ON THE OLIGOCENE STRATA OF THE HAMPSHIRE BASIN. 137 



12. On the Oligocene Strata of the Hampshire Basin. By Prof. 

 John W. Jtjdd, P.R.S., Sec. G.S. (Eead February 4, 1880). 



[Plate VII.] 



Contents. -p^^Q 



I. Introduction 137 



II. History of Previous Opinion 139 



III. Stratigraphical Evidence 142 



IV. Paleeontological Evidence 148 



V. Correlation of the Strata with Foreign Deposits 150 



VI. Subdivisions and Nomenclature of the Series 165 



VII. Thickness of the Strata and their Development in different Areas 169 

 VIII. Conclusion 173 



I. Introduction. 



There are perhaps few portions of the series of British strata which 

 have attracted so large a share of the attention of geologists, both 

 in this country and abroad, as the fluvio-marine formation which 

 constitutes the highest member of the Tertiaries of the Hampshire 

 basin. When we remember the numerous memoirs which, since the 

 commencement of the present century, have been devoted to a descrip- 

 tion of these strata and of their fossils, it might well be supposed 

 that little can remain to be done, either in working out the order of 

 succession of the beds, or in determining their relations to the 

 deposits of other areas. That such is not the case, however, I shall 

 have occasion to show in the memoir which I now submit to this 

 Society ; and it may be well that, at the outset of this inquiry, 

 I should briefly indicate the difficulties which beset the study of this 

 particular formation, and the causes which have led to the serious 

 discrepancies of opinion concerning the mutual relations and the 

 geological age of the strata which compose it. 



Among the difficulties which confront the investigator of the order 

 of succession in these fluvo-marine strata of the Hampshire basin, 

 the most serious is found in the tendency shown by the various mem- 

 bers of the formation to undergo rapid variations in mineral cha- 

 racters within short distances. As in the "VVealden and other similar 

 deposits formed in deltas, so here, we find the whole mass of strata 

 made up of lenticular patches of sediment dovetailing into one another 

 in the most complicated manner ; so much, indeed, is this the case, 

 that we seldom discover any bed in the whole formation exhibiting 

 such persistency of character as to allow of its being traced over any 

 considerable area. Hence the art of the geological surveyor and 

 map-maker — which, in dealing with the more uniformly deposited 

 marine strata, often aff'ords such valuable aid in making out the 

 order of succession of beds — is here comparatively useless. And 

 the perplexities of the geological sui'vcyor are greatly increased 

 by the fact that all over the northern half of the Isle of Wight and 

 the New Forest, where these strata arc developed, tliick superficial 



Q. J. G. S. No. 142. L 



