Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/363

 The structure of the rock varies, being compact, granular, or Oolitic limestone, or loose calcareous sand. Many of these masses are described as " intersected by narrow channels of deep water."

II. The sedimentary beds. — These were considered in reference to the influence their accumulation would have on the accumulation of the coral-masses. Many causes will from time to time change both the rate of accumulation and the extent of the area over which the sedimentary accumulation will take place. If such accumulation take place in the same sea with coral-growth, a turbid condition of water may alternately encroach on and recede from the coral-area. These occasional irruptions would then temporarily interfere with the coral-growth. In the case of the permanent encroachment of sediment on a district of coral islands and reefs, the contour of the gradually depressed islands would be protected by the sediment, which would also fill in the deep channels between them.

These considerations, it was suggested, may serve to explain the " dip towards the valley " as mapped by Mr. Sanders, or " the thinning out," which is believed by the author to occur. The hills were probably coral islands submerged, and covered up by sedimentary matter. When the whole area, after subsequent elevation , became subject to denudation, the yielding sedimentary matter being swept away left the limestone islands standing as hills.

This naturally leads to the question whether the limestones themselves have suffered denudation. The author thought there can have been but little, if any. He considered first the mechanical action of water. He pointed out that the Great Oolite is traversed by numerous vertical joints and fissures, which split it up into what may be roughly termed cubical blocks. The surfaces of the joints are generally covered with crystalline carbonate of lime. The rain falling on the plateaux would at once percolate through the surface- soil and " heading," and would pass off in the fissures. There would thus be no accumulation of water, therefore no body of water in motion, therefore no mechanical denudation. Elie de Beaumont's suggestion, set forth in 1843 in his ' Legons de Geologie pratique,' that where there is no mechanical denudation, there must be accumulation of vegetable soil, has not been objected to in principle, though the extreme lengths to which he carried the idea have been attacked. It is most probable that a protective covering of vegetable mould has everywhere covered these Oolitic hills.

Secondly. With regard to chemical denudation, as each block is bounded above and below by a parting of clay, and on its four sides by crystalline carbonate of lime, the author could not see how chemical denudation could take place except in an accidental way.

These remarks apply to the Great Oolite. The Inferior Oolite in this district nowhere forms the summit of a hill ; it is only to be seen in sections on the sides of the hills. If it also was formed in detached islands, some of the original edges of these islands may be still hidden from our view by sedimentary matter preserved from denudation by the Great Oolite above ; its origin must be dealt with in a locality where it forms the tops of the hills. It may, however, happen