704 REPORT—1899. 
levels up to 600 feet, and great interest is attached to the high level or plateau 
group as affording traces of man’s presence before the formation of the existing 
river systems. 
The implements of the plateau and hill drift are extremely rude, and bear 
evidence of rough usage and transportation ; while they differ in type from those 
of later Paleeolithic times, 
A large collection recently formed by Mr. R. Jones, of East Wickham, con- 
tains many interesting examples, which were exhibited by the author. Amongst 
the localities from which these have been derived are Swanscombe, Milton Street, 
Ash, Darent, Crayford, &c., and also a remarkable series found at Rainham, a 
new locality, here at only about 20 feet above high-water mark. A large 
number of Paleolithic implements were found on the surface, but they appear to 
have been derived from an old gravel; they are for the most part deeply stained 
white or yellow, and are highly patinated, whilst also showing signs of con- 
siderable wear on their worked edges. The question is, Have they been brought 
down to this low level from the high-level drifts of the Medway Valley ? 
5, Report on Photographs of Geological Interest. See Reports, p. 377. 
6. Report on Irish Elk Remains in the Isle of Man, See Reports, p. 376. 
7. Report on the Flora and Fauna of the Interglacial Beds in Canada. _ 
See Reports, p. 411. 
WEDNESDAY, SEPTEMBER 20. 
The following Papers and Reports were read :— 
1. Sigmoidal Curves.—By Maria M. Gorpon, D.Se. 
The phenomena of crust-torsion are produced when the wave-forms existing in 
an already folded region are altered as a result of the superposition of a new series 
of folds. Let a unit-area of rectangular folds be taken as a type, where two anti- 
clines and an intervening trough in east and west direction have been crossed by 
two anticlines and an intervening trough in north and south direction. Then the 
cross-arches are four in number, and are areas of uprise limiting obliquely an inner 
cross-trough, which represents a common reciprocal area of depression. The new 
anticlines east and west of the trough are areas of uprise, while north and south of 
the trough the old anticlines are broken by local areas of depression. 
The redistribution of the wave-forms in the area determines several distinct 
centres towards which crust-creep sets in, and the conflicting nature of the com- 
bined horizontal and vertical pressures in relation to the separate centres produces 
torsional phenomena. The inner trough is an area of involution, into which the 
higher horizons of rock sink and are carried obliquely downward, while each of 
the four cross-arches are areas of evolution where the compensatory opposite move- 
ments of torsion carry the lower horizons of rock or molten material from below 
the crust obliquely upward. ‘Streaming’ of the rock-particles is associated with 
this circulatory system of crust-movement. 
‘S’-folds gradually take shape around the cross-arches, and as these are 
specially liable to be fractured in their most warped, ‘middle limb’ portions, 
limiting-faults tend to form in oblique directions and to become continuous with 
north and southfaults between the simple lateral arches and the reciprocal troughs. 
The outcrop of the fault-zone in the unit-area, therefore, describes characteristic 
