504 W. J. SOLLAS ON THE SILURIAN DISTRICT 
The calcareous parts of the foregoing organisms retain, as a rule, 
their calcareous composition ; but the hollow spaces within them 
have become filled up with a material which has a greenish-white 
appearance by reflected light, and either a green or a brownish 
colour with transmitted light: the same material cements together 
the quartz which fills up the spaces in the rock not otherwise 
occupied by organic constituents. When it is green both by reflected 
and transmitted light, it is insoluble in acids and probably exists 
as a more or less pure silicate of iron; when it is brown by trans- 
mitted light it is soluble in hydrochloric acid and probably consists 
of calcite coloured by admixture with the iron silicate. It is the 
soluble material which chiefly binds the quartz sand together. 
The injection of the Encrinital skeletons by the green silicate 
has led to the preservation of their structure in a wonderfully com- 
plete manner. 
Scattered irregularly through the matrix of the rock, and arranged 
along regular lines within some of the constituent shelly fragments, 
occur a number of little spherules of iron-pyrites, precisely similar 
to those described in my paper on Pharetrospongia (Quart. Journ. 
Geol. Soc. vol. xxxiii. pp. 250, 251). I now find that these are 
very common constituents of certain caleareousrocks; they are beauti- 
fully exhibited by the Lias limestone of Aberthaw and Penarth, South 
Wales, and may easily be obtained for ebservation by treating the 
limestone with dilute hydrochloric acid, when they are left as an 
insoluble residue. Examined as opaque objects under the microscope, 
they are seen to be covered on the surface by brilliant crystalline 
facets, and look very much as the curious iron-pyrites concretions of 
the Chalk would do if reduced to the same insignificant dimensions ; 
nor do I doubt that the latter only differ from the former in this respect 
—that is to say, as regards relative size. Jn the matrix of the Lias 
limestone they can be traced, as in Pharetrospongia, passing into an 
oxidized state, staining at the same time the surrounding limestone 
with the products of their decomposition, and finally giving rise to 
globules of clear, transparent, blood-red hematite. ‘The size of the 
hematite globules is much more uniform than that of the iron-pyrites 
concretions, a large sphere of pyrites resolving itself during oxida- 
tion into a great number of smaller globules of hematite, which 
then remain all grouped together as if subordinate parts of a common 
spherule, thus giving rise to the idea that they might be spores or 
sporangia, as I suggested in the paper before alluded to (see figs. 15 
and 16, pl. xi. loc. cit.). 
Precisely similar but isolated globules of haematite are found 
within the canals of fossil Nummuline Foraminifera, and in this posi- 
tion have been mistaken for the ova of the animal. I have now 
seen them associated with concretions of pyrites in the crystalline 
calcite filling up the interspaces of Stawronema, inside the hollowed- 
out spicules of Pharetrospongia, dispersed throughout the Lias lime- 
stone of 8. Wales and the shelly limestone of the Rhymney Silurians, 
as well as in the canals of a fossil Operculina. In all these positions 
they present a nearly uniform size and the same essential characters. 
