Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/30

8 Here then we have designedly tabulated about sixty species of Cephalopoda from D'Orbigny's 'Paléontologie Francaise, Terrains Jurassiques,' as this author has referred fully one quarter of the species to the Lias. It is an extraordinary list (even though not yet fully made out) for about two feet of rock.

If we inquire how it is that so many of the species have been allocated to the Lias, we shall find that some few of them have undoubtedly ascended upwards from the lower stratum; but most of them have been called Liassic upon the assumption that our Bradford Cephalopoda-bed and our sands were the equivalents of those beds in Gloucestershire, and both supposed of Lias age. This we know, not only from references by D'Orbigny himself, but also from having seen fossils from my own quarry of the age of the Gryphite Grit labelled as from "the Upper Lias."

From all this it appears evident that while some English geologists have confounded two beds fully 100 feet apart, and made their lists of fossils harmonize with this view, both some foreign and home savans (taking, be it observed, these two beds to be one) have, in the same way, made them to harmonize with the Upper Lias of the Continent.

Now I have not had the pleasure of a personal examination of foreign oolites, but I can plainly see that they have been interrogated to support theories no less than have those at home; and I can well believe that if they at all harmonize with our Dorset strata, foreigners, like ourselves, may have confounded two beds widely apart.

That they do so harmonize we are strongly inclined to believe from D'Orbigny's drawings of Cephalopoda, as in Dorset we have not only a large number of species referred for the first time to our home rocks, but they are for the most part in a fine state of preservation—so much so, that the terminations of the Ammonites have in many cases been clearly made out.

It may be further remarked upon this list of Cephalopods, that although the bed in which they occur has been made out over a wide district, and in all cases it preserves its peculiar character, yet it differs at various points as to the prevalence of species.

Thus at Bradford Abbas the Ammonites subradiatus prevails. At Babylon Hill the A. Murchisonæ is more common, while midway the A. Sowerbii takes the lead. At Halfway House the A. Parkinsoni is the characteristic fossil for a part of the quarry, and the A. subradiatus for another part. Further to the east, at Sherborne, the A. Humphresianus assumes importance; whilst at Clatcombe, a mile from there, the A. Braikenridgii is not only a common but a most perfect fossil.

Now if it be assumed that this Cephalopoda-bed at these different points occupies a different horizon, of course we can recognize them as different zones, and name them after their prevailing Ammonites; but it is not so; and it is a remarkable fact that from 2 to 3 feet of the oolite rock in a limited area should present not only so great a crowd of individuals but such a variation in species.