128 president's address — section c. 



In the southern portion of the State the DarHng Range, the 

 Northampton district, &c., the fundamental complex is pierced by 

 a much later series of basic dykes, where they sometimes preserve 

 their dyke-Hke features across country without a break for miles. In 

 the north-west district, however, they occur on a scale of magnificence 

 as- yet unknown in any other portion of Western Australia. Owing 

 to the marked features which many of them exhibit, these dykes can 

 be readily followed across country, and in certain localities they are 

 of considerable value in working out the details of the geological struc- 

 ture. A feature of significance is the faulting these later dykes have 

 undergone since their injection and consolidation. In the Warrawoona 

 field — an important north-west mining camp — these basic dykes- 

 traverse the centre of the auriferous zone almost at right angles to its 

 general strike. The regular continuity of the system of dykes (which 

 extend across country for 30 or 40 miles) has been interrupted in the 

 vicinity of their intersection with the auriferous series, and they have 

 undergone considerable movement since their injection. The peculiar 

 dislocation and apparent displacement in short segments are probably 

 due to a development of step-faulting in a manner not as yet fully 

 understood. The dykes are apparently cut into a curved and distorted 

 segments, and displaced along more or less vertical planes which have 

 a general tendency to shift the separate portions bit by bit, in one direc- 

 tion, the dykes being dragged to the north-west of their course. 



A very important feature in the geology of the Pilbarra district 

 is the evidence of great earth movements that affected the district. 



In the neighborhood of the picturesque Doolena Gorge, on the 

 Coongan River, very impressive evidence of a powerful rupturing of 

 the crust is to be seen, represented, inter alia, by a line of dislocation 

 which has been proved to extend for nearly 100 miles. The large 

 quartz reefs to which allusion has just been made are seen to be abruptly 

 cut off by this powerful fault, which presents a steep vertical escarp- 

 ment — unscaleable in places — often over 150ft. in height. At some 

 distance north of this main fault an instance was observed, on the 

 eastern bank of the Strelley River, of one of these large reefs being not 

 actually truncated, though subjected to deflection by a powerful thrust 

 exerted in a direction approximately parallel to its strike. In this 

 instance the compression has been so great that this reef, which is about 

 30ft. thick, was reduced to Gin. to lOin., whilst the horizontal displace- 

 ment reached about 100ft. 



The quartz reefs of the district also afford evidence of overthrust- 

 ing, contortion, &c., and many of them present features which seem to 

 indicate that they have been wrenched apart by movement along shear 

 planes. A very important instance of this is to be seen at the mining 

 camp of Warrawoona, where a line of reef of a peculiar type traverses 

 the field along a powerful dislocation, which has been followed for twa 

 and a half miles. 



This dislocation occurs along a persistent reef, portions of which 

 have been torn apart and shifted in segments, producing the peculiar 



