SECTIONAL TRANSACTIONS.—C. 311 
AFTERNOON. 
Excursion to the Ordnance Survey Office. Leader, Sir John S. Flett, 
F.R.S. 
Friday, August 28. 
Morning. 
5. Dr. Epwarp GREENLY.—Orders of Validity in Geological Theory. 
The early days of geology were days of far-reaching theories. Progress began with 
patient accumulation of evidence, and has continued to the present hour. While facts 
without interpretation are lifeless, it may be salutary to recognise, among present- 
day theories, classes which are of different orders of validity. At one extreme are 
theories of cosmic evolution and kindred subjects, matters in regard to which 
information is but scanty, and theory far-reaching. At the other extreme is what may 
be termed the great body of the science ; matters on which the proportion of evidence 
to theory is very high. Between these extremes we may recognise an intermediate class, 
wherein may be placed certain great questions on which we have a considerable, 
though far from adequate, body of evidence. Tacit recognition of such differing 
orders of validity is, of course, no new thing. But explicit recognition and an attempt 
at classification may be of service to geological research. 
6. Dr. Epwarp GrREENLY.—The term ‘ Gottlandian.’ 
‘Gottlandian’ has been proposed as a substitute for ‘Silurian.’ The paper 
discusses the results which would ensue if the principle upon which this proposal is 
based were applied, with consistency, to the nomenclature of the systems. 
7. Mr. 8S. W. Wootpriper.—The Pliocene Question in the London Basin. 
This paper reviews the extensive literature dealing with the post-Eocene-pre- 
Glacial history of the London Basin, and presents certain new suggestions relative to 
the problems still at issue. The central problem is the age and origin of the deposits 
shown as ‘ Pebble Gravel’ on the maps of H.M. Geological Survey. These deposits 
are of great significance, and by no means to be dismissed as ‘ drift.’ Concerning 
them, two views have been propounded: (1) that they are of marine origin and Prestian 
age ; (2) that they are of fluviatile origin and of late Pliocene or early Pleistocene age. 
The acceptance of either view carries with it a definite chronology as regards late 
Tertiary earth-movements and the evolution of the river system. 
An attempt is made to decide between the two alternatives, making use of the 
evidence afforded by the petrography of the beds and by the general geo-morphology 
of the area. The general conclusion reached is that two distinct accumulations have 
been included under the head of Pebble Gravel, one possibly equivalent to the 
Lenham Beds, while the other, a more widely spread type, is definitely later. 
8. Dr. Atsert Hearp.—Early Devonian Plants from the Brecon District. 
This paper records the occurrence of several genera of an early Devonian Flora 
from the Brecon District. The fossils were first observed by Mr. F. N. Gossling, B.A., 
of the Post Office Survey Department. The plants occur in arenaceous green shales 
in the Senni Beds of the Old Red Sandstone, and appear to represent drifted material. 
The most abundant remains consist of dichotomously branched leafless stems, 
which in the apical regions are characterised by circinately coiled tips. These are 
referable to the genus Psilophyton. The plants have undergone varying degrees of 
pytitisation.. The pyrites can be removed by chemical treatment, and the perfectly 
preserved internal morphology of the plant is thereby revealed. 
The plant consists of :— 
(a) A relatively large protostele in which a thin phloem zone surrounds a ring of 
phloem parenchyma. The central part of the stele is occupied by a large xylem area, 
which consists of spiral tracheids surrounded by a ring of smaller spiral elements. 
(6) A Cortex differentiated into two marked regions, an inner and an outer cortex. 
* The inner cortex is imperfectly preserved in most specimens. The outer cortex is 
characterised by a great thickening of the parenchyma. 
