JURASSIC OF THE PAEIS BASEST. 



571 



traced the series from end to end of the Paris "basin, they seem to 

 have no weight. The Corallian series, as now to be limited, is one 

 of the best-marked gronps it is possible to conceive ; but, as Buvig- 

 nier (22) has so well expressed it, its constant feature is inconstancy. 

 Below this group we may have some continuous beds ; above it the 

 beds are markedly continuous : but within it discontinuity is the rule. 

 It is quite to miss the mark, therefore, to say, as Tombeck says 

 (50 &c), that its normal form is the compact limestone, and coral- 

 growths and Diceras-beds abnormal. Xo one kind of deposit is more 

 normal than another, except in this way, that the variety which is 

 most frequently repeated, and which occurs at several horizons and 

 under the most varied forms, is a coral-growth, and neither above 

 nor below the limits of this series can such coral- growths be found 

 in the country examined. Hence, unless one were to invent a new 

 term, such as Protean, to indicate variability, no better name could 

 be chosen than Corallian to indicate this most name-needing group. 



In the study of the English Corallian we have stated* that the 

 Coral Bag, with Oidaris jlorigemma, is always at the top of the 

 limestones, while the French geologists have asserted it to be as 

 constantly at the bottom. This anomaly is now explained. By 

 marking off so much of the so-called Coralline Oolite to place it in 

 the Oxfordian, we leave the Coral Bag nearly, and in most places 

 quite, at the base. Sometimes, as near Oxford, it lies on the 

 Oxford Grit : sometimes, as at Upware, on the Oxford Oolite ; and 

 only here and there can we suppose, and that doubtfully, that any 

 oolite belonging to this series rather than to the Oxfordian, and 

 hence to be called Coralline Oolite, intervenes. It may be so at 

 Malton and Seamer, in Yorkshire, especially where shell-beds are 

 developed : and it may be so at "Weymouth, where we may possibly 

 call the Trigonia-beds by this name. As for Oidaris florigemma, it 

 is characteristic of the Corallian as a whole, and is perhaps not con- 

 fined to it. Tombeck describes (55) a bed abounding with it at 

 St. Ansiau, in the Haute-llarne, supposed to lie in the Oxford Clay, 

 though a fault is here possible ; and the Houllefort limestone, which 

 cannot be called Corallian, contains it ; on the other hand, it is found 

 in Astartian beds at Weymouth. But, after experience of its habit, 

 it is impossible to say more than that it is commoner towards the 

 base than towards the top of the Corallian series, and is usually asso- 

 ciated with massive corals. As to there being a bed full of it con- 

 stantly marking the junction with the Oxfordian, as Hebert says 

 (21), this is a feature that seldom occurs. 



The upper line separating the Corallian from the Kimme- 

 ridgian is rather hard to draw. Tombeck says (50) that, if 

 we take the so-called Astartian as the base of the latter series, 

 it cannot be drawn palaeontologically ; and de Loriol (50, 56), 

 on the strength of this, wishes to call the Astartian and Coral- 

 lian by one name, Sequanian. On the other hand, Hebert (19, 

 31) says that one can put one's finger on the line of junction, and 



* Blake & Hudleston, Quart. Journ. Geol. Soc. vol. xxx. p. 313 &c. 



