224 KEY. J. E. BLAKE ON THE 



was killed off, and a wide area became exposed to the atmosphere. 

 The freshwater conditions of the rocks of the Isle of Purbeck in- 

 troduced a new era, which it is not my object to discuss, 



"With this history of the formation of the Portland rocks the 

 earlier local elevations affecting countries to the south of England 

 have nothing in common. They were not even the prelude to the 

 changes above recorded ; for after the Boulognian episode had begun 

 and ended, the sea-bottom once more assumed its former position, 

 and fresh deposits of clay (which had never ceased to fall on the 

 British area) commenced again, and only gradually through the 

 Portland Sands gave place to calcareous rocks. "We may therefore 

 rightly object to the use of the term Portlandian for rocks which 

 neither in organic contents, nor in physical history have any thing 

 to connect them, and restrict the name to those which have formed 

 the subject of the present description. We are even led to ask 

 what amount of justification there is for the title Portland Sands 

 as applied to the lower beds. If the choice lay between including 

 these in the Kimmeridge Clay and including some more of the 

 clay beds below them in the Portland series, I should unhesita- 

 tingly take the first alternative; for while the true Kimmeridge 

 Clay has very little in common with the Portland Stone, it has 

 much with the Portland Sand, and the latter has as much with 

 it as it has with the Portland Stone. Moreover, in dividing the 

 Portland Sand from the Kimmeridge Clay we have to draw a 

 somewhat arbitrary line ; but the line above is generally a well- 

 marked one. It is these facts which have led the Prench geologists 

 to include with the Portland Sand the upper part of the Kim- 

 meridge Clay in one group, called by them the " Middle Port- 

 landian." Yet, if there were no more to be said than this, the 

 logical result would be the abolition of the Portland Sands. We 

 have seen, however, that it is a variable mass developing local 

 features and expanding and contracting in thickness, thus acting 

 after the manner of an episodal formation and not like the normal 

 Kimmeridge Clay. We have seen, too, that at certain spots a well- 

 marked special fauna may be found in it. These features indicate 

 that at the period of its deposition in any locality (a period which 

 may have varied in absolute time from spot to spot) the tranquillity 

 of the Kimmeridge sea-bottom had been disturbed, and those changes 

 commenced which finally closed the Oolitic period in this country. 

 To the final deposits due to these changes we give the name of 

 Portland Stone ; and hence to the results of their precursors, and to 

 these alone, can we rightly give the name of Portland Sand. 



