ON THE UPPER JURASSIC OF THE PARIS BASIN. 



497 



38. On the Correlation of the Upper Jurassic Rocks of England 

 with those of the Continent. — Part I. The Paris Basin. By 

 the Eev. J. F. Blake, M.A., F.G.S. (Read April 27, 1881.) 



[Plate XXVI.] 



In former papers on the Portland Rocks*, the Kimmeridge Clayf, 

 and the Corallian Rocks J, as developed in onr own country, it has 

 been pointed out that, while the normal deposits of the period 

 commencing with the Oxford Clay and continuing to the close 

 of the Jurassic era were essentially argillaceous, the uni- 

 formity has been broken by certain episodes which have resulted 

 in the formation of distinct kinds of rocks, but that, in spite of 

 these episodes, there is a continuousness both in the physical and 

 biological features, uniting the whole into one great group, to which 

 the term Upper Jurassic is appropriated. The present study ought 

 therefore, logically, to include a correlation of the Oxford-Clay series ; 

 but though the upper portions of that series come to be incidentally 

 examined, the far wider range and greater constancy of the lower 

 portion would render its examination a more arduous and less 

 interesting task ; and it is found convenient to have for base a 

 thick mass of clay, which may almost everywhere be recognized, 

 however much the upper part may be encroached upon lithologically 

 by the various preludes to the Corallian series. In point of fact, 

 the rocks hitherto called Corallian in England comprise much that 

 is placed in the Oxfordian by the French geologists ; and our corre- 

 lation is therefore only stopped when the rocks universally called 

 Oxfordian are reached. 



The Upper Jurassic rocks of France lie in two distinct areas. 

 The more northern is that which is drained by the Seine and the 

 Loire and smaller rivers having a similar direction ; the more 

 southern is a continuation of the Swiss Jura, or lies to the south of 

 the central plateau of Auvergne. The former constitutes the basin 

 of Paris, round which city the Jurassic rocks form an irregular 

 curve ; and their development in this range forms the subject of the 

 present study, the Upper Jurassic rocks of other districts being 

 left for a future occasion. 



Much good work has been done by the French geologists in the 

 description of the various portions into which this basin may be 

 divided, and in the correlation of the rocks with one another, by 

 the aid of which it is possible for a foreigner to pass from spot to 

 spot, appreciating, confirming, or even correcting the stratigraphical 

 succession and its interpretation. The advantage that such a 

 student has is that, instead of being confined to one area, and 

 being obliged to obtain his ideas from it, he can make the several 



* Quart. Journ. Geol. Soc. vol. xxxvi. p. 189. 

 t Op. cit. vol. xxxi. p. 196. 

 \ Op. cit. vol. xxxiii. p. 260. 



Q. J. 0. S. No. 148. 2 l 



