OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 503 
ble that the oolitic granules were formed so long as organic matter 
existed in the mineralizing waters, and that ordinary crystalline 
dolomite began to be deposited as the organic matter disappeared. 
(A qualitative analysis which I made of this rock, showed the pre- 
sence of a considerable quantity of both magnesia and phosphoric 
acid.) 
Reviewing the knowledge we have acquired of the ferruginous 
Wenlock limestone, we are now in a position to give a short history 
of the course of its formation. At first there existed a plentiful 
growth of very various calcareous organisms, which by their death 
and subsequent decay furnished a considerable quantity of calcareous 
fragments to the deposits accumulating on the sea-floor. Some of 
these fragments were rounded into minute pebbles either by solution 
in carbonated waters or by mechanical attrition—most probably by 
attrition, since they are now often only small fragments of what 
were once large shells, and their surfaces are smoother than they 
would be had they been eroded by solution. Others of the frag- 
ments retained completely their original organic form; and yet 
others again were broken up into fine angular detritus, and mingled 
by a slowly moving current of water with occasional grains of 
broken quartz, or finely divided ferruginous mud. The calcareous 
sand thus produced appears next to have been permeated by an 
infiltration of ferruginous water, which thus made incursions into 
the Silurian sea long before the time of the Old Red Sandstone. 
This led to the injection of ferruginous matter into the canals of the 
Brachiopod shells and the interstices of the Encrinital skeletons, 
and also to the building-up of successive coats of iron oxide around 
many of the rounded calcareous fragments. Grains of glauconite, 
which had probably been previously formed, were also enveloped in 
this material. At the same time, or perhaps previous to the influx 
of ferruginous waters, oolitic granules were being formed within the 
closed chambers of such coral-hke structures as now contain them. 
Finally the incoherent mass of sand and mud and granules was 
cemented together in places into compact nodules and patches, by 
the deposition partly of iron oxide, but chiefly of carbonates of lime 
and magnesia. 
2. Band of Limestone above the Rhymney Grit—The cut and 
polished surface of this rock shows a number of white, green, and 
pinkpatches set in a greenish-grey ground, corresponding to a num- 
ber of fragments of organic calcite imbedded in a matrix of very fine 
quartz sand, the whole being cemented together by an impure 
glauconitic mineral. 
The organic remains revealed in these sections by the microscope 
are very numerous and interesting: good sections of Bryozoa are 
frequently seen; Brachiopoda are plentiful, very often in a frag- 
mentary state, but never incrusted with iron oxide, or rounded at 
the edges; univalves with their shells converted into the crystalline 
state are not rare; Encrinite skeletons are abundant; and Forami- 
nifera resembling Nodosaria and Lotalina are observable now and 
then. 
