316 A. J. Jukes- Browne—Relative Age of Flints. 
Prof. Hébert as aids in dividing one zone from another. Further 
observations on this succession of forms of flint, with the view of 
determining how much dependence can be placed upon them, would 
be very useful; but the fact of there being differences is unquestion- 
able and is evidence for the contemporaneous origin of the nodules, 
The flints with hollow tubular cores and spaces to which Mr. 
Abbott refers are specially characteristic of the Micraster corangui- 
num zone, and have generally been formed round the bifurcating 
branches of Doryderma ramosum (Spongia ramosa, Mantell), not round 
Siphonia stems as he imagines. Doryderma was a siliceous (Lithis- 
tid) sponge and its subsequent solution has left the flints in the 
condition which Barrois calls cariés. 
Now it is clear that in such nodules the flint must have been 
formed around and attached to the body of the siliceous sponge before 
its solution, and must have been as hard as it is now when the 
sponge tissue was dissolved out. The solution is not likely to have 
been accomplished until after the upheaval of the Chalk, ergo, the 
flint was there before. It is quite true that sometimes the cavities 
are partially or even wholly filled with subsequently-deposited 
siliceous matter, but this is usually chalcedony and is quite 
different from the external flint. 
Mr. Abbott’s suggestion to aceount for the supposed absence of 
flints in the Lower Chalk cannot possibly be accepted. He does 
not seem to know that what was formerly called “ Lower Chalk ”’ is 
now divided into Middle and Lower Chalk. Flints are common in 
the upper part of the Middle Chalk and though they do not occur 
in the Lower Chalk of the South-east or of the Midlands, they do 
occur in it and very low down in Devon, Dorset, and Yorkshire. 
Mr. Abbott is quite right, however, in saying that flints can be 
found in every stage of development, and a study of these incomplete 
flints with the aid of a microscope ought to give us some insight into 
the manner of their construction. My friend Mr. W. Hill, F.G.8., 
has for some years been occupied in the study of such flints, and I 
understand that he intends to publish some of his observations at an 
early date. He agrees with me in regarding the imperfect flints as 
cases of arrested development apparently for want of a sufficient 
supply of silica. 
The co-existence of flint and chert with a prevalence of sponge 
spicules is too frequent to be a mere coincidence. All who have 
recently studied siliceous nodules, notably Dr. Hinde, Prof. Sollas, 
and Mr. W. Hill, have connected the formation of the concretions 
with the occurrence of the siliceous sponges. But as to the details 
of the processes whereby the silica of the sponges has become con- 
centrated in the siliceous nodules and layers we are at present very 
much in the dark. More observation and experiment is required 
before Nature yields up this long-treasured secret; but I can see no 
good reason why the initial formation of all the siliceous nodules, 
which occur in limestones, should not have been due to the same 
causes which have led to the production of calcareous and phosphatic 
nodules in sands and clays; and in the formation of these it is clear 
