300 C. EF. De Rance—Age of Clwydian Caves. 
lighter varieties of the rock exhibit under the microscope no indica- 
tion of the colouring matter, except, perhaps, that the boundary of 
grains appears somewhat more definitely marked than is usual in 
a colourless quartzite, but in some of the darker specimens a thin 
greenish-coloured film can be detected round each grain, and the same 
tint is disseminated through the microcrystalline ‘cement.’ It would 
appear then that the first stage of consolidation was the deposit of 
a film of iron-silicate, minute quantities of which were from time to 
time precipitated during the formation of the chalcedonic quartz. 
The figure represents, somewhat diagrammatically, the general 
structure of the rock, omitting the organisms and darker grains. 
The fragmentary organisms contained in this rock are not very 
common. Those which I have noticed are not much larger than the 
sand grains, and are often somewhat cylindrical in form. They can 
be readily detected with a good lens. With the exception of a few 
which resemble Annelid tubes, but may possibly be inorganic, they 
are of a pale green colour, and appear to have a slightly roughened 
surface, one or two much resembling bits of small spines of an 
KEchinid. When seen under the microscope, they exhibit a peculiar 
reticulate structure, dark in colour, with the interspaces occupied by 
microcrystalline silica. I have no doubt they are really from an 
Kchinid, some being probably bits of the test, while others are from 
the spines. One slide gives a very good transverse section of a spine 
about 14” in diameter.! I note also a longitudinal section of a 
moderately elongated spiral Gasteropod, about °25” long, with one or 
two other fragments, of the nature of which I am doubtful. 
So far as I am aware this conversion of asandstone into a quartzite 
by the deposition of chalcedonic silica is rare among the older rocks. 
In my own researches I have never come across a case, but Prof. R. 
D. Irvine mentions its occurrence among the older American 
quartzites, and figures an example from a cherty Potsdam sandstone 
from Wisconsin.? It occurs also in the ‘Sarsen stones’ and in the 
matrix of the Hertfordshire ‘Puddingstone,’ but in all these cases 
(so far as I know) the growth of the chalcedonic silica is much less 
regular than in the rock which I have described. But after I had 
examined the above described specimens, Miss C. A. Raisin, who at 
my request had undertaken the investigation of a parcel of rocks 
from Somali-land, found a very similar case, which will be noticed 
in the account which she is preparing. 
IV.—AGE oF THE CiwypIAN CAVES. 
By C. E. De Rancz, F.G.S., A.I.C.E. 
he following abstracts of early papers, on the Cefn Caves, throw 
a most interesting light on the sequence and method of occur- 
rence of the deposits found at the Tremeirchion Caves, which are 
1 T have to thank Dr. G. J. Hinde for specimens for comparison. The genera 
Pseudodiadema and Peltastes seem to be most common in the Upper Neocomian rocks 
of England. So far as I can judge, I should refer these spines to the former genus. 
2 Fifth Annual Report of the U.S. Geol. Survey, plate xxxi. 
