ON A SECTION AT RAYLEIGH.
115
of shelly detritus on the lee-side of Septaria, a fact which points
to the same conclusion.
2. The Passage Beds are clearly marked off from the London
Clay below by a colour change. There is, at first, but little
change in the character of the sediment itself, but the lower part
is rendered brown by a multitude of included ferruginous films.
This may be due in part to a definite change in the conditions
of deposition, but is largely the work of subsoil waters penetrating
the beds along the sandy seams. There is no reason to doubt
that the same solutions have removed the fossils originally present
in the beds ; delicate stellate groups of selenite crystals are
frequently found, and represent the redistributed calcareous
matter.
Taken in the mass, the beds show a typical rhythmic banding,
closely resembling, if not identical with, that of the Pleistocene
"varve" sediments. If it is to be so interpreted, it. points to
a climate with marked seasonal differentiation. The beds agree
in all particulars with the Claygate Beds of Surrey, but it is
preferable to use the general term Passage Beds to designate
them, since the assumption of exact equivalence in age is thus
avoided. The beds represent a facies which ascends in the time
scale from west to east, and which includes two distinct litho-
logical types—the thin-bedded or Claygate type and the massive
or Brentwood type. The former appears to occupy the southern
margin of the London Basin, while the latter occurs in the more
central tracts. It would appear that two similar and similarly
related types can be distinguished in the Basement Bed of the
London Clay.
3. The Bagshot Sands are not clearly delimited from the
Passage Beds. There is a progressive diminution of argillaceous
material as the sequence is ascended, and it becomes necessary
to select some arbitrary line of division. In practice it is not
difficult to adopt a line that can be mapped consistently, for by
the time the sand bands have attained a thickness of 1ft., the clay
seams are much reduced in number and thickness, and the mass
characters of the beds are definitely those of the Bagshot Sands.
The lithology of the sands has been described elsewhere,2 and
it will suffice here to call attention to their exceedingly fine
grain and the fact that they are definitely marine in origin.
2 Proc. Geol. Assoc., vol. xxxv (1924). p. 359.