866 On a Ternary Geological Classification. [July, 



find it necessary to confine the middle division to the Ilfracombe 

 Hmestone series alone, the passage from the sedimentary conditions 

 of the lower into the calcareous conditions of the middle, and from 

 these latter into the sedimentary conditions of the upper series 

 appears in this district to have been gradual and uninterrupted. 



Uerefordshire. — The Old Ked Sandstone in this district and 

 South Wales appears, from the observations of Sir Henry De la 

 Beche, to consist of two members lying discordantly on each other, 

 the upper member appearing to pass upwards into the Carboni- 

 ferous series, the lower to graduate downwards into the Silurian. 

 A possible explanation which I venture to offer is, that we have here 

 the representatives of the upper and lower sedimentary members of 

 the ternary group, of which the central calcareous member has not 

 been deposited in this region. In illustration of this view I may 

 point to the Trias of England, which, as we shall presently see, 

 afibrds a similar example capable of ready explanation. 



Scotland. — In Scotland we seem to have a succession of beds 

 somewhat parallel to that of South Wales. Mr. A. Geikie has 

 divided the Devonian series into three groups, each resting uncon- 

 formably on that below it. But as the upper member of this three- 

 fold series is stated by him to graduate upwards in the lowest 

 beds of the Carboniferous rocks, I have ventured in this classifica- 

 tion to place this " Upper Old Eed Sandstone " at the base of the 

 Carboniferous series, with which it seems to be more closely aUied 

 than with the " Middle Old Ked Sandstone." We have then two 

 sedimentary members resting discordantly on each other, and 

 leaving an intermediate gap in the succession, which the middle 

 member ought properly to have filled, as in the case of South Wales. 



Ireland. — The investigations of Mr. Jukes seem to have brought 

 to Hght in Ireland, as in Scotland, a double series of sedimentary 

 strata resting discordantly on each other. 



To the unrepresented space between these members may, I 

 think, be referred the position of the central calcareous member, 

 and the two divisions to the upper and lower sedimentary members 

 of a natural group. Thus we have the "Dingle and Gleugariff 

 Grits " of the lower stage surmounted discordantly by the Coomhola 

 Grits of the upper. If the above explanation of a highly diflicult 

 problem regarding the relations of the Devonian series in Wales, 

 Scotland, and Ireland has any approach to the truth, the entire 

 absence of the middle calcareous member may probably bo accounted 

 for by supposing that during this stage in the geological cycle 

 these tracts had been elevated into laud surfaces, while in the 

 lihenish and Devonian regions the limestones of the middle 

 Devonian group were in course of formation. Such oscillations of 

 the sea-bed would serve also to account for the break and un- 

 conformity between the upper and lower members of the group. 



