B. Snuth—Ball or Pillow-form Rocks. 155 
the Mesozoic and Tertiary rocks, especially the latter. I am not 
aware, however, that much attention has been paid to them, although 
I am convinced that a strict examination of ripple-marks, current- 
and drift-bedding, internal creeping-movements, and removal of 
sediment by current-action would yield important results if studied 
in conjunction with present-day sedimentation in, and beyond, the 
mouth of one of our great tidal estuaries. 
We want to know, for example, the extreme depth at which ripple- 
marks can be formed, and to what extent depth is a deciding factor. 
We require reliable data as to the rate at which sandy sediment may 
be accumulated and again removed by current-action; the relative 
effects of a flood- and ebb-tide on sedimentation; and if a layer of 
drift- or current-bedded sand is laid down by the one, how far it may 
be wiped out by the other, or covered up by further sediment. 
When we find the lamin of false-bedding or ripple-drift (in a rock 
exposure) dipping in a certain direction, are we to assume that the 
sediment was carried directly to that spot from the land? These 
and similar questions may be difficult to solve, but would give scope 
to an investigator provided with a suitable boat, and ample leisure for 
the task. 
Tae Misstsstppr Detra. 
Since writing the above it has been my good fortune to light upon 
an instructive paper dealing with the conditions obtaining in the 
region of the Mississippi Delta.' Investigations were undertaken 
with the object of discovering the nature and mode of formation of 
the mud-lumps that obstruct the navigation at the river mouths. 
The results achieved, although they have not fully settled the origin 
of the mud-lumps, are enlightening. 
It has been shown, for example, that the deposits near the mouth 
of the river consist of lenticular layers of dark-blue clay, fine sand, 
and silt, and a great many beds of intermediate character, each of 
which grades into the adjacent beds. The sandy beds, and those of 
mixed sand and clay, are much more rigid than nearly pure clay. 
The most rigid material is a mixture of sand and clay in certain 
definite proportions, whilst some of the clays are very fluid. The 
resemblance between these deposits and some of those of Ordovician and 
Silurian age, as they would have been at the time of their formation, 
is striking; although this, of course, does not imply that the Lower 
Palzeozoic deposits were of deltaic origin. 
Just off-shore, and adjacent to the mouths of the river, silt is 
accumulating at the rate of several inches a year, and the character 
of the deposit varies from season to season. uring high-water—the 
first half of the year—the sediment is coarser than during the low- 
water period. The apparent result is astructure somewhat resembling 
the annual rings of growth of trees. This is a suggestive fact, if we 
bear in mind the sequence of sedimentation of some of the older rocks, 
in e.g. the Wenlock—Ludlow Series (see p. 153). 
Again, certain features of the Delta suggest that it is affected by 
a process of bodily flowage towards the sea, giving rise to readjustment 
1 The mud-lumps at the mouths of the Mississippi, by E. W. Shaw, U.S.G.S., 
Professional Paper 85-8, 1913. 
