502 W. J. SOLLAS ON THE SILURIAN DISTRICT 
bluish tint they exhibit with reflected light may be correlated with 
the amber or brownish tinge they give to transmitted light, the rays 
reflected and absorbed being, it would seem, complementary to each 
other. 
Between crossed Nicols they remain dark, though the thicker 
layers appear a little brighter than the rest; the darkness is also of 
different intensity along certain lines, producing a very dark cross 
where deepest. Besides the concentric structure, they exhibit no 
other—not a trace of radiating fibres, such as one commonly sees in 
concretions of an oolitic nature. 
In one or two instances some of the concentric layers have begun 
to be stained by iron oxide, which in one case is present in suf- 
ficient quantity to form a aa ale of hematite. 
In size the grains vary from [7 toy}; in form they differ con- 
siderably, some giving an almost ‘perfectly circular outline in section, 
others a polygonal, oval, or even heart-shaped one; but whatever 
their external contour, they nearly all possess a spherical nucleus, and 
the nearer the concentric layers are to this and the further from the 
exterior the more they approximate to the spherical form. As re- 
gards the true characters of these grains one feels a certain amount 
of difficulty. The first point to decide is whether they have been 
formed in situ, or washed in from the exterior. The fact that they 
only occur in such cells as were shut off from communication with 
the muddy sea-bottom, and so had become infiltrated with pure 
crystalline dolomite, seems to point to their formation in situ; and 
this supposition is strengthened by the way in which some of them 
appear to have adapted themselves in their later stages of growth to 
the form of the cellules in which they lie, and also to one another 
when two or more lie in close proximity. Admitting that they were 
formed in the places where they are now found, one can only conclude, 
further, that they must have been formed by the successive deposition 
of coats of carbonate of lime, one over the other concentrically, on 
some original nucleus. They are therefore oolitic grains without a 
fibrous structure, and they must have grown by a deposition of 
carbonate of lime from solution. But why, one cannot help asking, 
should the deposition of calcareous material have taken place in two 
very different ways in the same cellule of the Vavosites (at first 
about nuclei concentrically, without apparently giving rise to a crys- 
talline structure, and afterwards generally, filling up the cellule 
with a confusedly crystalline mass of dolomitic calcite)? One almost 
feels tempted to regard the oolitic granules as “‘ caleuli” formed during 
the lifetime of the Havosites; and though such a supposition is not 
probable, it yet may afford us a hint as to the true explanation; 
for immediately after the death of the Mavosites a good deal of 
organic matter would be set free, and would pervade not only the 
surrounding water, but the several cells of the organism. Un- 
fortunately we do not know much about the precise mode in 
which the presence of organic matter influences the mode of deposi- 
tion of mineral SHS ames: but that it does exert a.special influence 
of some sort is pretty generally admitted ; and it may be just possi- 
