BY THE REV. W. B. CLARKE, M.A., F.G.S., &c. 303 



the site of trappean influence, the transmuting agent is concealed. 

 But, as the upper portions mark the coming in of the Wianamatta 

 beds which near by are well developed, and as the basaltic trap 

 of Burwood is but two miles distant to S.W., the conclusion we 

 mast naturally come to is, that that trap has extensive ramifications 

 under the surface and has produced the transmutation at Five Dock. 



It is, therefore, only reasonable to consider, that where similar 

 changes have been noticed, a similar cause for them exists ; and 

 as the processes connected with excavations of the soil go on, 

 these dykes may be hereafter discovered. 



There are, however, other visible demonstrations of igneous 

 rock than those mentioned ; as near Canterbury, and along the 

 old Liverpool road, where, in one part, probably connected with 

 the Prospect and Bull's Hill district, blocks of porphyritic diorite 

 lie by the way side. 



How far these eruptions may have been contemporaneous 

 with the peculiar formation of the country near Sydney, it is 

 impossible to say. But the broken and fragmentary, insulated or 

 peninsulated masses of sandstone, with cliffs and ledges 

 apparently successional, and the peculiar parallelism which exists 

 in the direction of the outlines of the harbour coasts, all point to 

 a series of prolonged changes by which the deposited and once 

 more highly elevated Hawkesbury rocks were subsequently 

 depressed, cracked and broken down along certain lines of joints, 

 thus allowing the intermediate masses to be swept away, leaving 

 only such landmarks as Spectacle Island, Cockatoo, Snake Island, 

 Garden Island, Pinchgut, Clarke's Island, Shark Island, the 

 patch off Grey Cliff and some other minor features. These point 

 to a once general extension of rocks over the whole harbour, of 

 which these small islands were summits ; whilst the coast is 

 marked by the larger and more striking islands, now peninsulated 

 by sandy beaches, of the South Head, North Head, and other 

 masses of like kind, both north and south of these. 



When we look to these features and observe the agency of 

 igneous forces we may naturally conclude that the irruption of 

 basalt and other rocks of like kind may have been the result of 

 the depression of the country along the sea line, in turn producing 

 the effects 011 the rocks which we have been considering. 



