MIDDLE CHALK—CONDITIONS OF DEPOSIT. 
549 
In a still more recent volume* M. L. Cayeux, though not in¬ 
clined to regard such nodular structure in chalk as always the 
result of contemporaneous current-action, remarks that “ the 
nodular structure of the Inoceramus labiatus zone is that which 
has the greatest resemblance to the conglomerate of Station 196 
(Challenger Expedition). The study of this chalk has led me to 
the conclusion that its peculiar texture cannot be explained without 
admitting a strong dynamic action operating on the sea-floor at 
the very time of its accumulation.” 
The conglomerate referred to by M. Cayeux is described on 
pp. 99 and 171 of the Challenger Report on Deep Sea Deposits. 
It was found on hard ground at a depth of 825 fathoms, the trawl 
bringing up large “ irregular fragments of a honeycombed con¬ 
glomerate, overgrown with Serpula, Polyzoa, and Sponges. . . 
The sections showed that the conglomerate was composed of Fora- 
minifera and calcareous Algae, cemented together into a hard 
crystalline limestone.” Although, as Sir J. Murray observes, 
“ this rock would seem to have been formed in comparatively 
shallow water near land ;” the occurrence is important, as indicat¬ 
ing a hard, bare, current-swept floor at a depth of 825 fathoms. 
At another locality, moreover, the Challenger seems to have 
found a really nodular and indurated Globigerina ooze. This 
was at Station 192a, off the Ke Islands, west of New Guinea. 
Here the bottom at 140 fathoms was blue mud, but from 129 
fathoms the trawl brought up many rounded nodules of very 
hard, calcareous rock, consisting entirely of the shells of Globigerina, 
Pulvinulina, and Orbulina, in a calcareous matrix, and containing 
79’56 per cent, of carbonate of lime. It is suggested that these 
concretions “ are hardened portions of a deep-sea deposit, formed 
at a much greater depth, and subsequently elevated into the posi¬ 
tion in which they are now found.” (Op. cit., p. 171.) 
The occurrence of “ rounded, compact chalky nodules appar¬ 
ently formed of the deposit ” is recorded at Station 44, the deposit 
being described as “ blue mud ” with 24 • 61 per cent, of calcium 
carbonate, depth 1,700 fathoms. Again, at Station 191a, near 
the Arron Islands, at a depth of 580 fathoms the green mud, with 
40 per cent, of calcium carbonate, contained “ some concretions of 
Globigerina ooze cemented into a fine, almost opaque paste of 
carbonate of lime.” (Op. cit., p. 95). A chalky Pteropod ooze 
containing “ hard chalky concretions ” was found by the American 
Blake Expedition near Cuba in 860 fathoms. 
Hard ground, presumably swept clear of deposit by bottom 
currents, was found at various depths by the Challenger Expe¬ 
dition, as, for instance, between 325 and 950 fathoms off the Spanish 
* Contribution a l’etude Micrographique des Terrains Sedimentaires, 
p. 557 (1897). 
N 2 
4219. 
