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These layers sometimes interbranch and form loops; but I did not see any case of actual intersection. They form the passage from the perfectly glassy portions, to those nearly homogeneous and stony, with only an obscure concretionary structure. In the same specimen, also, sphaerulites differing slightly in colour and in structure, occur embedded close together.
Tenthly: a conformable layer, less than two feet in thickness, of pitchstone, generally brecciated, and traversed by veins of agate and of carbonate of lime: parts are composed of apparently concretionary fragments of a more perfect variety, arranged in horizontal lines in a less perfectly characterised variety.
Near Wenlock it consists of thick masses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed "ball-stones" in Shropshire, are often enormous, even 80 feet in diameter. Halysites catenularius, Linn. sp. Favosites Gothlandica, Lam. Dudley. a. Portion of a large mass; less than the natural size. b.
These layers, beginning at the bottom, are as follows; igneous granite, unstratified; limestone laid down from life in the ocean, metamorphosed by heat and all fossils thereby destroyed; limestone highly crystallized, composed of fossil shells and very hard; sandstone, made under the sea from previous rock powdered, having huge concretionary masses with a shell or a pebble as a nucleus around which the concretion has taken place; shale from the sea also; conglomerate, or drift, deposited by ice in the famous glacial cold snap; alluvium soil deposited in fresh water and composed partly of organic matter.
On the contrary, in this lake ore, as it is called, we have an excellent illustration of what is called concretionary action that is, the tendency of matter when in a fine state of division to aggregate its particles into masses about some central nucleus, which may be a fragment of sunken wood, a grain of sand, or indeed a pre-formed small mass of itself."
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.
Hence I conclude, that the foregoing concretions have been formed by a process of aggregation, strictly analogous to that which takes place in aqueous deposits, acting chiefly on the silica, but likewise on some of the other elements of the surrounding mass, and thus producing the different concretionary varieties.
The river was very winding, so that we did not advance more than 7 or 8 miles W.N.W.; the Bricklow scrub compelled us frequently to travel upon the flood-bed of the river. Fine grassy forest-land intervened between the Bricklow and Myal scrubs; the latter is always more open than the former, and the soil is of a rich black concretionary character. Oct. 9.
Most of these sedimentary strata are much indurated, and no doubt have been partially metamorphosed: many of them are extraordinarily heavy and compact; others have agate and crystalline carbonate of lime disseminated throughout them. Some of the beds exhibit a singular concretionary arrangement, with the curves determined by the lines of fissure.
From the close similarity in so many respects, between the obsidian formations of Hungary, Mexico, Peru, and of some of the Italian islands, with that of Ascension, I can hardly doubt that in all these cases, the obsidian and the sphaerulites owe their origin to a concretionary aggregation of the silica, and of some of the other constituent elements, taking place whilst the liquified mass cooled at a certain required rate.
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