Geological Society of London. 81 
freshes, into the sea, rather than to repeated changes in the relative 
level of land and sea. 
It is certainly the safer course to incline to this hypothesis when- 
ever there are no unequivocal signs, as in the Purbeck strata in Port- 
land, of land plants having become fossil on the very spots where 
they grew. For although there may be many river deltas like that 
of the Indus, where the land is subject to be alternately upheaved 
above, and then let down below the waters of the sea, yet such os- 
cillations of level must be considered as exceptions to the general 
condition of the earth’s surface near the mouths of rivers at any 
given period. Even in a case like the delta of the Indus, both the 
causes above alluded to may be expected to co-operate in producing 
alternate fluviatile and marine strata; for in the long intervals be- 
tween great movements of the land, the river will annually advance 
upon the sea with its turbid waters, and then retreat again as the pe- 
riodical flood subsides, and the salt waters, after being driven back 
for a time, will re-occupy the area from which they have suffered a 
temporary expulsion. 
In the conclusion of his valuable paper, Mr. Prestwich observes 
that the carboniferous strata of Coalbrook Dale must onee have 
been entirely concealed under a covering of new red sandstone, and 
they owe their present exposure partly to those movements which 
have shattered and elevated the coal measures, and partly to exten- 
sive denudation. It is natural therefore to inquire how many other 
coal-fields may still lie buried beneath the new red sandstone of the 
adjoining district. 
In relation to this point of ereat practical importance, Mr. Mur- 
chison formerly offered some conjectures, when speaking of the 
probable passage of the ten-yard coal of the Dudley field beneath 
the new red sandstone, which there flanks it on the east and west. 
That geologist now informs us that his conjectures have been verified, 
and that at Christchurch, one mile beyond the superficial boundary 
of the coal-field, the ten-yard and other seams have been reached 
by borings carried down to the depth of nearly three hundred yards. 
Adverting to this discovery, he directs attention to the possible ex- 
tension of other carboniferous tracts beneath the surrounding new 
red sandstone of Shropshire, Worcestershire, Staffordshire, and other 
central counties. 
Vou. XXXIII.—No. 1. 11 itt 
