STRATA OF THE HAMPSHIRE BASIN. 173 



on the Osborne estate on the east side of the Medina*. It is not 

 improbable that under the extensive beds of gravel that almost 

 everywhere conceal the Oligocene strata in the northern part of the 

 Isle of Wight, the Hempstead beds may be present at many points. 

 They have not, however, been detected on the north side of the Solent. 

 The portion of the Hempstead series, as now limited by me, 

 which is exposed at Hamstead Cliff, is about 100 feet in thickness. 

 The incoming of brackish-water conditions at the base of the series 

 is marked by the appearance of numerous forms of Cerithium and 

 marine Mollusca. At the top of the series the marine forms become 

 much more numerous. The Hempstead beds represent, as we have 

 pointed out, the lower part of the Middle Oligocene ; whether the 

 representatives of the remainder of the Middle Oligocene and of the 

 Upper Oligocene were ever deposited in the Hampshire basin we 

 have unfortunately no means of determining. It is interesting, 

 however, to notice that the Lower Oligocene and the inferior portion 

 of the Middle Oligocene deposits are in this country more than 900 

 feet in thickness. 



Ym. Conclusion. 



"Whether we regard the enormous thickness of the beds deposited 

 during this portion of the Tertiary epoch',"the marked and distinctive 

 characters of both the marine and terrestrial faunas, or the vast 

 changes in the distribution of land and water, of which we have 

 such clear proofs in the deposits of this period, it must be admitted 

 that the Oligocene is worthy to rank among the great divisions of 

 the Cainozoic epoch, and must be regarded as of equal value with 

 the Eocene, the Miocene, or the Pliocene. 



It is clear that at the commencement of the Oligocene period 

 great changes must have taken place in the physical geography of 

 Europe and Asia. Large areas, in which marine deposits had been 

 slowly accumulating during the Nummulitic period, were now up- 

 heaved and formed dry land ; and though the sea from time to time 

 re-invaded these areas, the deposits formed in Europe during the 

 Oligocene period were to a great extent of terrestrial and lacustrine 

 origin, while the marine strata were, for the most part, quite sub- 

 ordinate to these. During Eocene times marine conditions, due to 

 continued subsidence, prevailed ; and during Miocene times terres- 

 trial conditions, resulting from elevation, existed : the Oligocene 

 was deposited in a period of oscillation (one of enormous duration) 

 which separated these two epochs. 



The reason why the importance of the marine fauna representing 

 the Oligocene was so long overlooked, is to be sought for in the cir- 

 cumstance that marine strata of this age are usually thin and sub- 

 ordinate to intercalated freshwater or estuarine beds ; and the fact 

 that the strata of this age are very frequently covered by thick 

 superficial accumulations long prevented the collection and study 

 of the fossils of the period. 



It was during the Oligocene period that those great movements 

 commenced which resulted in that folding and crumpling of strata, 



* Proc. Geol. Assoc, vol. i. p. 194. 



