720 TRANSACTIONS OF SECTION K. 
glacial boulder clay, this development is of great importance, for if the streams 
had not cut their way down to the solid rocks below, and deposited alluvium, 
&c., in their course, the fora would have been much more uniform. 
In Leicestershire, for instance, the consequents or dip-streams flow from 
the north-west to the south-east, with the dip (originally entirely). They give 
rise to a series of parallel valleys, with intervening hills, developing lower lias 
each side of their course, on each side of patches of alluvium. The later strike 
stream or subsequent of the River Soar (in the first part of its course) was 
developed along a line of outcrop, and captured the streams to the east, so 
causing them to flow to the west, and cut back their bed towards the divide 
formed by the high escarpment of the Middle Lias and Oolites of East Leicester- 
shire. River development thus depends largely on the junction of soft and hard 
strata, and this again leads to diversity in the natural habitats for plants. 
Streams also have a radial arrangement subsidiary to this, thus complicating the 
outcrop conformation. Moreover, the little obsequents with laterals cut back like 
strike streams are like the multifid lobes and saddles of the suture of an Ammon- 
ite, and it would be extremely difficult to map the plants found on such irregular 
formations except by choosing the actual geological formations on which they 
occur as a basis. But as altitude also enters to some extent into the effect of 
river development (since some valleys are very deep), and there is the influence 
on the soil by the action of the river on the area of each soil, there is some- 
thing to be said for the retention, for floristic purposes, of the old hydro- 
graphical districts adopted in some floras, for they serve as (i) an indication 
of the effect of some one stream or river in the district in developing the soil 
and controlling plant dispersal, and (ii) as a ready means of referring to the 
area, by a process of subdivision, for the station of a plant, but it is admitted 
that this is an entirely artificial advantage. 
For ecological purposes these divisions are of no use, as a description of the 
district should be by the natural plant formations. The contour formed by the 
outcrop of each formation can readily be defined in the valleys cut through by 
streams, except where denudation has obscured them. 
The flora of the boulder clay is composite, and though certain differences 
can be noted between the different types it includes, it is, as a whole, common- 
place. That of such a formation as the Lower Lias can be distinguished, and 
even made use of, to determine the junction between the outcrops of the two. 
The slope, aspect, and relative moisture, &c., imposed on a valley by river 
development have also all an effect on plant distribution. 
5. Some Features of the Sand-dunes in the S.W. Corner of Anglesey. 
By W. H. WortHam. 
The sand-dunes in the S.W. corner of Anglesey form a spit of about five 
miles in area, bounded to N.W. and to S.E. by river estuary and strait respec- 
tively, and to the S.W. by open sea; the sand is aggregated about a high spit of 
schistose rock, often over 100 feet high, which forms a backbone running from 
S.W. to N.E.; parts of it are uncovered by sand. The fixed dune association, 
where the sand forms a thin covering over the summits of the schistose rocks, 
is a Caricetum arenarie, which forms a close sward. 
The region to the N.W. of this ridge, owing to the frequent rebound of wind- 
driven sand from the ridge, is occupied entirely by white dunes, of which the 
only colonist is marram grass, alternating with embryonic stages of Salix repens 
marsh. These dunes are increasing in extent. 
The dunes to the S.E. of the ridge are stationary in extent, and wind- 
destruction is marked. The marram of the white dunes here shows associates, 
of which Huphorbia paralias is the first. Salix dunes are here extensive, and 
have a twofold origin : (1) The inundation of a dune-marsh with sand, a process 
which results in the extinction of all but the Salix repens, which grows up 
through the sand, forming low dunes; (2) the invasion of a stretch of white, or 
even grey, dune by salix seedlings, often from a distance, which grow up, collect 
blown sand round them, and form dunes in exactly the same way as marram. 
Wind-action is destroying these salix dunes in many places, and blow-outs, 
