380 Reports & Proceedings —Miner alogical Society. 



of their growth and decline, of a method of determining their depth 

 below water, and of the probability of specialized life existing 

 beneath such formations. 



The President expressed to Sir Douglas Mawson the thanks of 

 the Fellows and visitors for his lecture. 



II. MlNERALOGICAL SoCIETY. 



Jmie 18, 1918.— W. Barlosr, F.ll.S., President, in the Chair. 



W. A. Eicbardson : "On the Origin of Septarian Nodules." 

 Septarian structure consists not of a simple combination of radial 

 and concentric circles, but of irregular polygons closely simulating 

 mud-cracking. By experiments with clay balls and films and 

 comparison with timber cracks it was shown that radial cracks 

 widening inwards are produced by internal circumferential contrac- 

 tion, radial cracks widening outwards by internal expansion, con- 

 centric cracks by contraction towards the centre, and polygonal 

 cracks by either free or chemical desiccation. Moreover, analysis 

 shows that septarian nodules are more aluminous towards the centre 

 than the outside, and are therefore capable of contraction. The 

 evidence disproved the expansion theories, and showed that con- 

 traction on numerous centres in a colloidal medium caused the 

 cracking, and desiccation by chemical agents the contraction. The 

 central portions are not merely enclosed clay, but clay that has 

 undergone considerable chemical modification, and the original 

 colloidal nature of the medium is so changed that closing of the 

 cracks by absorption when placed in water cannot take place. 

 Finally, the occurrence of the nodules suggests their origination by 

 rhythmic precipitation according to the laws of Liesegang from 

 solutions of bicarbonates diffusing through a colloidal medium. 



Dr. G. T. Prior: "The Composition of the Nickeliferous Iron of 

 the Meteorites of Powder Mill Creek, Lodran, and Holbrook." 

 A simple and expeditious method of determining the amount and 

 chemical composition of the nickeliferous iron of a meteorite was 

 described. The method depends upon the us^ of dimethyl glyoxime 

 for the separation of nickel. Its application to the meteorites 

 of Powder Mill Creek, Lodran, and Holbrook gave percentages 

 respectively of about 42, 30, and 6^ of nickeliferous iron, in which 

 the corresponding ratios of iron to nickel were about 13, lU, and 5. 



COI?,I?,E!SI='03SriI)E]ISrOE. 



MOUNTAIN BUILDING. 

 Sir, — The aim of my article in the Geological Magazine for 

 May was to point out that those data for the earth's thermal condition 

 and past history that agree best with evidence derived from totally 

 different sources lead directly to an amount of compression of the 

 earth's crust in cooling that is of the correct order of magnitude to 

 account for mountain building. Mr. Deeley in his reply makes no 

 attempt to answer this statement. What he does is to suggest that 



