Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/121

1872.] SOLLAS UPPER-GREENSAND COPROLITES. 77 between calcic phosphate and animal matter in the case of the "Coprolites."

The Blackdown silicified shells were next explained; and it was reasoned that the state of their silica offered arguments tending to prove a passage of silica from the colloidal to the crystalline state.

Part II.

Coprolites, as has already been stated, are the result of the phosphatization of organic matter. They may be classed according as their origin is known or obscure. The progress of discovery transfers the obscure forms into the class of known forms; but there will always remain a certain number which cannot be thus transferred— those, for instance, which have been produced from soft-bodied animals in the last stage of decomposition, all traces of their structure having been obliterated. The fossil remains of Reptiles, Fish, and Crustacea, the casts of Mollusks, and the perfectly preserved Ventriculites are instances of coprolites about whose origin there is no uncertainty; but besides these easily recognizable kinds there remain a vast majority of forms which, from their want of any striking characters, have always been obscure. In this paper the author proposes to transfer these in great part among the fossil sponges.

General Character of the Coprolites.

In the Greensand, most of the coprolites are of a black or deep brown colour, while in the Gault they are greyish-white on the surface, but brownish-black internally. By etching the greensand coprolites with acid they change, however, to the same greyish- white colour as the specimens from the gault; and masses of agglomerated coprolites are met with in the Greensand which, when broken open, reveal nodules of their original light colour.

The surfaces of most of the coprolites are variously marked with (i.) fold-like depressions, (ii.) osculiform pits, (iii.) puncta, and (iv.) contraction cracks.

(i.) The depressions occur as fold-like markings, which sometimes run longitudinally with remarkable constancy in size and direction for nearly the whole length of the fossil; besides this they may take any other direction. These grooves are marked by very minute wrinkles, which give the whole depression the appearance of one of the creases stretching from button to button in the leathern back of an easy chair. The better-marked of these grooves scarcely appear to be due to contraction consequent on fossilization.

(ii.) The circular or oval osculiform pits vary in size, frequently being 1/20 of an inch in diameter. Their margins are often depressed into a concave border, which is striated by regularly radiating "groovelets." These commence at the sides of the osculiform pit, pass across the concave border, and either stop there or pass to a greater or less distance on the surrounding surface. In some specimens these little striations are more restricted and better- defined than in others, and in some they are altogether absent.