Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/616

520 character of the rock and its components. The most interesting are four well-marked seams of nodules, which differ considerably. Many of these lumps contain casts of Mollusca, and display appearances of having been rolled. They may be irregular in shape and consistence, or polished and rounded. The following is the result of very many careful examinations of cliff- and horizontal sections of this formation made by me during five years' work on the Maltese deposits.

The uppermost portion of the bed is composed of a pale grey freestone, soft and easily worked. Traversing this bed is a band of nodules, for the most part rounded and loosely arranged: it often thins out to a mere indication of a bed; indeed it would seem to be sometimes wanting. This, the First Nodule seam, is generally characterized by the abundance of casts of what have been supposed to be a Pteropod allied to Hyalea, and a Vaginella undistinguishable from V. depressa.

About eight feet below the last nodule-seam, in a fawn-coloured sandstone, is the Second Nodule seam, which is readily distinguished, not only from its position but from the small round nodules and their loose arrangement. They are usually of a brown colour; and when broken present no apparent characters distinct from those of the parent rock. The thickness of the band is often from three to four feet. It abounds with organic remains, and has produced nearly all the Vertebrata and the majority of the Invertebrata of the Calcareous Sandstone. It is a famous horizon from which the teeth of Squalidæ are obtained.

About the middle of the bed a few scattered nodules of a light green colour extend in broken lines, but rarely agglomerated; and they are not unfrequently absent.

About thirty feet below the second seam is the Third, distinguishable by the irregular shape of its nodules, which are of a dark brown colour, firmly cemented together, and apparently of the same mineral structure as the parent rock. They repose on a surface broken up by pot-holes and crevices, in which many of the nodules are contained. This stratum is highly fossiliferous; but, from the firmness and hardness of the matrix, organic remains are extracted with difficulty. It varies in thickness from 1 to 4 feet, and may be seen to the greatest advantage on the shore-line westward of the lighthouse of Gozo.

Prom twenty-four to forty feet below the last is the Fourth Seam in a pale-coloured sandstone. It is made up of light-brown nodules of irregular shape and of variable thickness. It marks the point of transition between the Calcareous Sandstone and Lower Limestone beds. Sometimes a seam of rounded nodules of limestone traverses the rock in place of these calcareous nodules; the former differ in their waterworn aspect and the great firmness of their matrix, which is composed of fragments of shells of various forms found in both formations. Moreover the fourth nodule seam may be replaced by lines of broken shells.

As to the mineral composition of the nodules generally, I repeat