deposits, that one is at once tempted to attribute to them an analogous
origin. They resemble ordinary concretions in the following respects: in
their external form,--in the union of two or three, or of several, into an
irregular mass, or into an even-sided layer,--in the occasional
intersection of one such layer by another, as in the case of chalk-flints,-
-in the presence of two or three kinds of nodules, often close together, in
the same basis,--in their fibrous, radiating structure, with occasional
hollows in their centres,--in the co-existence of a laminary,
concretionary, and radiating structure, as is so well developed in the
concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in
sedimentary deposits, it is known, are due to the separation from the
surrounding mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I have
endeavoured to discover whether obsidian and the sphaerulites (to which may
be added marekanite and pearlstone, both of them occurring in nodular
concretions in the trachytic series) differ in their constituent parts,
from the minerals generally composing trachytic rocks. It appears from
three analyses, that obsidian contains on an average 76 per cent of silica;
from one analysis, that sphaerulites contain 79.12; from two, that
marekanite contains 79.25; and from two other analyses, that pearlstone
contains 75.62 of silica. (The foregoing analyses are taken from Beudant
"Traite de Mineralogie" tome 2 page 113; and one analysis of obsidian from
Phillips "Mineralogy.") Now, the constituent parts of trachyte, as far as
they can be distinguished consist of feldspar, containing 65.21 of silica;
or of albite, containing 69.09; of hornblende, containing 55.27 (These
analyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and
of oxide of iron: so that the foregoing glassy concretionary substances all
contain a larger proportion of silica than that occurring in ordinary
feldspathic or trachytic rocks. D'Aubuisson ("Traite de Geogn." tome 2 page