1865.] WHITAKEE ISLE OP WIGHT CHALK. 405 



Fig. 4. — General Section across the Chalk of the Isle of Wight. 



a. Flint-gravel, often occurring on 



the highest downs. 

 h. Woolwich and Reading Beds. 



c. Chalk with frequent parallel layers 



of flint or fliits. 



d. Chalk with few flints, and of a 



somewhat nodular structure. 

 e. Position of the " Chalk-rock." 

 /. Massively bedded chalk mthout 



flints, but with thin marly layers. 

 g. Evenly bedded hard Chalk-marl. 

 h. Upper Greensand. 



Junction of the Chalk and the Tertiary beds. — With all due respect 

 to the opinions of other geologists who have written on the subject, I 

 must venture to state my doubt of there being clear proof of any 

 " eroded surface of the Chalk " at the junction of that formation with 

 the overlying Tertiary beds *. That there is sometimes an " uneven 

 surface " cannot be gainsaid by anyone who knows the Alum Bay 

 section. There the junction is most irregular, the sand at the bottom 

 of the Keading Beds filling hollows of all shapes in the Chalk ; this 

 irregularity, however, is not, I take it, owing to " erosion " of the 

 latter, that is to say, to its having been worn away before the depo- 

 sition of the Reading beds ; but is merely a good specimen of those 

 " pipes," or irregular-shaped funnel-like hollows, that have been dis- 

 solved out in the Chalk by the action of carbonated water after the 

 deposition of the beds above. 



If, after studying these pipes, the geologist will " right-about 

 face " and look at what the sea is now doing along the chalk-coast, 

 he will see the great difference of these two agents ; for along the 

 shore west of Alum Bay the sea is constantly attacking the Chalk, 

 planing it down, and forming flat or shghtly curved surfaces and 

 shallow basins, but nothing like the irregular-shaped and branching 

 pipes of the cliff. 



As far as I know, it is only where the surface of the Chalk is 

 easily got at by water that pipes occur of any great size or in any 

 great number, — that is to say, where there is but a somewhat thin 

 capping of Tertiary or Drift beds, or where, as in the present case, 

 the beds are enough tilted to allow of a fairly free access of water 

 along the junction. On the contrary, where there is a good thick- 

 ness of the beds above the Chalk, and where the dip is low, pipes 

 are both rare and small, as may be seen in the chief junction-sec- 

 tions of the Chalk and the Tertiary beds in the London Basin, at 



* See the Geological Survey Map of the Isle of Wight, Mr. B)'istow's Memoir, 

 p. 29, and Mr. Prestwich in Quart. Journ. Geol. Soc. vol. viii. p. 256. 



