1846.] PRESTWICH ON THE ISLE OF WIGHT TERTIARIES. 251 



range as the sea deepened, or else by a contrary movement gradually 

 replacing the deep-sea testacea by others inhabiting shallow waters, 

 estuaries and fresh waters. When the sea was once more deepened, 

 the original fauna which had been preserved in a part of the neigh- 

 bouring seas not subject to the same disturbances might again be 

 distributed over the sea-bottom, so that there might be formed a 

 deposit of small thickness and uniform palaeontological character, 

 the equivalent of a series of greater thickness and very variable 

 fauna, but exhibiting nevertheless, as a condition of synchronism, 

 and at each return to similar conditions, a tendency to the develop- 

 ment of similar animal life. 



With regard to the disturbances which have affected the Isle of 

 Wight tertiary deposits, I have endeavoured to show the probability 

 that a slow and quiet subsidence of the sea-bottom was taking place, 

 commencing at least as early as the deposit of the Bognor beds, and 

 continuing throughout the overlying marine sands and clays, gra- 

 dually diminishing or ceasing as we reach the fluvio-marine strata, 

 and at that period filling up the estuary and occasionally barring 

 out the sea. 



On the subject of the effects of the powerful disturbance by which 

 the strata at Alum Bay and White Cliff Bay have been placed in a 

 vertical position, it is evident that no unconformability of" superposi- 

 tion has thence resulted. Commencing at the chalk, the strata 

 are at first inclined at an angle of about 75°. This inclination shortly 

 increases to 88° ; and at the upper part of the Bognor beds the strata 

 are slightly reversed. They then continue nearly vertical to the 

 fossiliferous clay stratum " B," where their dip is 80°. From this 

 point upwards through the 250 feet of the London clay a gradual 

 decrease in the dip may be observed. At a distance of 100 feet from 

 the bottom of this bed it is 75° : eighty feet further it is 70° ; it then 

 rapidly decreases, first to 68°, and then to 60°. At the junction of 

 the London clay and Headon Hill sands it is only 55° ; twenty feet 

 higher it is 4<5° ; and 100 feet above this last point the green marls 

 and limestones have a dip of 21° in the same direction. (See PI. IX. 

 fig. 1.) It is clear, therefore, that although not affected to the same 

 extent, they are so in equal ratio, the force of the disturbance acting 

 laterally from the chalk at the present sea-level to the lower part 

 of the green marls and limestones. In further evidence of this it will 

 be observed that the end of the green marls and limestones nearest to 

 Alum Bay dips at 21° for a short distance, and that the strata are 

 then fractured — the disjointed edges of the njass nearest to Alum 

 Bay being eight feet below the level of the nrass from which it is 

 separated, this mass being prolonged northward at a very slight and 

 uniform dip. (See PI. IX. fig. 1.) Now this detached mass bears 

 in itself evidence of having experienced the action of a protruding 

 force; for if we prolong the plane of the larger and less-disturbed 

 portion of Headon Hill, we shall find it intersect the j)lane of this 

 small mass at angles of about 16°, and that the angle of depression, 

 below the principal plane, subtends northward, and that of elevation 



