TRANSACTIONS OF SECTION C. 489 
original sponge) is readily understood ; where, on the other hand, a slight lateral 
flow of the water occurred, a tabular form, with its slightly laminated structure 
(determining often the mode of fracture), would naturally occur. Vertical walls 
of flint in fissures in the extensively abraded Upper Chalk seem to tell of decom- 
position of the alkaline silicates during the deposition of ‘the Lower London 
Tertiaries,’ in which some of the liberated SiO, has often taken up Fe and K to 
form glauconite. In such cases decomposing Algwe, &c., may have furnished the 
acids required ; or CO, furnished from the chalk itself might replace the Si0O,. 
In areas where the alkaline silicates were wanting we should have the skeleton 
of the siliceous sponge preserved without any flinty investiture;'* or even cal- 
careous fossil sponges, as in the well-known ‘ Faringdon Sponge Beds.’ Silici- 
fication of calcareous fossils, Foraminifera,’* Echinoderms, &c., can be under- 
stood as a ‘mass-reaction’ of the alkaline silicates in the presence of a large 
excess of water, the alkaline base going for the CO, of the calcareous fossil, while 
the CaO became hydrated, leaving the SiO, to take the place of the original 
CaCO,. 
Thus :— 
n H,O +- CaCO, + K,Si0, = K,CO, -+ Ca(OH), -+ SiO, + (» — 1)H,0 
(dissolved) (dissolved) (precipitated) 
(where x = an indefinitely large number) ; 
or (in some cases ?) 
CaCO,-+-K,Si0, = K,CO, + CaSiO,. 
(dissolved) (Wollastonite). 
The theory here propounded for the genesis of flint is the same as that sug- 
gested years ago to account for the formation of the ‘ woody opal’ of the silicified 
trees of the lower Nile Valley, the sequence of chemical changes being similar in 
both cases (see paper by Capt. H. G. Lyons, R.E., F.R.S., in ‘Q.J.G.8.,’ Vol. 1, 
November 1894, and the discussion thereon). 
6. How the Relation between the Horizontal and Vertical Movement 
of the Water in Tides and Waves causes them to drive Sand for- 
wards. By Vaucuan Cornisu, D.Sc., F.R.G.S., F.G.S., F.C.S. 
The author dealt with the conditions of the transport of detritus superficially 
and in suspension. He pointed out that the rate of subsidence is the constant 
which best defines the behaviour of a granular material with respect to trans- 
portation by currents. He showed how detritus may be classified in three 
groups according to the value of this constant, these groups being familiar as 
shingle, sand, and mud, in the case of water-borne material, and gravel, sand, 
and dust in the case of wind-borne detritus. 
It was pointed out that the change of direction of the vertical currents in sea- 
waves does not occur simultaneously with the change of direction of the hori- 
zontal currents, and it was shown that the result of the sequence of the changes 
is to endow waves with a shoreward action upon shingle and the coarser kinds of 
sand independently of any motion of translation in the water. 
In tides also rise does not commence simultaneously with flow, nor fall with 
ebb, and the author showed that the sequence of these changes is such as to 
make the flood tide more effective than the ebb as an agent of littoral drift, 
apart from any greater speed of current. 
Examples wére given of the different positions in which deposits of detritus 
accumulate according to the rate of subsidence of the particles. 
An explanation was given of the effect of a change in the inclination of 
current to the horizontal in sorting heterogeneous detritus, and examples were 
given for wind-borne material. 
15 See Rupert Jones’ edition of Dixon’s Geology of Sussea. 
16 Sollas, Age of the Harth, chap. vi., pp. 150 ff., fig. 43. 
