Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/384

 regard to other dolomites. That the Magnesian Limestone was deposited in great part from solutions may partly account for the absence of fossils in so much of that formation ; and the uncongenial nature of the waters of a salt lake may account for the poverty-stricken character of the whole molluscan fauna.

One other group of red or purple strata remains, older than the Old Red Sandstone, and of this I speak with more doubt ; I mean the Cambrian rocks. Neither in North Wales nor in the Longmynd do these rocks afford any indications of life, excepting annelide- tracks and burrows ; for, with some other persons, I consider that the so-called trilobite Paloeopyge Ramsayi is only an accidental marking simulating the form of a trilobite*. The general absence of Mollusca in these strata, and the sudden appearance of shells and trilobites in quantity in the succeeding Lingula-flags, indicates a sudden change of conditions. There is perfect conformity between the two formations ; but the change of lithological character is rapid, and akin to that which marks the change from Upper Silurian rocks into Old Red Sandstone ; only the order of change is reversed. Believing that the red colour of rocks is apt to be connected with their deposition in inland waters, I conceive it to be possible that the absence of marine Mollusca in the Cambrian rocks may be due to the same cause that produced their absence in the Old Red Sandstone. The presence of sun-cracks and rain-pittings in the Longmynd beds favours this suggestion. In Pembrokeshire, however, Mr. Hicks has discovered a Lingulella (L. ferruginea) and Leperditia cambrensis on two horizons, in strata well down in the purple Cambrian series. It is, however, worthy of note that the lowest slaty bed in which the fossil occurs, directly overlies " olive-green grits and shales"†. These may possibly mark occasional influxes of the sea into inland waters, due to oscillations of level. I do not wish, however, to speak positively on these Pembrokeshire strata. It is nearly thirty years since I mapped the country, and I should like again to see the rocks in place before doing any thing more than merely hinting at the subject. It is worthy of remark, however, that the possible inland origin of the Cambrian deposits in general is quite consistent with and throws some light on the statement by Dr. Otto Torell of the occurrence of land-plants in these strata, supposing this statement to be correct.

If I am right in the deductions I have drawn in treating of the red rocks and magnesian limestone of older date than the Trias, some remarkable conclusions may be arrived at in connexion with the inland character of the waters of the epochs to which this and my previous paper on the New Red Marl relate.

The old palaeozoic area of Bristol and the Mendip Hills and of adjacent districts now partly concealed by secondary strata stood above water during the whole of the periods of deposition of the Permian and Bunter beds, and was not covered with water till these were more

p. 340).
 * "The mythical Paloeopyge" (Salter, Quart. Journ. Geol. Soc. vol. xxiii.

† Geol. Mag. vol. v. p. 306.