Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/77

 Discussion.

Sir R. I. Murchison differed from the author on one point only. He regarded the upper member of these saliferous rocks, which is overlain by 1500 feet of Bunter Sandstone, as completely separated from that formation, and as forming the upper part of the Permian group.

3. On the Formation of Deltas ; and on the Evidence and Cause of GREAT Changes in the Sea-Level during the Glacial Period. By Alfred Tylor, Esq., F.L.S., F.G.S., &c.

(Abstract.)

The first portion of this paper is devoted to a comparison of the delta- deposits of the Po, Mississippi, and Ganges, by means of the descriptions of the strata obtained from borings in their deltas for water.

The surfaces of these deltas and the alluvial plains above them are compared together with reference to their inclination and height above the sea-level ; and it is found that a parabolic curve drawn through the extremities of each river, and through one point in its course, nearly represents its longitudinal section, the greatest deviation being 30 feet in some of the largest deltas.

The delta-deposits are found to be coarser and more sandy near the bottom, indicating that the rivers were more rapid during the earlier portion of their existence.

Messrs. Wheatley and Abbott's descriptions of the delta of the Mississippi are compared with those of earlier writers ; and a description is given, from their work, of the late extensions of the delta into the Gulf of Mexico.

The formation of delta-deposits is explained by the hypothesis of a change in the level of the sea, instead of in the level of the land and sea-bottom.

The littoral deposits around Great Britain are investigated by the author, to ascertain if his hypothesis, of a fall in the sea-level of 600 feet during the Glacial Period, is tenable.

Some evidence of the extent of the Glacial Period is given ; and the ice-cap hypothesis advocated by Mr. Croll is alluded to as a probable cause of a great reduction in the level of the sea through abstraction of water from the sea, and its deposition at the poles in the form of ice.

The positions of the fossiliferous strata of the Quaternary Period are discussed with relation to Mr. Godwin- Austen's former suggestion of a great river where the German Ocean now is, formed by the junction of the Rhine, Thames, and Humber. The probable age of the Straits of Dover is also alluded to.

Prof. Forbes's examination of the fauna and flora of the British Isles, with a view to the determination of the sources of Alpine plants, induced him to believe that the British plants and animals migrated from Scandinavia, Germany, and France at different periods, some before and some after the Glacial Period; and the