H. B. Woodivard — On the Midford Sands. 515 



regard to the indication of a slight lapse of time between the con- 

 clusion of the Upper Lias and the commencement of the Midford 

 Sands in the Somerset and Dorset area, where we have such afeeble 

 representative of the former. Indeed, but for Mr. Moore's patient 

 investigations it would be but little known in this area, for his 

 observations show that it attains but an average thickness of 8 feet 

 in Somersetshire and the adjoining portions of Dorset. It has 

 therefore been generally too thin to trace out on the small scale of 

 the Geological Survey Map ;. but its presence was indicated and a 

 small exposure mapped near Sandford Orcas more than twenty years 

 ago by Mr. Bristow. 



Although, as Mr. Moore points out, these Uj)per Lias beds "present 

 a remarkable variety in lithological condition, indicating that they 

 must have been deposited slowly, and that there were probably 

 periods of rest during their accumulation," ^ yet it becomes an 

 interesting question when we compare this feeble representative 

 of the Upper Lias with the great thickness it reaches in the north 

 of England, to speculate as to whether Oolitic conditions may not 

 have commenced somewhat earlier in this part than elsewhere. 



Professor Buckman has recently given his opinion in regard to 

 the Sands near Sherborne in Dorsetshire, that they belong to the 

 Inferior Oolite series ; that the concretionary masses interbedded 

 with the sands contain fossils essentially Oolitic, and not Liassic. He 

 considers them to be the equivalents of the Middle and of a portion 

 of the Upper division of the Inferior Oolite of Grloucestershire, and 

 the " Cephalopoda Bed " of this district he states to be at the top, 

 instead of at the base of the Oolite.^ 



This opinion shows the difficulty of correlating these divisions of 

 the Secondary rocks with any exactness. Indeed we are inclined to 

 think it impossible, and to look upon the whole of the rocks, the 

 Keuper, the Lias, and the Oolites, as one conformable series, marked 

 for convenience into certain stages of sedimentary deposit, which are 

 also characterized by certain assemblages of organic remains ; but we 

 do not think that these stages continued uniformly, or that they were 

 of a like duration over any large area, the conditions changing at 

 different times in different places. In this way we may understand 

 some of the marked differences in thickness of the rocks, which do 

 not seem always to be accounted for by rate of deposition, and 

 certainly do not admit of denudation to account for their attenuation. 

 We may mention the Fuller's Earth, which attains a thickness of 

 400 feet in Dorsetshire, which entirely disappears to the north-east 

 beyond Gloucestershire, as an instance of a deposit contemporaneous 

 with others of a different character. 



The subject requires much study before we can look upon the 

 Lower Mesozoic rocks as a series of interlacing sediments.^ 



1 Op. cit., p. 17. 



2 Proc. Geol. Assoc, toI. ii., p. 249. 



3 Mr. Bristow has prepared an interesting Table, showing the thicknesses of the 

 Secondary strata in the Southern counties of England, -which is published iu the 

 Report of the Coal Commission. 



