9S Br. BetlgE'R on tbe physical Structure 



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 cUffs 

 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 ;* this last I did not see /// s'ltu^ 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 fine grain, and the particles distinct. It contains petri- 

 factions, 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 Nor- 

 mandy, f 



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-ferruglnous cement, first suc- 

 ceeds 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 dia- 

 meter ; it then approaches to a conglomerate puddingstone, but near 

 Exeter, it assumes the character of an arenaceous sandstone, and be- 

 comes 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 bounda- 

 ries of primitive countries, or on the passage to the secondary countries. Traite Ele- 

 mentaire dc Mineralogie, torn. i. p. 527. 



t Journ. de Physique, Mars 1807. 



