654 DR. J. A. THOMSON' ON THE [Dec. I913, 



alteration by later solutions took place, leading to more exten- 

 sive formation of sericite and carbonates at the expense of the 

 albite and chlorite. The term ' bleached greenstone ' has been 

 applied, not only because it avoids the necessity of some such 

 phrase as ' metasomatized quartz-dolerite -without chlorite ' ; but 

 also because I believe that the rocks thus named have actually 

 passed through the greenstone stage, and have been dechloritized. 

 The amount of free quartz is much greater than is to be expected 

 in a fresh quartz-dolerite ; but it may probably be explained by 

 the liberation of secondary quartz during the alterations sketched 

 out above, without any extensive addition of silica to the rocks. 



Lode-Matter in the Quartz-Dolerites. 



The term ' lode ' or ' lode-formation ' is used in Western Aus- 

 tralia for a body of ore which is not clearly a quartz-vein or reef, and 

 has the general appearance of a rock rather than of a fissure-filling. 

 It is generally agreed that the ' lode-formations ' are zones of rock 

 that have been strongly fissured or sheared, and impregnated or 

 replaced in part by valuable minerals. 1 The only criterion of 

 whether a given piece of rock is ore or country is the economic one 

 of whether it will be profitable to mine or not. Such lodes are 

 found in several kinds of rocks in Kalgoorlie ; but those in the 

 quartz-dolerite are the richest, and alone carry persistent shoots 

 of ore. 



Most of the bleached greenstones bear some percentage of 

 pyrite, and are never without at least a low tenour in gold ; whereas 

 the greenstones are predominantly barren. The greater part of 

 the actual ore mined consists of bleached greenstone-schists or 

 much-silicified replacements of them ; nevertheless, some of the 

 high-grade telluride-ores are almost perfectly-massive green- 

 stones or bleached greenstones such as have been described above. 

 Detailed descriptions of the various kinds of ores would occupy 

 too much space in a paper that is already of great length. 



(_;') The Porphy rites. 

 The rocks here classed together present considerable variety 

 when compared one with the other, and range from grey rocks 

 of dioritic facies to light-coloured, eminently porphyritic types, 

 hardly distinguishable from the albite-porphyries. The porphyries 

 and porphyrites differ from all the other rocks of the field in 

 containing phenocrysts of felspar, and are thus easily recognized. 

 The distinction drawn between them rests chiefly on the presence 

 of hornblende and biotite in the porphyrites, and on the absence of 

 these minerals in the porphyries. As a rule, too, the porphyrites 

 consist more preponderatingly of phenocrysts, with a consequent 

 diminution in the amount of the groundmass, and thus approach 

 more nearly to plutonic rocks. 



1 See H. P. Woodward, ' The so-called " Lode-Formations " of Hanuan's, 

 & Telluride Deposits' Mining Journ. vol. lxvii (1897) pp. 1369-70. 



