288 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 18, 



marlstone and limestone (belonging to the Upper Lias) varies from. 

 2 to 3 feet *. This extraordinary band of stone, thns including the 

 bonndary-line of two formations, is interposed between portions of 

 the same great clay-deposit, since above it there is a mass of grey 

 micaceous marl (c) precisely similar in character to that below. This 

 marl is continued upwards for about 70 feet. 



We have already seen that blue marl, similar to that of the Lower 

 Lias, is continued some 20 feet into the Belemnite-beds of the Middle 

 division ; here, on the contrary, we find the higher limit of the 

 latter, as defined by fossils, carried upwards into a clay-deposit most 

 certainly belonging in the mass to the Upper Lias. We thus see 

 that the organic and inorganic conditions characteristic of two con- 

 tiguous groups do not always possess the same boundary-line. 



Over the great clay-bed last mentioned there are about 140 feet of 

 sands (h) — brown, micaceous, and with close-set tiers of large nodules, 

 and layers of indurated sandstone. These are the sands which, till 

 within the last few years, have been called " the Sands of the Inferior 

 Oolite," but which are now, as is well known, generally assigned 

 to the Upper Lias. The highest of these beds immediately under- 

 lying the Inferior Oolite have been, in the section under consideration, 

 completely denuded, the total height of the hill being completed by 

 a few feet of drift (a). Passing under the height of Thorncombe 

 Beacon, we find the Down Cliffs section sloping away gradually to 

 the eastward. The beds are cut through by the Valley of Eype, but 

 without any fault. 



5. Fourfoot Hill. — In this hill, at a little distance to the eastward 

 of " Eype's Mouth," we come suddenly to a very strong fault, which 

 carries out of sight not only as much of the Middle Lias as is not de- 

 nuded, on the one side, but, on the other or eastern side, brings down 

 the highest beds of the Inferior Oolite below the level of the beach, 

 with upwards of 200 feet of Fuller's Earth and Forest Marble resting 

 upon it. As there is a thickness of more than 200 feet of Lias in 

 the one cliff, and as the interval between those beds and the upper- 

 most of the Inferior Oolite cannot be much less than 300 feet, we 

 may estimate this fault as a downthrow of nearly, if not more than, 

 500 feet, and this too in one great fracture. 



The Lias beds of the western side of the fault include the brown 

 sands nearly up to the Upper Lias Stone ; but, as the only portion of 

 the section not obscured is to be seen in the faulted face, where the 

 beds are much displaced and coated with the debris of the fault, it 

 is difficult to say much about this, the most easterly extension of 

 the Middle Lias on the Dorsetshire coast. 



No more of the Lias is seen till we approach Bridport Harbour, 

 where the Upper Lias Sands are brought up again by a fault corre- 

 sponding to that of Fourfoot Hill, though not equalling it in mag- 

 nitude. 



* R. Etheridge, Esq.. of the Geological Survey, being engaged, in the summer 

 of 1861, in making a section of the Lias strata near Lyme Regis, was the first to 

 discover the existence and true position of the Upper Lias in this cliff', though 

 he did not then suspect the character of the under part of the block in which he 

 found the Upper Lias Ammonites. 



