OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 501 
afford us a complete section (longitudinal or transverse, as the case 
may be)through one of the disks of the Encrinital stem, their structure 
being usually wonderfully well preserved—though in some cases, 
when the ferruginous injection is absent, it has become nearly obli- 
terated by blending as it were with the enclosing calcitic matrix. 
The Bryozoa sometimes form the nucleus of one of the lentil-like 
bodies ; but more usually we observe them as uncoated sections in 
the matrix, of considerable size, and, as we have said, only partially 
enveloped in iron oxide, their cells being filled up with calcite 
or dolomite, or in many cases with an opaque brownish granular 
ferruginous material. 
Many of the lentil-like bodies show no organic structure within ; 
but in some of them a concentric structure is more or less observable, 
probably indicating the previous existence of an oolitic granule. 
Thus alternating opaque brown and transparent colourless or yel- 
lowish layers are visible in the sections shown by figs. 24 and 25. 
Some few of the lentils enclose irregular, frequently angular, frag- 
ments of glauconite, having optical characters identical with the 
glauconite of the Cambridge Greensand. But while the glauconite 
in the latter case contains clear and transparent granules, many of 
which have the form of Coccoliths and Foraminifera, in the former it 
contains only black and opaque granules of no characteristic form. 
On treating the rock with dilute acid the calcareous cement which 
binds its parts together is dissolved away, and the ferruginous con- 
cretions are set free; when washed and dried they have very much 
the appearance of little black pebbles, but exhibit no further 
characters than those already described. We have seen enough, 
however, by this, to decide as to their true nature. They are not 
oolitic granules which have become replaced by iron oxide (or at 
all events very few of them are so), but simply rounded fragments 
of organic calcite which have become enveloped in a ferruginous 
covering. ‘Though these coneretions are not oolitic in the ordinary 
sense of the term, yet oolitic spherules do really occur in the lime- 
stone. Thus, on breaking open a compact nodule of carbonate of 
lime which had been formed about a mass of Favosites fibrosus lying 
in the bed, I noticed a number of little spherical bodies bearing a 
great resemblance, both on account of their faintly bluish colour 
and slight translucency and their beautiful pearly lustre, to a num- 
ber of minute pearls: a thin slice was prepared from this nodule ; and 
the “ pearls’ were then found lying within the cells of the Fuvosites ; 
they only occurred, however, in those cells which had been filled up 
with clear transparent dolomitic calcite, and were constantly absent 
in the cells which had been filled in with ferruginous sand. They 
were found to consist of a great number of very fine wavy layers or 
concentric shells of transparent calcite, having a faint brownish 
colour by transmitted light. Some of the layers were much thicker 
than others, appearing as comparatively broad bands in section, the 
rest merely as lines of inappreciable thickness. The pearly lustre of 
the spherules seen by reflected light appears to be due to the super- 
position of these immeasurably thin layers one upon another; the 
