SECTIONAL TRANSACTIONS.— C. 323 



II. The Stagnant-glacier Form. 



This type of tectonic structure is associated •svith the retreat of an ice-sheet. The 

 disturbed beds have a uniform direction of dip and the structure is progressively 

 developed backwards, i.e. in the opposite direction to that of movement of the ice. 

 As in the roche-moutonnee type I, arrestation of movement leads to the formation of 

 the sigmoid-curve which marks a zone of direct and concentrated pressure, relief from 

 which is obtained as follows. 



Movement against the plane of the sigmoid-curve results in upward flow, the 

 curve being similar to the bent fingers of the hand. This flow-curve may be subse- 

 quently squeezed into an anticline having a pointed apex. Its form, therefore, is 

 essentially different from an ordinary anticline. Moreover, it has no complementary 

 syncline. Increased pressure from behind ultimately leads to relief being obtained 

 by the formation of a thrust-plane, a process which may be repeated, at intervals, 

 many times. 



Imbricate or Schuppen-struktur. 



The repetition of the same bed or series of beds associated with thrust-planes, 

 leads to the development of imbricate, or schuppen-struktur. This zone, in turn, is 

 associated with a series of domes and basins, the formation of which marks a decrease of 

 pressure. The final stages are shown by the deposition of boulder-clay containing 

 far-travelled erratics. 



[See Reports Brit. Assoc. 1923-28 for abstracts of papers by the author on Glacial 

 Tectonics.] 



Prof. P. G. H. BoswELL. — The Action of Colloids in precipitatitig Fine- 

 grained Sediments. 



Mr. W. N. Edwards. — Triasso-Rhaetic Floras of the Southern Hemisphere. 



The close resemblance of the Thinnfeldia floras in different parts of Gondwanaland 

 has recently been emphasised by Dr. A. L. Du Toit, and further resemblances are now 

 brought forward, based on material in the British Museum (Natural History), e.g. the 

 occurrence of the South African genus Moltenia in New Zealand. The southern 

 Thinnfeldia floras are compared and analysed, and contrasted with Triasso-Rhaetic 

 floras of the northern hemisphere. 



Afternoon. 

 Excursion to Sea Point. 



Wednesday, July 24. 



Dr. A. V. Krige. — The Geomorphology of South Africa. 



Mr. E. McKenzie Taylor. — Base Exchange between Clay and Solutions of 

 Sodium Salts and its Relation to the Formation of Coal and Petroleum. 



Bituminous coal and petroleum occur in unaltered sedimentary strata with similar 

 biological, chemical and physical properties. The material forming the roofs of coal 

 seams and the cap-rocks of oil-sands has been deposited under such conditions that it 

 came in contact with solutions of sodium chloride forming a sodium clay. The sodium 

 clay in contact with water breaks down, a solution of sodium hydroxide being formed. 

 This solution of sodium hydroxide is a suitable medium for the decomposition of organic 

 matter by bacteria. A further property of the sodium clay is that it is impermeable 

 to water and gases. This ensures that the decomposition of organic matter situated 

 beneath it is carried out in the absence of oxygen and results in the formation of 

 residues containing little or no oxygen. It is suggested that coal has been formed by 

 the bacterial decomposition of land plants under a sodium clay roof, and that petroleum 

 has been formed by a similar decomposition of marine plants and animals. As coal 

 and petroleum occur in strata of various geological ages and as the deposition of silt, 

 under such conditions that it came in contact with solutions of salt, has been taking 

 place from the earliest times, the suggested mode of formation of coal and petroleum 

 accounts for the occurrence of these materials in rock-systems of various geological 

 ages. 



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