353 
LOWER CHALK—SEDIMENTATION. 
extensive deposit of decomposing organic matter is thus formed. 
In this way alternations of beds with and without phosphatic con¬ 
cretions may be produced. He is also of opinion that the concre¬ 
tions are formed in the deposit as it accumulates, the phosphatic 
material being separated out of solution and deposited round various 
kinds of foreign bodies present in the sediment. 
Walther, however, does not explain very clearly how the solution 
of phosphate of lime may be formed, and on this point my colleague, 
Mr. J. J. H. Teall, sends me the following note “ Decomposing 
organic matter gives rise to carbonic acid and ammonia. These 
will readily dissolve the bones of fish, and thus bring phosphate 
of ammonia and carbonate of lime into solution. If, then, 
this solution comes into contact with another containing an excess 
of carbonate of lime, precipitation of phosphate of lime will occur.” 
Chalk above the Totternhoe Stone .—In the higher part of the 
Lower Chalk there is clear evidence of the declining power of 
both the currents which seem to have played a part in the form¬ 
ation of the lower beds. The chalk above the Totternhoe Stone 
becomes gradually less grey and less shelly, till it merges into a 
nearly white chalk, and it was pointed out long ago by Mr. Hill * 
that this was due to the increasing preponderance of fine chalky 
matter and of calcareous spheres. The deposit finally assumes the 
character of an oceanic ooze, without any glauconite, and with no 
quartz grains which could not have been carried by the wind. 
In the “ rag-bed ” of the country between Bisborough and Dun¬ 
stable we have evidence of a local and temporary recurrence to the 
conditions of the Totternhoe Stone, but in the whiter and upper 
part of the Lower Chalk there is no trace of current action and 
every indication that the deposit was formed in the open sea at some 
distance from land. In other words we may reasonably conclude 
that the Cenomanian sea had become both larger and deeper in con¬ 
sequence of a continued subsidence of the whole region, so the 
currents which set off the shore could not carry any terrestrial 
detritus into the more central parts of the sea. 
Belemnite Marls .—Under these conditions some thickness of 
chalky ooze was slowly accumulated, but instead of a passage into 
the equally calcareous material of the Melbourn Bock, as we 
might have expected, we find nearly everywhere an interposition 
of argillaceous marl either in one or in two beds. It would seem, 
indeed, as if for a brief time there was a recurrence to the con¬ 
ditions of the Chalk Marl, mud-bearing currents being 
re-established over nearly the whole of the area of deposition. 
There are, however, a few districts where there is either a com¬ 
plete or nearly complete absence of marl, and just such a passage 
from the Lower to the Middle Chalk as we should have expected to 
be the more general type of sequence. Thus in the north-west of 
Norfolk, where all marly beds are so conspicuously absent, there 
is no representative of the Belemnite marls. In South W ilts and 
* See Quart. Journ. Geol. Soc. Vol. xlii., p. 228 ( 1880 ). 
