REPORT'ON GEOLOGY. 
377 
the subjacent Purbeck limestone ; thus showing that the alterna¬ 
tion of oceanic and lacustrine deposits was certainly not confined 
to the tertiary epoch, but had equally occurred in the more an¬ 
cient periods*. Messrs. Murchison and Sedgwick have ob¬ 
served similar lacustrine deposits in still older rocks in the Isle 
of Sky. The elaborate Memoir of Prof. Buckland and Mr. De la 
Beebe on the Weymouth district is not only valuable as having 
imparted minute and accurate precision to our knowledge of the 
interesting geological phenomena exhibited in the western ex¬ 
tremity of the tract affected by the great convulsions which have 
elevated the chalky ranges of Purbeck and the Isle ofWight, but 
it informs us that this tract also, like the Weald, furnishes facts 
pregnant with remarkable consequences, as to the circumstances 
of the general service at the period when the strata of Port¬ 
land limestone were deposited ; for we find interposed between 
these and the superincumbent Purbeck limestone, a bed of 
black vegetable mould, full of the stems of Cycadeae and of large 
Coniferae, many of their roots being fixed upon and still adhering 
to the subjacent limestone, so as to evince that they must have 
originally grown in their present position : the surface of the 
Portland limestone must therefore at that time have been dry 
land, bearing a thick growth of tropical vegetables f. 
In the inferior oolite of Yorkshire, associated with the coaly 
* A similar conjecture had indeed been previously entertained concerning 
the fluviatile origin of still older rocks, including portions of the coal measures; 
but the evidence resting only on the occurrence of obscure shells, referred per¬ 
haps too hastily to the fluviatile genus Unio, must be regarded as very insuffi¬ 
cient, and appears opposed by the undoubtedly marine shells of the associated 
carboniferous limestone. It is however certainly by no means improbable that 
the coal strata have in part at least originated in the drift timber of vast 
eestuaries like that of the Mississippi; and in such localities this intermixture of 
fluviatile shells might naturally be expected. 
f It must surely be unnecessary to insist on the fundamental importance of 
a fact thus affording direct evidence of the repeated oscillations by which the 
relative level of the ocean and land has been affected; for the Portland beds, 
subsequently to their having been thus exposed as a dry continental surface, 
appear to have been again submerged, first by an aestuary in which the fluvia¬ 
tile deposits prevailed, although with a partial intermixture of marine fossils 
as is shown in the Purbeck beds; in the adjoining district of the Weald the 
fluviatile character of the beds is more unmixed; a second oceanic submer¬ 
sion must have produced the vast mass of the cretaceous superstrata. And 
lastly, the alternating oceanic and fluviatile deposits of the Isle ofWight seem 
to attest a recurrence of similar oscillations. Prevost indeed, in a Memoir on 
this question, opposes the idea of reiterated oceanic submersions, and endeavours 
to explain the phaenomena of the basin of Paris by the hypothesis of a basin 
originally oceanic, but converted by the gradual subsidence of the sea level suc¬ 
cessively into an aestuary and inland lake of brackish water, subject occasionally 
to accidental irruptions of the oceanic water on one hand and of the land'floods 
on the other; but it does not appear possible thus to explain the vast fluviatile 
