466 W. A. E. USSHEK ON THE CHRONOLOGICAL VALUE OF THE 



correlating the Upper Marls and Sandstones of Devon with the Upper 

 Keuper Marls and Lower Keuper Sandstones of the midland counties, 

 each with each, and am very much inclined to consider the con- 

 glomerate subdivision as equivalent to the base of the Keuper in the 

 midland counties. 



(d) Beyond this it is impossible to correlate the beds in the two 

 areas, either by lithological characters or position. For, whilst in 

 the midland counties a distinct hiatus has been proved by Professor 

 Hull in the frequent unconformities exhibited between the Keuper 

 and Bunter (often amounting to the entire absence of the upper di- 

 vision of the latter and the unconformable overlap of the Keuper 

 upon the Bunter Pebble-beds), in Devon we have a series of marls 

 at least 500 feet thick on the south coast, conformably underlying 

 the base of the Keuper*. Prom the Pebble-bed subdivision down 

 to the base of the series, we have in Devon no evidence of uncon- 

 formity or eroded surfaces. 



Unfortunately the Lower Marls are cut off at Straight Point by 

 a fault bringing up marly beds on sandstones. Prom this point to 

 Exmouth, the constituents of the cliffs vary from marls in sandstones 

 to beds of sandstone in marl, marl predominating toward Exmouth. 

 These beds deserve the epithet of Waterstones, applied to them by 

 Mr. Ormerod. The break in the sequence at Straight Point might 

 be said to conceal an unconformity; but I have traced, between 

 Thorn St. Margaret and Wiveliscombe, in very clear sections, a 

 downward passage from Lower Marls, through an intercalated series 

 of marls and clays with beds of sand, the latter predominating 

 downwards, into the sandstones generally representing the basement 

 beds in that neighbourhood. 



The fault at Straight Point can scarcely have cut out less than 

 20 feet of the Lower Marls ; so that the beds underlying the pebble- 

 beds and overlying the basement sandstones and breccias can scarcely 

 be less than 850 feet thick on the south coast, of which 500 feet are 

 represented by pure marl without beds of sandstone underlying the 

 Pebble-beds. The remainder may be regarded as a passage series 

 into the lowest division. In their northerly extension these beds 

 gradually attenuate ; and they do not probably exceed 200 feet, taken 

 together, to the north of Thorn St. Margaret. 



(e) A fault occurs at Exmouth bringing up breccia against the 

 Passage-marls. This, again, might be taken as a case of concealed 

 unconformity, were it not for the presence, just north of Exmouth, 

 of the beds cut out by the fault, and that they prove to be a series 

 of sands passing upwards, as we have seen (locally), into Lower 

 Marls, and passing downwards, generally by intercalation, into 

 breccia, as may be seen between Dawlish and Langstone Point. In 

 many places these sands represent the major part of the division ; 

 but near their old coast-line a beach-deposit of rubbly gravel seems 



* Mr. Vicary has given a description of the relations of the Pebble-bed (Quart. 

 Journ. Geol. Soc. vol. xx. p. 283, &c). The conglomerate against which the marls 

 are said to be faulted is a thin impersistent band of breccia in the Straight- 

 Point sandstones, nowhere exceeding 10 feet in thickness. 



