' On the Cotteswold, Midford, and Yeovil Sands. 361 



writers, the author proceeded with the evidence on which his own 

 views are based. He described a series of sections of the typical 

 exposures of ■' Sands " and contiguous strata, commencing near 

 Stroud and termiuating on the Dorset coast. Dividing the series, 

 into seven horizons, characterized by their distinctive Ammonites, viz.- 

 Amm. communis, variabilis, striatulus, dispansus, the genus Diimor 

 tieria, Amm. Moorei, and opalinus, and taking the Striatidits-beds as a 

 fixed starting-point, the author demonstrated how the strata varied 

 in regard to that horizon. The Cotteswold Sands, containing the 

 Variabilis- and part of the Co?3iwn«}u"s-horizons, were below the Stria- 

 tidus-heds, ; the Midford Sands, containing the Dispansus-'h.OT\2on, 

 were above, Gramm. striatulum occupying a thin bed at the base ; 

 the Yeovil Sands, containing the Moorei- and DumortierictAxoxviow?,, 

 overlay a bed containing Ammonites of the Bispansus-hoYVLon, and 

 were consequently still later deposits. 



Since the different sands were deposited not on a horizontal plane, 

 in point of time, but, as it were, obliquely, the deposit of Cotteswold 

 Sands having ceased before that of Yeovil Sands commenced, it was 

 incorrect to lump all the " Sands " from the Cotteswolds to the Dorset 

 coast under the single local name "Midford Sands," thereby implying 

 a contemporaneity which did not exist, while the use of the present 

 restricted local names was defended. 



The Ammonites were apparently uninfluenced by changes in the 

 character of the deposit, since the same species are found in Lime- 

 stone in the Cotteswolds, in Sands at Midford, and in argillaceous 

 Marl at Ilminster. The change from argillaceous to arenaceous or 

 calcareous deposits has been looked upon as so distinct a feature, 

 that it has been utilized as a great argument in favour of drawing 

 the line between Lias and Oolite at that point ; but if this be done, 

 the line is always drawn at different horizons in different districts. 



If lithology furnishes no reason for a dividing-line at this point, 

 it was shown that neither did palaeontology. It was also shown 

 that the Ammonite family Hildoceratidse dominated the period from 

 the Falcifer- to the Concaviis-zones, and that with the close of the 

 latter zone they died out with singular abruptness, and that, further- 

 more, there exists, both in England and upon the continent, a marked 

 hiatus at the same point due to the absence of a zone or a number of 

 zones. 



On account of these facts the proposal was put forward that d'Or- 

 bigny's term " Toarcien " should be emyloyed to designate the strata 

 from the Falcifer-zone to the Concavus-zone inclusive, that this term 

 should not be used in the sense of merely an extended " Upper 

 Lias," but to mark an entirely distinct transition-formation, — a 

 definite part of the Jurassic period, — separating the typical Lias 

 from the mass of thoroughly Oolitic strata. 



