Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/496



Plate 18. Sketch of the ground explanatory of the several appearances described in the paper. The cross lines refer to the sections in Plate 21. The upper part represents such profiles of the lines as seemed most necessary for the elucidation of the subject.

Plate 19. A Map, which serves to represent the several vallies that communicate with Glen Roy at the altitude of its lines. It also points out the communication which it would have with the sea were it now filled with water to the level of its uppermost line, the colour indicating both.

Plate 20. A Map, for the purpose of pointing out on a larger scale the communications of Glen Roy with the vallies in its vicinity that bear the marks of the lines.

Plate 21. Sections referred to in Plate 18.

Plate 22. Ideal sections referring to the circumstances represented in Plate 19.

PLATE 23.

Porphyritic veins traversing the schist of St. Agnes in Cornwall, described in the Rev. J. J. Conybeare's paper, page 401.

Fig. 1. Is a view of Chalk Point, the promontory of which is formed of granite resting upon the schist. The vein of elvan is seen passing through the schist dipping at a smaller angle than that of its stratification.

Fig. 2 & 3. Are different examples of the veins of elvan in the schist, representing some of the irregularities that characterize them.

PLATE 24.

Appearance of the Paramoudra, and of other siliceous veins and nodules in chalk.

Fig. 1. Part of a vertical section of a chalk pit, near Moira, shewing in their relative proportions the chalk alternating with flinty nodules, with three specimens of Paramoudræ in their matrix.—(Scale half an inch to a foot).

Fig. 2. Specimen of a Paramoudra from the same place, presented by Dr. Bruce to the Museum at Oxford.

Fig. 3, 4, 5, 6. Other specimens seen in the same chalk pit near Moira. (Scale of Nos. 2, 3, 4, 5, 6, an inch and a half to a foot.)

Fig. 7. Fragment broken from a Paramoudra inclosing a small cluster of hexagonal cells, supposed to have been accidentally introduced from some extraneous body. (The drawing is magnified much beyond the natural size.)

Fig. 8. Veins of plated flint at Hurley Bottom, near Henley, cutting the beds of chalk and flunty nodules. (See Note, page 417.)

Fig. 9. Veins of flint cutting the chalk at Rottingdean, with strata of plated and nodular flints in the same section of the cliffs.—The lines represent strata and veins of plated flints. The dots express siliceous nodules. (See Note, page 417.)