File:The Quarterly journal of the Geological Society of London (13206034394).jpg

ROBERTSON ON THE WEALDEN BEDS OF BRORA. 121
encroachments of the salt-water lakes during storms, on the other*
To the effects of the latter agent may be attributed the rolled peb-
bles of Portland stone found in the " dirt beds*."
It may seem necessary in this place to consider the question,
what has become of the beds of marine organic remains formed in
these inland seas ? To this I regret my inability to offer ai.y satis-
factory reply, but I may suggest, as a possibility, that the strata ac-
cumulated for some time after the separation of the seas from the
ocean were identical with the Portland stone, while, subsequently,
the rivers in draining so extensive a surface may have supplied
enough of fresh water to render the seas brackish, and so permit the
association of marine and fluviatile species (as in certain lagoons on
the Mediterranean delta of the Rhonef); in which case all, except
the lowest deposits of the basins, would be classed as Wealden. '^*
It is needless to enter upon any further discussion regarding
the circumstances which attended the accumulation of these de-
posits. They present, I think, no phsenomena incompatible with
the hypothesis, that they resulted from the deposition of sediment
conveyed by rivers liable to be copiously flooded and opening into
inland seas, the latter being occasionally subject to storms and cur-
rents produced by the action of winds or earthquakes. The strata
accumulated in these detached basins of the continent and those
formed along its coasts would go on increasing independently of,
but contemporaneously with each other, and could not come in con-
tact until the barriers which separated the inland seas from the
ocean had sunk below the level of the latter. When this happened,
the oceanic beds would extend themselves over those which had
been formed in the inland seas, and would rest conformably upon
them in the same way as the lower greensand does on the subjacent
Wealden deposits.
ffi fMiBiaoo ii giadjo^iioio^ fcuiiX .jCiSiiiiii ^iHrniOt
In this diagram the lower bed represents Jurassic rockisV' the l)€?a' §Hadea"'B^"
straight dark lines, Wealden, and the dotted bed at the top the Lower Greensand.
The preceding theory will be rendered more intelligible by an inspec-
tion of the ideal section, fig. 2, in which the lower bed represents the
continent, composed of Jurassic rocks, its uppermost member being of

• The " top cap " of the Isle of Portland, on the surface of which Prof. Henslow
  

observed root-shaped cavities, produced by the trees of the " Black dirt," belongs,
together with several feet of the subjacent strata, to the Purbeck series, and not
to the Portland stone. (Dr. Fitton, Trans. Geol. Soc. 2nd series, vol. iv. p. 219.)
The inference which Mr.Lyell (Elem. of Geology, 2nd ed. vol. i. p. 425) draws from
the observation alluded to, viz. that the Portland stone was in a " soft and pene-
trable condition" at this time, is, therefore, not supported bv it.