Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/364

 that the edges, or one of the edges, of an Inferior-Oolite island may be coincident in position with a Great-Oolite island above.

There remains the question of the meaning of finding on opposite sides of a valley zones containing assemblages of similar organic remains. The author believed that this in no way bears on the question of the original continuity or otherwise of the two sides. If there are beneath the sea two spots (a and b) near together, having exactly similar conditions for life, the assemblage of forms at both places might be the same, whether a and b are separated by a channel or not.

In conclusion, the author remarked that, though he had arrived at this idea inductively, yet he believed it would be fair to start with the statement that coral islands when covered up would retain their contour ; and it would rest with those who insist on continuous limestone-beds in all cases to show why the islands with deep channels between them must have been broken up and spread out as a continuous sheet before the sedimentary deposit was accumulated on them. He also expressed his belief that the present view commonly held affords a correct explanation of the phenomena of Oolite valleys in many cases.

Discussion.

Prof. Morris did not consider that the author's views as to the oolitic masses round Bath being originally isolated coral banks with clay beds, although suggestive, were quite satisfactory. He pointed out that the strata on each side the valleys were similar in structure, mineral character, and fossil contents, and were once continuous ; and the present intervening deep valleys were rather due to the movements which the area had undergone in producing lines of weak resistance, subsequently assisted by the erosive action of percolating and running water, both in excavating and undermining the harder rocks, so as to cause them to bend towards the hill-sides, or fall in larger or smaller masses on their slopes.

Mr. Seeley thought that Mr. Mitchell was justified in applying considerations drawn from the formation of coral islands to the elucidation of the phenomena under discussion. He maintained that shallow- water limestones must always occur in isolated masses with intervening masses of clay, and that the clay might be washed out, leaving the limestone as hills.

Mr. Whitaker held that when like beds cropped out on the tops or flanks of opposing hills it was a logical inference that the said beds had once spread across the space between, and that there was no need to call in the agency of supposed coral islands to explain the occurrence of isolated masses of limestone, which were perfectly accounted for by denudation, an agency that involved no supposition, and was quite equal to the work.

Mr. Etheridge remarked that the mollusca of the outliers of the Oolites in the Severn Valley were constant in beds at the same relative level. He also referred to the sliding of the oolitic strata of the Cotteswolds upon the subjacent clays as accounting for the dip