472 TRANSACTIONS OF SECTION C. 
certain sections along the eastern seaboard between Tolsta Head and Loch 
Bhrollum. 
In the areas examined, the north-west and south-east strike, referred to by 
Murchison, is not characteristic of the gneisses of Lewis. It is prevalent 
immediately to the west of Stornoway, but exceptional in other tracts. In this 
connection the observations of Professor James Geikie in the Eye peninsula 
were confirmed. The dominant strike over extensive areas runs a few degrees 
east of north and west of south; in certain localities it is north-east and south- 
west, and in others nearly east and west. * 
The prevalent types of gneiss closely resemble those to be found on the main- 
land between Loch Laxford and Cape Wrath, without the great series of acid 
intrusions. The structure is coarsely granular, or granulitic, the mineral grains 
being rounded and not elongated. The range of rock types seems to be com- 
paratively limited, for there is a marked absence, in the areas examined, of the 
pyroxene-gneisses with blue quartz, of pyroxene-granulites, and other basic 
and ultrabasic materials, which are so characteristic of the Fundamental Complex 
between Lochinver and Scourie on the mainland. The remarkable series of 
basic and ultrabasic dyke intrusions in the west of Sutherland has not been 
detected in Lewis. 
The flaggy granulitic gneisses of the Butt of Lewis which appear to run 
southwards along the belt of high ground between Stornoway and Barvas are 
of special interest. In structure they closely resemble the Moine gneisses east 
of the Moine thrust-plane, but they differ petrologically from the rocks of 
sedimentary origin that form the Moine series of the Geological Survey. 
The system of over-folding and the direction of the axial planes of the folds 
approximate to those found in the Moine rocks on the mainland. 
The platy rocks or mylonites, noted by Macculloch, occur along definite lines 
of movement, trending a few degrees east of north and west of south. Actual 
thrust-planes have been detected, which are inclined to the south of east 
at low angles, as if the displacements had been in a westerly direction. Various 
stages in the development of mylonites from the acid and basic gneisses are 
represented. 
5. The Settlement and Transport of Sand in Water. 
By J. 8. Owens, M.D., A.M.Inst.C.E., F.G.S. 
This paper described experiments on (a) the effect of the water temperature 
on the rate of settlement of sand, and (b) the rate of transport of sand along 
the bottom under the influence of a current, and its relation to the velocity of 
the current. ; 
It was shown that there is a definite relation between the rate of settlement 
and the temperature of the water, and tables and curves were given showing 
what this relation is. ‘The experiments described were made with siliceous sand 
from the sea-shore in most cases, and dealt with grades from a diameter of three 
up to fifty-five thousandths of an inch; that is, from an almost inpalpable powder 
to coarse sand. The curves show that velocity of fall varies almost directly 
with the water temperature, the rate being always increased by rise of tem- 
perature, but that as the diameter of grain increases the temperature effect 
becomes less, until for grains over one-tenth inch diameter the effect is prac- 
tically negligible. The bearing of the investigation on silting and suspension of 
matter in water was dealt with. Warm water would have less suspending power 
than cold, and therefore less erosive power. Silt-laden rivers must deposit their 
load more rapidly in warmer than in colder seas; this must affect the growth of 
deltas. The sea has probably less erosive power on the coast in summer than in 
winter, other conditions being the same. 
The experiments on sand transport along the bottom, which were described, 
refer only to that stage in which sand moves in ripples; that is, under the 
influence of currents of from about 1 to 24 feet per second. At the former 
ordinary sea-sand begins to move in ripples, and at the latter the ripple move- 
ment ceases and a new method of transport in a sheet, or long flat waves, begins. 
Tables and curves showing the relation between the velocity of ripple move- 
ment and that of the current were given, and this relation was shown to be 
