258 Palaa-Geological and Geographical Maps of the British Islands. 
coal-formation does not exist, and what kinds of strata occupy its place. Now, 
nearly all the borings which have thrown any light on the internal structure of 
sub-Jurassic or sub-Cretaceous areas have been made within the last quarter of a 
century ; and until they had been made, geologists were not in a position to deal 
with any degree of certainty with the problems I have referred to, much less to 
represent their views on physiographical maps. 
In tracing out the range of special formations over tracts of country of consider- 
able extent and noting the changes they undergo, the mind is naturally led to 
speculate on their original extent and distribution before they had undergone the 
denudation or waste, to which they have subsequently been exposed. ‘The forma- 
tions of the British Isles and of the adjoining parts of the European continent are 
only fragments of the original masses; they have been disturbed from their once 
horizontal position, tilted into various angular inclinations from the horizon, and 
have undergone much waste, so that we seldom meet with the original “shore 
beds” which were deposited in the waters of the sea or lake (as the case may be), 
along the margins of their hydrographical areas. Hence we are led on to speculate 
regarding the position of the old lands which yielded the sediment of which the strata 
are formed, or which bounded their areas of deposition. In order to arrive at 
conclusions on these subjects, we have to note the directions in which these special 
formations expand in thickness, and those towards which they appear to thin away. 
jenerally speaking, and within certain limits, formations composed of sedimentary 
materials, such as gravel, sand, clay or mud, tend to increase in thickness in the 
direction of the Jand from which these materials were carried down and spread over 
the area of deposition. On the other hand, formations of this kind rapidly thin away 
in the direction of any tract of contemporaneous land, against the shelving bed or 
shore of which they were deposited, but which may not itself have contributed 
much, if any, sediment during their deposition. The old ridge of Lower Palzeozoic 
rocks which stretched across the centre of Kngland during the Carboniferous and 
Permian periods was of this kind.* Itself of too limited an extent to be a source of 
sediment, the newer formations simply tail out towards its margin both to the 
north and to the south. The sub-Cretaceous ridge of a subsequent period was of 
a similar kind.t While it formed a land-surface against which the Triassic and 
Jurassic formations successively wedged out, it was not itself a source of sedi- 
ment, and these formations terminate along its border, but little changed in their 
mineral characters on approaching its position. On the other hand, limestones 
formed over the bed of the sea by organic agency, become split up by bands of 
sandstone or shale in the direction of the lands of the period from which sandy or 
muddy sediment was being carried down by streams; but on approaching barriers 
of land, or isolated tracts of older rock of limited extent, they wedge out towards 
* Plate XXVIII, Fig. 2. t Plate XXXL, Figs. 1 & 2. 
