Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/148

 The marine strata associated with the various groups of estuarine beds in the Scotch Jurassic system also exhibit many very interesting characters. As compared with their equivalents in England, they usually show indications of having been of more shallow- water origin, and accumulated under conditions of a much more local character. While on the one hand there is a general absence of the thick masses of clays, formed of fine sediment and crowded with pelagic forms of life, like large portions of the Lias, and the Oxford and Kimmeridge clays, we find in many parts of the series great accumulations of conglomerate made up of the local rocks. At the same time there are not wanting proofs that, during certain portions of the Jurassic period, marine conditions prevailed over a very considerable area; and it is in these that the strata are found to assume the comparatively deeper-water and more normal characters.

The remarkable feature of the frequent recurrence of estuarine strata, though characteristic of the Scottish Jurassic series, is not peculiar to it. In the southern province of Sweden (Scania) we find a precisely similar set of phenomena to those which we have been noticing as so strikingly displayed in Scotland.

In Sweden the Secondary strata are exposed under the same disadvantageous conditions as in Scotland. Almost everywhere the surface of the country is concealed by great masses of drift of various kinds, above which a few hard ridges of Mesozoic rocks rise in isolated patches. Some of these patches are composed of Chalk and Upper Greensand; others of Jurassic strata presenting very peculiar characters. The exact geological relations of these singular fragments of Secondary strata have not apparently been fully determined, but, like the similar beds of Scotland, they are developed in the immediate vicinity of great masses of Silurian and granitic rocks. The Jurassic strata of Sweden consist of alternations of sandstones, shales, grits, quartzose conglomerates, impure lignites, and workable seams of coal : in some places these beds yield a beautiful flora ; in others they contain bands with marine shells. These strata have been studied by Wahlenberg, Nilsson, Hisinger, Murchison, Braun, and others : and by some authors, as Brongniart and Mantell, they have been regarded (as were the estuarine Jurassic beds of Scotland) as representing the Wealden.

The two most important patches of these strata, those of Hogonas and Hor, have lately been made the object of careful and exact study by M. Hebert, who has shown that the marine strata at the base of the former contain a fauna which enables us to assign them to the base of the Lower Lias, while the evidence with regard to the latter, though less decisive, is such as to lead us to consider them to be of nearly the same age *.

Thus we see that there are reasons for believing that over a vast area, comprising the northern limits of the Anglo -Parisian basin, a

(Suede meridionale), par M. Hebert," Annales des Sciences geologiques, tom. i. p. 117; Bull. de la Soc. Geol. de France, 2 e serie, tom. xxvi. (1870) p. 366.
 * " Recherches sur l'age des gres combustibles d'Helsingbord et d'Hogonas