LOWER GliEENSAND. 19 



(2.) The base of the Lower Greensand is a thin line of coarse 

 grit, containing rolled fragments of fossils (Ammo- 

 nites and other marine forms) which must have been 

 derived from some marine heds, ex|)o>ed outside tl e 

 limits of the freshwater Wealden Bed^, together with 

 an occasional pebble of sandstone lai-ger in size, nnd 

 resembling the sandstones which are interstratified in 

 the Wealden Beds. There are also in this grit numerous 

 broken bones, teeth, and scales of fish, and at Atherfield 

 it contains fragments of Vicarya stromhiformis, the 

 gasteropod which is so abundant in the top of the 

 Wealden Shales at this spot. The fragments occur only 

 in the grit, which is about three quarters of an inch 

 thick, and have doubtless been washed out of the sur- 

 face of the Wealden Shales. At Punfield this grit has 

 yielded a well-rounded pebble of white sihcified wood, 

 precisely similar to the wood in the Lower Purbeck 

 Beds. 



(3.) The Wealden Shales, where the junction is exposed, often 

 present the appearance of having been disturbed and 

 broken up for a distance of a foot or two below the base 

 of the Lower Greensand. 



(4.) In Wiltshire the Lower Greensand overlaps the Wealden 

 Beds so rapidly as to indicate an actual unconformity.* 

 As a result of this overlap it passes westwards on the 

 Upper Oolites, a fict which provides a clue to the 

 source of the rolled fossils of marine species, which 

 occur as pebbles at the base of the Perna Bed in the Isle 

 of Wight. 



As far as the Isle of Wight is concerned, however, there is 

 not sufficient evidence to establish an unconformity between the 

 Lower Greensand and Wealden Beds. That the bedding of the 

 two is strictly parallel is proved by the persistence of the Wealden 

 Shales at the top of this formation, not only in the Isle of Wight, 

 but both to the east and west on the main-land. The change of 

 sediment is such as might have been produced by the sudden con- 

 version of a partially land-locked estuary or lake into a bay open 

 to the sea, whether by subsidence or by the washing away of a 

 barrier. 



On this theory we must suppose that a Lower Greensand sea 

 with its* proper fauna was in existence at the time when the 

 Wealden Shales were still being deposited in the land-locked area. 

 This supposition is in accordance with the sequence observed in the 

 north of England. For the Upper Neocomian deposits of Yorkshire, 

 as shown by Professor Judd, contain the same fauna as the lowest 

 of the Isle of Wight Neocomian beds, namely, the Atherfield 

 Clay. We are thus compelled to suppose the Middle and Lower 



* Geology of England and Wales, by H. B. Woodward, 2nd Ed. 1887, pp. 352, 

 354,375. 



B 2 



