286 Palwo-Geological and Giographical Maps of the British Islands. 
have been shown by Professor Judd to be present in the Isle of Wight,* and consist 
of alternating sands, clays, shales and limestones of marine and estuarine origin. 
All these beds were deposited either at the mouths of rivers flowing from the 
north and west into a sea, which was generally open in the direction of France, 
but often very shallow, and sometimes converted into estuaries and even lakes"of 
limited extent. Oceanic water, such as that of the Atlantic, probably never 
occupied the area in question. The fauna indicates successive stages of depression 
or elevation, and the alternation of fresh water, estuarine, or marine conditions. 
Distribution of Land and Sea.—I have already to some extent dealt with this 
subject, and will, therefore, only here observe that as a long interval of time 
elapsed between the formation of the Upper Chalk and of the Lower Hocene strata,t 
during which land conditions prevailed over the British area, much of the Chalk 
formation itself was denuded away; and consequently the Tertiary beds are 
unconformable to the Cretaceous, and rest sometimes on higher, sometimes on lower 
strata of that formation. 
The position of the northern limit of the sea margin, even during any given epoch 
of the Tertiary period, is a question of much uncertainty. In Figure 2, Plate XX XIIT, 
I have attempted to show the physical geography of the epoch of the London clay, 
when the sea had its greatest extension over the area here described. At the same 
time, I have considered it necessary to leave a considerable tract of uncoloured 
debateable ground between the respective margins of land and sea. 
(2.) Miocene Strata.—With the exception of some lacustrine beds of gravel, clay 
and lignite at Bovey Tracey in Devonshire, all the British representatives of the 
Miocene epoch are restricted to the north-east of Ireland, and the west coast 
and isles of Scotland, and are of volcanic and lacustrine origin. 
These beds consist of great sheets of augitic and felspathic lavas, with 
intervening beds of ashes, lapille, pisolitic iron ore (in Antrim), and lignite beds 
with plants, the examination of which enabled the late Professor Edward Forbes to 
determine the Miocene age of these rocks. These volcanic sheets rest generally on 
a floor of chalk, or of some older formation, and all observers are agreed that they 
have been poured out upon a land surface. It is probable that, at one or more 
intervals, lakes were formed, and the valuable pisolitic iron ore of Antrim may 
be reterred to this mode of origin. 
These volcanic products have undergone enormous denudation since the Miocene 
period, and on the little map Figure 2, Plate XX XIII., I have endeavoured to show 
the original area overspread by them. 
* Quart. Journ. Geol. Soc,, 1880. 
+ ‘This interval is partly represented by the Maestricht beds of Belgium. 
