Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/541

1870.] SHARP NORTHAMPTONSHIRE OOLITES. 375 to the total thickness of D and E, the sequence of the beds of the wells-section being also favourable to such an assumption.

It is noticeable, however, that at the " Old " Duston pit the green colouring of the beds begins with the Astarte zone, while in this Ironstone section it is only observable lower down. The absence of the green colouring in the higher beds of both sections is probably due to oxidation, the effect of atmospheric influence. In the " old" pit, a very thick mass of rock is superimposed upon the Astarte zone ; and consequently such atmospheric influence has not permeated that and lower beds ; but in the Ironstone quarry the Astarte zone lies comparatively near to the surface, and considerably above the level to which that influence has penetrated. This difference in circumstances of position will probably account for the apparent discrepancy in the range of the green colouring of the beds in different sections.

The parallelism of these beds with the lower beds of Bass's pit (l) is not only suggested by position, but is confirmed by the fact of the identity of some characteristic fossils found in both sections, notwithstanding the absence from the last-named pit of the Astarte zone : for instance, in the lower beds of Bass's pit are found two large species of Pleurotomaria (armata? and a species near to Marcousana of D'Orbigny) ; and the same forms (unmistakably identical with those from Bass's pit) occur in the Duston Ironstone beds. The lowest bed, also, like that in the last-named pit and in the Kingsthorpe pit, is full of rounded pebbles or concretionary nodules.

That these beds are not higher up than the Inferior Oolite is abundantly shown by the fossils exhibited ; while the view that these beds are possibly equivalent to Dr. Wright's Frocester Cephalopoda beds is strengthened by the occurrence of Ammonites jurensis (?), and in three other pits of Rhynchonella cynocephala ; and that they have a transitional character, by the probable presence of Pholadomya ambigua, Ammonites bifrons, A. opalinus, and some other fossils which would tend to such a conclusion.

It cannot be conceived that there could have been a point in time at which the period of the Upper Lias definitely ceased and the period of the Inferior Oolite as definitely commenced. One must have merged into the other, and life-forms have been gradually transmuted into or superseded by other life-forms, during a connecting period of longer or shorter duration ; and my suggestion is, that we have in the lower beds of the Northampton Sand a stratigraphical representative of a portion of such transitional interval *.

Before leaving this remarkable section, I may be excused if I offer a few words upon the ferruginous character of these beds. I need not say that they are not in their original condition. The numerous living organisms of which these fossils (many of them, as it were, cast in iron) are the enduring monuments, could not possibly have existed in waters charged with iron to the degree apparently indicated by the present condition of the rock. The iron must have been introduced after the deposition of the sedimentary


 * See Dr. Lycett's opinion in note to the Blisworth Area.