AUSTEN— EXTENSION OF THE COAL-MEASURES. 60 



form of a thin band of limestone, included in dark Wealden shales, 

 with much vegetable matter, and containing a curious assemblage of 

 marine, brackish, and freshwater shells. From this point there ^is 

 no evidence of the presence of any beds of Lower Greensand along 

 the base of the Chalk-escarpment of Dorsetshire, nor in the A^ale of 

 Wardour, nor yet at Warminster ; and this arrangement is just what 

 should take place, supposing that the axis of Frome should represent 

 that of Artois, or any of the anticlinals which form part of the same 

 system. 



True littoral Lower Greensand shingle occurs near Devizes, and 

 thence on to Calne. The precise age of these beds is that of 

 M. d'Orbigny's Urgonian series, or upper division of the Neocomian 

 group*. I am still of opinion that the weight and the worth of the 

 evidence of the fossils are in favour of the Neocomian age of the 

 Farringdon gravels, and that in their inquiries geologists must be 

 guided by the forms which do not pass up into higher members of 

 the series, rather than by those that dof : if so, the following species 

 determine the question : Terebratula Celtica, T. oblonga, T.prcBlongay 

 T. sella, T. tamarindus, RhyncJionella Gibbsiana, R. parvirostrist 

 Salenia punctata, Goniopygus peltatus, NucleoUtes Neocomiensis. 

 The beds containing these are situated on the N. of the Frome axis, 

 so that it would seem that that line had caused an advancing ridge 

 into the area of the lowest Cretaceous deposits, and produced an 

 irregular boundary-line on the W. From Punfield (Dorset) a line 

 may be drawn passing outside the Isle of Wight, and to the S.W. of 

 the Pays de Bray, beyond which no Lower Greensand occurs. Ac- 

 cording to a recent discovery by M. Cornuel J, freshwater shells have 

 been washed into the Neocomian marine beds of Champagne, as has 

 happened at Atherfield, indicating the proximity of land on either side; 

 so that the Neocomian area of the N.of France and S.E. of England 

 resolves itself into a channel, having a main direction at right angles 

 to our present English Channel, and open only at its S.E. extremity, 

 where it communicated with the great expanse of the ocean in which 

 the earlier Neocomian groups of the S. of Europe were deposited. It 

 will be thus seen that the two areas of the Anglo-Gallic Wealden 

 and Neocomian groups are nearly co-terminous ; in other words, an 

 area of fresh water, bounded by ranges of land having a main direc- 

 tion of W. 31° N., w^as, by a process of gradual subsidence, converted 

 into an arm of the great Southern-Europe ocean, and it was on such 

 views that the lines drawn on the Map (pi. I.),were arrived at. 



It remains that we should inquire as to the amount of evidence 

 which can be arrived at as to a boundary-line in the North. As in 

 all cases the materials of a detrital-sedimentary group have been 

 derived from coast-line waste, an examination of these, particularly 

 when of the character of coarse sands and conglomerates, will never 

 fail to afford curious results. 



* Quart. Journ. Geol. Soc. vol. vi. p. 469. 



t See Sharpe, Quart. Journ. Geol. Soc. vol. x., p. 176 ; and Davidson, * British 

 Cretaceous Brachiopoda,' (Pal. Soc.) p. 106 et seq. 

 X Bull. Soc. Geol. de France, 2nd series, t, xi. p. 47. 



