565 



the development of a system of smooth jointing, by which 

 the rock splits up easily into rectangular pieces, causing the 

 lower slopes of the spur to be strewn with its fragments. 



In the area surrounding this remarkable porphyritic 

 development the basic dyke has to some extent an 

 amygdaloidal structure. Numerous gas cavities occur in the 

 rock, of various sizes up to half an inch in diameter. Some 

 of these have an elongated outline, suggestive of flow, and 

 many of the cavities have received a secondary deposit, and 

 are now filled by aggregates of small crystals of rhombohedral 

 calcite. It is difficult to account for such a scoriaceous 

 feature in the rock if formed at depth. 



The dyke, resuming its normal features, follows the slope 

 of the spur below the porphyritic inclusion, cuts through an 

 impure limestone, and ceases to be visible on reaching a 

 grassed terrace about 12 feet above the level of the stream. 

 There is, however, an outcrop of igneous rock seen in the 

 cliff face of the creek, not much out of line with the dyke 

 just described. This is marked on the map as No. 5, and is 

 probably an extension of dyke No. If.. The exposure is about 

 18 feet in width, but as the rock is much decomposed it is 

 difficult to define its exact limits. This is the only instance 

 in which an igneous rock was observed to be cut by the 

 Cathedral-rock Creek, and even in this case it did not appear 

 to occur on the opposite bank. 



No. 6. — This dyke is also seen on the before-mentioned 

 ridge, a little east of Nos. 3 and Jj., from which it is separated 

 by a zone of calcareous and slaty rocks, about 80 yards in 

 width. The dyke goes over to the southern face of the ridge 

 for a distance of from 60 yards to 70 yards, having a strike 

 W. 20° S. On the northern side of the ridge a strike is 

 maintained almost parallel with that of the adjoining No. If 

 dyke. It is in this igneous intrusion that the old Mount 

 Remarkable Copper Mine ( 4) is situated. At the mine 

 costeen pits have been cut into the dyke, and at a lower level 

 a tunnel has been driven into it for about 50 feet in length. 

 In the section thus exposed the dyke is seen to be greatly 

 broken and mashed under pressure, sometimes even reduced 

 to a crush-rock composed of angular, recemented fragments. 

 It is in this broken rock that the copper (in the form of green 

 carbonate) occurs. A shaft, said to be 20 feet in depth, has 

 been sunk in the contact rock, and from the dumps at the 

 surface it is seen to be in laminated and banded shales 

 belonging to the Tapley Hill-Brighton series. These shales 

 have been indurated and heightened in colour by igneous 



(4) See Records of Mines of South Australia (official), 4 ed., 

 1908, p. 94. 



