1871.] MITCHELL OOLITE -DENITDAXIOir. 229 



The structure of the rock varies, being compact, granular, or Oolitic 

 limestone, or loose calcareous sand. Many of these masses are de- 

 scribed 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 accumula- 

 tion take place in the same sea with coral-growth, a turbid condi- 

 tion of water may alternately encroach on and recede from the 

 coral-area. These occasional irruptions would then temporarily in- 

 terfere with the coral-growth. In the ease of the permanent en- 

 croachment of sediment on a district of coral islands and reefs, the 

 contour of the gradually depressed islands would he 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 sedi- 

 mentary 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 them- 

 selves 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 OoKte 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 Kme. 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 ' Logons de Geologic pratique,' that 

 where there is no mechanical denudation, there must be accumula- 

 tion 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 denu- 

 dation 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 



