SECTIONAL TRANSACTIONS.— C. 325 



Sunday, July 28. 



Under the guidance of Dr. A. L. du Toit, members were conveyed 

 in private motor-cars to Barkly West to examine the high-level 

 diamondiferous gravels, which yielded large numbers of palseolithic 

 implements. 



Thence the party proceeded to Sydney on the Vaal, where the alluvial 

 workings proved of great interest. In the afternoon a visit was paid to 

 Nooitgedacht Farm where the famous glaciated pavements with coverings 

 of Dwyka Tillite (Permo-Carboniferous age) were inspected. The occupiers, 

 Mr. and Mrs. Dold, kindly provided refreshments. 



Monday, July 29. 



The Section visited the plant of the de Beers group of diamond mines, 

 where the full operations for the recovery of precious stones were examined. 

 Special facilities were afforded to the geologists for the acquisition of 

 specimens from a very complete collection of rocks in the diamond pipes. 



JOHANNESBURG. 

 Tuesday, July 30. 



Presidential Address by Sir Albert E. Kitson, C.M.G., C.B.E., on 



The Utility of Geological Surveys to Colonies and Protectorates of the 

 British Empire (see page 64). 



Mr. E. D. Mountain. — The Relations between Ray-Surfaces and Optic- 

 Pictures. 



A method is described by which the sign of minerals may be determined in sections 

 parallel to the optic axis or the optic axial plane. The method is appUcable even to 

 minerals possessing an optic axial angle approaching 90°. The determination is based 

 on the comparison of the birefringence in certain directions in such sections. 



Prof. K. B. Young. — The Geology on the Neighbourhood of Johannesburg. 



Mr. F. P. Mennell. — Suggestions on the Origin of Diamond Pipes. 



The general features of pipes and their characteristic inclusions were discussed ; 

 also the field relations of the granulite-eclogite group, to which many of the xenoUths 

 belong, which appear to indicate that they are the result of the alteration of older 

 rocks, including sediments, invaded by granite, rather than evidence of the character 

 of the layer below the continental crust. The nature of the pipe-fillings would seem 

 to show that they had their origin at comparatively limited depths, where explosive 

 phenomena had full play and where, moreover, materials existed which would account 

 for the explosions. Water is doubtless the usual agency in causing paroxysmal 

 eruptions, l)ut in this case carbonic acid seems demanded by the facts. The source 

 of this may well be sought in the crystalline limestones so frequently found among 

 the gneisses and schists of the region through which the 'pipes are distributed. The 

 circumstances may be imagined as follows : During the earth movements at the 

 close of the Cretaceous period, of which evidence is seen in the strata of Portuguese 

 East Africa, the ancient rocks were no doubt subjected to the same gentle movement 

 of elevation as the coastal sediments. This permitted the upward migration of 

 probably basic magma at various points. These intrusions seldom reached the 

 surface ; in the exceptional cases when they did so it was in consequence of their 

 penetrating and reacting with the ancient limestones, which led to their assuming an 



