284 Correspondence — ^-1. R. Hunt. 



(9) Albite-porphyrites (with hornblende and biotite) and al bite-porphyries. 

 Secondary minerals are common in the rocks of this group. 

 (10) Jaspers and graphitic schists. These traverse all the other rocks, 

 occurring as lodes or bands, and may closely resemble sedimentary 

 schists. They sometimes are found running on each side of dykes 

 of albite-porphyry. Their mode of origin is not quite certain, but 

 they are intimately connected with the igneous rocks. 



The relation and the sequence of the rocks of Kalgoorlie are next 

 discussed. 



The greenstones, fine amphibolites, and calc-schists are regarded 

 as the old 'country rocks', into which the others are intrusive. 

 They are probably a complex of basic lavas, ashes, etc., greatly 

 altered. 



The quartz-dolerites, horublende-dolerites, and pyroxenites are 

 very closely related one to the other, and show every grade of 

 transition. Probably the peridotite group is merely the early basic 

 facies of the quartz-dolerite series, and the porphyries and porphy- 

 rites, which were the last rocks intruded, are regarded as being 

 derived from the same magma. 



The metasomatic changes and origin of the ores are then considered. 

 The great characteristic of this gold-field is the prevalence of albitiza- 

 tion in the auriferous districts. From this, and from a general 

 consideration of the rock-facies developed from the magma, it seems 

 probable that we have in Kalgoorlie an instance of the production of 

 auriferous lodes by rocks belonging to the same class as the pillow- 

 lavas and their diabases and soda-granite-porphyries (the spilitic suite 

 of igneous rocks). 



The paper contains a large number of chemical analyses, principally 

 carried out by the chemists of the Geological Survey of Western 

 Australia. By the kindness of the Director of that Survey the 

 author has also been able to make use of the specimens in the Survey 

 cabinets, in addition to those collected during his own examination of 

 the gold-field. . 



coi?.iiE!s:poisr3DEisrcE;. 



GEANITE AND CEITICAL TEMPEKATUEES. 



Sir, — I have to thank Dr. Johnston-Lavis for two reprints, and 

 especially for a marked list of 161 of his papers. 



As my critical-temperatures inquiry has been confined to low- 

 temperature plutonic rocks, down to their associated quartz-veins in 

 adjacent sedimentary rocks, I cannot discover how I have in any way 

 invaded Dr. Johnston-Lavis' field of work, either for agreement or 

 dissent. There is no issue between us. His temperatures are far 

 above the critical temperature of water, mine are almost entirely 

 below it. 



"With respect to the possibility of granite cavities and fissures at 

 great depths, which Dr. Johnston-Lavis denies, Sir T. H. Holland, 

 in the Magazine for last month, cites Professor F. D. Adams as 

 having proved that empty cavities can exist in granite under pressures 

 equivalent to a depth of at least 11 miles, and at still greater depths 

 if filled with liquids (Geol. Mag., 1913, p. 170). 



