190 KEV. J. F. BLAKE ON THE 



local development in a great pelolithie formation, and deprecated the 

 extension of the name "Corallian" to beds of probably various ages, 

 which only agreed with our own in occurring in the Oxford-Kim- 

 meridge seas, and in containing corals. Just the same might be said 

 of the Portland rocks ; only I should now express it differently. The 

 '* normal"* deposits throughout the European area in the later 

 Jurassic times were argillaceous ; and in England we have two impor- 

 tant " episodes," the Corallian and the Portlandian. In the various 

 countries of the continent they also have episodes, two, or more than 

 two ; but how far they agree in age with ours has yet to a great ex- 

 tent to be determined. At present all the later ones have been called 

 Portlandian; and thus confusion has arisen. I shall now endeavour 

 to show that, as far as the rocks which have been called Middle and 

 Lower Portlandian at Boulogne are concerned, their normal repre- 

 sentatives exist in the mass of the Kimmeridge Clay, and are rightly 

 classed as part of that formation— and also to describe the relations 

 of the various parts of the true Portland episode to each other, and 

 thus to obtain an insight into those final oscillations which con- 

 verted the open argillaceous ocean into the lake-bearing and cycad- 

 growing continent of the Purbecks. 



1. The Island of Portland. 



It will first be necessary, at the risk of repeating well-known 

 facts t, to describe in detail the typical section in the Isle of Port- 

 land, in order to place in their proper relation facts which are not so 

 well known. Pig. 1 (PL YIIL) gives the generalized section of 

 the several exposures in the island. 



The line of demarcation between the Portland and the Purbeck is 

 very clear and constant, consisting of a layer of clay, not lying 

 exactly on an eroded but on a very uneven surface. The lower 

 Purbeck beds known as the " Cap " and the *' SkuU Cap " are botry- 

 oidal limestones or indurated calc-tuffs. They have therefore been 

 derived from the denudation of the Portland rocks which had else- 

 where emerged from the sea at an earlier date, though here the 

 emergence had been so recent that very little atmospheric action 

 had occurred. The separating layer, however, contains remanies 

 Portland flints. This gives us the expectation of finding here a 

 more complete development of Portland rocks than at such places 

 as show more erosion. 



Ko. 1 is the " Roach," which may be characterized as a sheU- 

 limestone. It is to be noted that throughout it is distinctly oolitic, and 

 especially so at the top ; it contains chalcedonic masses which can 

 scarcely be called flints. It is so variable in thickness and so incor- 

 porated with the bed below that it must be taken as forming part of 

 it. The shells which characterize this are : — first, the Cerithium 

 jjortlandicum, which is not, to my knowledge, found in any other 



* See my paper on Geological Episodes, abstracted in ' Nature,' Sept. 4, 1879. 

 t Damon's description of these beds is most accurate, and draws attention to 

 points which are insisted on below. 



