394 
MEMOIR OF CALEB B. ROSE. 
cretaceous, the strata consequently exhibited a dissimilar minera- 
logical character.” Subsequent researches fully hear out this view 
of the case.* With regard to the “ ramose Zoophyte ” of the 
so-called Sponge Bed, he cautiously remarks that the nature of the 
organism is “ not satisfactorily determined.” It is now regarded 
as an inorganic structure by Professor Hughes, f 
Bose also mentions that “ The singular strue observed by 
Mr. Mantell to occur at tlie natural separations of the chalk in 
Sussex, are also seen here, particularly in the pits at Marham and 
Westacre. Mr. Mantell thinks these strue were ‘produced by 
a subsidence of the strata which caused them to slip over each 
other before they were entirely consolidated.’ May they not also 
be the result of concussion from remote volcanic action 1 ” Like 
appearances have since been noticed in Yorkshire and elsewhere ; 
and it is thought that in some cases they are due rather to 
“incipient crystallisation” than to agents producing slickensides. £ 
Bose mentions the various kinds of flints, including thin tabular 
flint : and he asks, “ Does it not favour the opinion, that the 
siliceous molecules, by elective attraction, separate themselves from 
the calcareous matter, above and below certain parallels determined 
by the proportion of silex contained in the chalk, and approach 
each other until they arrange themselves in the tabular form ? ” 
This explanation would apply rather to the nodules that occur 
in layers than to the tabular flint that occurs in veins that cross 
the beds obliquely; but, in illustration, Bose quotes from the 
‘Penny Magazine’ of October 4th, 1834, the statement that when 
the masses of clay mixed with ground flints, prepared for making 
fine pottery and china, “ are allowed to stand unused for some 
time, it often happens that the particles of the powdered flint 
separate from the clay into detached hard stony nodules. The 
observation of this fact has thrown considerable light on the 
probable origin of the nodules of flint in chalk, a subject which 
was very obscure, and of which no satisfactory theory had pre- 
viously been proposed.” This suggestive statement was based on 
* A. J. Jukes-Browne and W. Hill, Quart. Journ. Gcol. Soc. vol. xliii. 
p. 544. See also C. Reid and G. Shannan, Geol. Mag. 188tj, p. 55. 
f Quart. Journ. Geol. Soc. vol. xl. p. 273. 
£ Quart. Journ. Geol. Soc. vol. xxix. pp. 417—419. 
