37 6 SCKENCE PROGRESS 



here. Its aim, briefly, is to apply metallurgical principles to 

 the problem of the separation of ores from igneous magmas, 

 and to petrogenic problems in general. 



In a study of the great Insizwa intrusion of Natal (Inst. Min. 

 & Metall. 1916, Bull. No. 147, pp. 1-47) Goodchild shows that, 

 while Insizwa has been regarded as the " Sudbury of South 

 Africa," there is really only a distant geological similarity 

 between the two eruptives. They are more or less alike in size, 

 thickness, mode of differentiation, and ore minerals ; but the 

 Insizwa intrusion is, on the whole, much more basic than that 

 of Sudbury, its differentiation is due to the sinking of olivine 

 rather than pyroxenes, and its ore minerals show a different 

 sequence in time. The Sudbury basin is believed to have been 

 formed by subsidence subsequent to emplacement ; the Insizwa 

 mass, however, is believed to have been intruded into soft clay 

 which yielded under its weight, causing the most decided 

 subsidence towards the centre of the intrusion where the 

 thickness was greatest. The picrite differentiate which carries 

 the ore minerals is thought to be due to gravitative concentra- 

 tion of olivine, which naturally took place with the greatest 

 facility in the thickest part, of the mass. The similar concen- 

 tration of the ore minerals is regarded as due partly to gravita- 

 tive descent, comparable to the settlement of matte in smelting, 

 and partly to mechanical entanglement of the sulphides with 

 sinking olivines. 



Goodchild applies metallurgical conceptions to the problem 

 of the Sudbury nickel eruptive (Econ. Geol. 191 8, 13, I37 _ 43)« 

 He regards the separation of the sulphide ore as a process 

 similar to " matte smelting " under hydrous conditions. A 

 reaction between the sulphur of the dissolved sulphuretted 

 hydrogen, and the iron oxides of the basic magmatic silicates, 

 results in the formation of metallic sulphides. A volume 

 increase of sulphides relative to silicates takes place, giving 

 rise to silicate replacement, and producing brecciation and 

 expulsion of the matte into the adjacent country rock. Water 

 formed as a by-product of the above reaction gives rise to the 

 local appearance of hydrothermal alteration which has led some 

 investigators to the view that the ores were introduced by 

 circulating waters after the consolidation of the rock. Good- 

 child thus supports the view that the sulphide ores are of direct 

 magmatic origin. In a quantitative petrological study of the 



