56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4,
will be found to be filled either with clear amber-coloured phosphate
or else (less commonly) with glauconite. But towards the interior it
ramifies into many minute fissures, which appear black, and, losing
themselves and reappearing in parallel lines with cross cracks connecting them, produce altogether a ramose structure, whose twigs are
generally directed in the course of the crack.
I have noticed that, bordering their course, the phosphate is clearer and more free from the ramose shading than it is at a distance from them, and that the finer ramifications of the cracks assume very much the character, on a larger scale, of that shading. This has led me to doubt whether the shading in question may not be, at any rate partly, due to a physical cause, and not to organic structure. These three facts, however, appear to come out clearly from the
study of the cracks:—first, that phosphate continued to be deposited after the mass was already to some extent mineralized and had begun to contract from that cause, and that slightly open spaces became thus infiltrated by it; secondly, that glauconite was sometimes, but more rarely, deposited in a similar manner and in like situations, chiefly by open communication from without; and, thirdly, that the more minute vacuities within the mass were generally filled by a dark mineral, which I presume to be ferruginous.
"When the nodules are dissolved in hydrochloric acid, a small residuum is left which does not commonly reveal any organic structure in the shape of siliceous spicula or otherwise. In the indurated matrix which fills the hollow axis of the cylindrical nodules a few spicula, apparently siliceous, may be here and there detected, along with the usual grains of glauconite.
In order to pass from the known, or partly known, to the unknown, I will describe the appearance of a few recognized organic forms, as seen in section under the microscope, before speaking of the ordinary nodules, the nature of which is more obscure.
The recognized organic forms, three in number, which I have examined have passed by the names of Porospongia and Scyphia. They appear to be all Ventriculites, and reveal very plainly the quadrate, reticular structure described and figured by Toulmin Smith*. And since this structure is seen equally well whether the section is horizontal or vertical, it affords a strong presumption that the true "octahedral" structure of that author is present.
An examination of these specimens, however, has led me to a somewhat different conclusion as to the nature of the fibres disposed in this quadrate arrangement from that arrived at by Mr. Smith. And this need not surprise one, because the state of preservation of the fossils is entirely different in the two cases. The most instructive of Mr. Smith's specimens, which may be seen in the British Museum, consist of Ventriculites preserved in chalk, of which the organic structure is mineralized by iron oxide. Some of these have been dissected out by the action of weak acid, and show beautifully the quadrate arrangement in minute strings of that mineral.
The bars, however, which compose the quadrate figures in our
- 'Annals & Magazine of Natural History', 1st series, vol. xx. p1. vii. fig. 9.
