94 



SCIENCE-GOSSIP. 



GEOLOGY OF THE ISLE OF WIGHT. 



By Thos. Leighton, F.G.S. 



(Concluded from page 64.) 



HpHE Chalk in the Isle of Wight forms the central 

 hills (or northern downs) and caps the hills 

 above the Undercliff (the southern downs). The 

 beds may be examined both at the coast and in 

 the numerous quarries which are found along the 

 outcrop. The' usual Chalk fossils occur, probably 

 as plentifully as anywhere else ; the fact that they 

 are somewhat uncommon in collections is doubtless 

 due to the distance from London rendering search 

 and preservation unprofitable to the workmen. 

 The Chalk Rock, and also, possibly, the Melbourn 

 Rock, have been recognized, whilst the follow- 

 ing Table exhibits the zones as worked out by 

 M Barrois : — 



fZone of Belemnitella 

 ts| 



Chalk with flin 

 1,016 feet 



Micraster cor-anguinum 

 Micraster cor-testudinarium 



Holaster planus 

 Terebratulina gracilis 

 Inoccramus labiatus 



Scaphites aqua! is 



Chalk without 1 

 flints, 196 feet 1 

 GreyChalk.Chalk 1 

 Marl, 115 feet i 

 These divisions do not exactly coincide with those 

 of the Geological Survey, whose thicknesses are 

 as follows : — 



Upper Chalk . . 1,370 feet 



Chalk Rock . . a line 



Middle Chalk . . 1S0 



Melbourn Rock . . 14 



Lower Chalk . . 206 ,, 



The Eocene Beds of the island occupy a narrow 

 tract at the foot of the northern slope of the 

 central hills. Striking east and west, although 

 between 1,500 and 1,600 feet thick, their outcrop 

 is extremely narrow by reason of their vertical 

 position. (See figs. 2 and 3.) On this account 

 they can perhaps be more fully examined in other 

 localities, although they are well exposed on the 

 east coast in Whitecliff Bay, and on the west coast 

 in Alum Bay. The Eocenes, however, are nowhere 

 lacking in interest — an interest indeed almost 

 poetical — best illustrated by the appropriate name 

 conferred upon them by Sir Charles Lyell, i.e. 

 Eocene — " the dawn of recent [times] ." 



Resting on a slightly eroded surface of the Chalk, 

 are the plastic clays of the Woolwich and Reading 

 Series, estimated at S4 feet in Alum Bay and at 

 163 feet in Whitecliff Bay. The thicknesses of the 

 lower beds of the Eocene in the Isle of Wight are, 

 however, scarcely comparable with those in other 

 localities, since it is impossible to say to what 

 extent the beds, particularly those near the Chalk, 

 have been squeezed up by folding. The magnitude 



of the pressure to which these beds have been 

 subjected can be somewhat realised at Alum Bay 

 by careful examination of their junction with the 

 Chalk. A mass of crushed flints will be found here 

 welded into a compact bed, perhaps slightly altered 

 in texture, and, from later infiltration of iron, form- 

 ing a curious store. 



The London Clay follows above the Woolwich 

 and Reading Beds, 233 feet thick in Alum Bay, 

 and 320 feet thick in Whitecliff Bay. It contains 

 occasional lines of flint pebbles and is more sandy 

 throughout than in the London area, conditions 

 pointing to shoaling waters in this direction. 

 Ditrupa plana occurs at the base, and other fossils 

 may be found throughout, which, from not being 

 pyritized, as in the London area, can be readily 

 preserved. 



The Lower Bagshot Sands follow next : they are 

 unfossiliferous except in the leaf bed of Alum 

 Bay, at which place they are also followed by other 

 unfossiliferous beds. If at Alum Bay the whole of 

 the celebrated coloured cliff is placed in this series 

 it attains a thickness of 662J feet, against 100 feet 

 in Whitecliff Bay, whilst in the series immediately 

 above, these thicknesses would be nearly reversed at 

 the two localities. Accordingly, Mr. Clement Reid 

 suggests that the upper 450 feet of the coloured cliff 

 at Alum Bay may be the decalcified equivalents of 

 fossiliferous beds at Whitecliff Bay. Mr. Starkie 

 Gardner again separates certain of the higher 

 beds as equivalent to the freshwater Brackleshams 

 of Bournemouth, and his dividing lines do not in 

 other respects coincide with those of the Geological 

 Survey. The leaf bed of Alum Bay has been 

 admirably described by Mr. Gardner, who states 

 that the flora is the " most tropical of any that has 

 so far been studied in the northern hemisphere." 

 The plant remains cannot now be readily found, 

 they occurred mainly in a lenticular bed of pipe- 

 clay which has been worked out. 



The marine Bracklesham Beds lie above the 

 Lower Bagshot Sands. They are highly fossili- 

 ferous at Whitecliff Bay, where they are 653 feet 

 thick. The fossils can be best obtained by digging 

 on the foreshore at low water ; so collected, however, 

 the sea-salt must be abstracted before they can be 

 safely preserved. In Alum Bay only a few casts 

 of fossils occur at this horizon, the series, however, 

 contains four beds of lignite of some interest, since 

 each bed rests on a regular underclay with rootlets, 

 as in the coal measures, showing that the vegetation 

 of which the lignites were formed actually grew on 

 the spot. 



