it is totally lost, so that setting out from London, and going in a direct line from east to west, this formation extends one hundred and fifty miles between these two boundaries. The structure of the cliffs on the sea-shore, it is true, changes several miles eastward of the western boundary now stated. Thus the Island of Portland furnishes a grit, having a calcareous cement; and Lyme Regis, a little farther west, a shell limestone;[1] this last I did not see in situ, but from the specimens I procured in that neighbourhood, it appeared to me rather to deserve the name of a shelly-calcareous grit. It is of a bluish colour, of a line grain, and the particles distinct. It contains petrifactions, particularly very beautiful ammonites, which are semi-transparent. This rock, in many respects, very much resembles that which forms the cliffs of Tracy on the coast of Bayeux in Normandy.[2]
Immediately upon quitting the chalk district, we enter upon a transition country, of which Exeter may be considered the centre, and as it is yet little known in a geological point of view, it deserves a more particular examination.
The red sandstone, having an argillo-ferruginous cement, first succeeds the chalk and flint. Towards Honiton, it is in the state of a coarse-grained gravel, almost entirely disintegrated. It contains rounded pebbles, some of which are two or three inches in diameter: it then approaches to a conglomerate pudding stone, but near Exeter, it assumes the character of an arenaceous sandstone, and becomes more compact and uniform in its texture and composition.
- ↑ There is also along the coast of Dorsetshire, a range of argillaceous hills, belonging to a kind which, according to M. Brongniart, are to be observed either on the boundaries of primitive countries, or on the passage to the secondary countries. Traité Élémentaire de Minéralogie, tom. i. p. 527.
- ↑ Journ. do Physique, Mars 1807.
