Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/294

 Messrs. Riley and Stutchbury in tho year 1836, the additional interest now attached to these remains being due to fresh researches, by Prof. Huxley, into the history and distribution of the Dinosauria through the lower Secondary rocks of the globe. It therefore becomes a matter of interest, if not necessity, to fix the geological age of the reptilian remains — the more so, in this case, on account of the numerous opinions held by Continental and British geologists as to the exact equivalents of this conglomerate, and its place in the Triassic group.

6. Position in time of the Reptilian or Dolomitic Conglomerate.

It is at all times difficult, if not, perhaps, almost impossible, to assign an approximate age to remains found in an extensive, continuously and contemporaneously forming and associated conglomerate belonging to any age or deposit; it is especially so with the Dinosauria under consideration ; and whether they inhabited any certain or given area, or at any given period during the immense lapse of time these magnesian breccias were under accumulation, is a problem important to solve, both as to space and time,— the first as bearing upon the habitat, locality, or area occupied by these genera prior to, and at the time of their deposition ; the latter as affecting their relation to that particular horizon of the conglomerate, and the assignment to these remains of an early or late date in relation to the mass. Two great periods of oscillation and associated phenomena have been assigned to the area under notice, — the first a downward movement of the palaeozoic land with its consequent loss of material, which must have commenced after its consolidation and elevation to the position it occupied at the time the New-Red sea began to denude its mass, and lasted through the whole of that long period of depression which was sufficient to allow of fully 1000 feet of New Red marl and sand to be deposited over the depressed palaeozoic rocks. The second period was one of elevation, during which the accumulation of the first period was again partly, if not almost entirely, denuded or removed (this is of comparatively recent date) ; and this again exposed the old land-surfaces, nearly as we now see them. Certainty, then, at some period during the depression and accumulation of the dolomitic conglomerate these reptilian remains were deposited ; and it is equally certain that these two genera (Thecodontosaurus and Paloeosaurus) inhabited the area or region where found.

The occurrence of these Triassic Dinosauria at the elevation of 300 feet above the present sea-level, and on the general tableland now occupied by the Carboniferous Limestone, and apparently that portion of it last influenced by the New-Red sea, would lead me to infer that it was late in the history of the Keuper that their deposition was effected. I assume this from the different stages and relative levels now occupied by the conglomerate, which necessarily elucidate their position in time at the date of deposition. The whole period of elevation is measured by the successive steps and stages occupied by