosphoric acid seems undoubtedly to lie in the 

 remains of fish which lived on the spot, and which are unlikely to 

 have been inhabitants of the deeper part of the ocean. This accords 

 also with the view that has been suggested by what we know of 

 the limits of the Chalk-sea, namely, that no part of the English Chalk 

 was formed far from land, and presumably, therefore, none in any 

 great depth of water. 1 We may perhaps infer with some probability 

 that the Southerham phosphatic chalk was formed at a depth of 

 between 200 and 600 fathoms. 



In the same volume (' Deep Sea Deposits,' pp. 396, 397) it is 

 pointed out * that phosphatic nodules are apparently more abundant 

 in deposits along coasts where there are great and rapid changes of 

 temperature, arising from the meeting of warm and cold currents. 

 .... It seems highly probable that in these places large numbers of 

 pelagic organisms are frequently killed by these changes of tempe- 

 rature, and may in some cases form a considerable layer of decom- 

 posing matter on the bottom of the ocean.' The same thing is 

 stated to occur where large quantities of fresh water are thrown 

 into salt water by floods. 



The facts observable in connexion with the phosphatic chalk 



1 On this subject see also ' On the Microscopic Structure of the Zones of the 

 •Chalk,' by A. J. Jukes-Browne, Proc. Yorks. Geol. & PoL Soc. vol. xii. part v. 

 (1894). 



