PRESIDENTIAL ADDRESS. 1 1 



(5) The prevalent strike of the long axes of masses of 



eruptive rocks such as granite, quartz-porphyry, 

 diabase, dolerite, etc. 



(6) Prevalent strike of quartz reefs. 



(7) Trend of joints in rocks. 



(8) Trend of rivers and lakes. 



The banded jaspers and hornstones are such a conspicu- 

 ous feature in the geology of West Australia, and so wonder- 

 fully persistent for distances of hundreds of miles, that 

 they deserve special mention here. 



At Northampton in West Australia, there is a great 

 development of what Mr. Maitland has termed sheeted 

 zones of micaceous and garnetiferous granulite, traversed 

 by much puckered and contorted veins of quartz. "These 

 sheeted zones trend generally north-west and south-east. 

 These are not planes of bedding, but they represent gliding 

 planes, along which the rocks have yielded to the irresis- 

 table lateral pressure, resulting, inter alia, from the con- 

 traction of the earth's crust. The result of this lateral 

 earth creep is that many of the rocks have been milled 

 down, as it were, and in some cases rocks having all the 

 external characters of finely banded slates or schist have 

 resulted. An excellent instance of this occurs in the valley 

 of the Helena River where the normal granite as a result 

 of the operations of the great earth mill has been ground 

 to powder or rock flour, producing a rock termed mylonite." 



That the mylonites represented by the banded red jaspers 

 and hornstones have been subject to earth movements since 

 their formation is proved by the fact mentioned by Mr. 

 Maitland, that at Boogardie and on the Murchison field, as 

 well as at Tuckanadra, 26 miles N.E. of Cue, they have 

 been thrown into a series of gentle curves. They are 

 crossed by numerous faults almost at right angles, and 

 pockets of gold ore occur at the intersection. 



