BY THE REV. W. B. CLAEKE, M.A., F.G.S., &c. 299 



the whole mass of the harbour was filled in with continuous 

 deposits of sandstone, and, therefore, of far greater antiquity 

 than the present features of the land. Generally it can only 

 be most distinctly made out where the land is lowest ; but at 

 Greenwich the existence of the dyke is traceable to a height 

 of full 200 feet above the sea level, and on the water edge 

 at one side of the isthmus the sandstone has been prismatised, 

 though not to the same extent as in the localities photographed. 



There are abundance of specimens here assembled to illustrate 

 this dyke. 



The occurrence and phenomena of the iron dykes, such as 

 those near the Racecourse at Rand wick, and on the North Shore 

 will be fully treated of hereafter by Mr. Miller, to whom I leave 

 the further details of the examples cited ; but I may mention 

 that he has discovered gold in very minute proportion not only in 

 the masses of iron ore, but in this very dyke of silicate of alumina, 

 as well as in other patches of North Shore iron. 



At the sea extremity of this dyke near Meriberi, the occur- 

 rence of silicate of alumina and various intermediate changes of 

 the sandstone, which is there cemented by it, are particularly 

 striking ; a more extraordinary locality than that between the 

 north head of Bondi Bay and the main coast line towards the 

 South Head of Sydney nowhere is met with ; for the fissured 

 character of the cliffs and the transmutations of the rocks are 

 most remarkable. 



No doubt hundreds of other localities exist along the coast, 

 which have not received yet, but which deserve examination . At 

 any rate, with the present remarks as a guide, it is possible many 

 of the open dykes on our harbour will, by close attention, be 

 found to be of the same character as those which occur on the 

 beach at Newcastle and at Wollongong, and at intermediate places, 

 and which are, undoubtedly, open dykes from which the trappean 

 matter has been removed. It is probable that in these, and many 

 others in the Carboniferous formation, the igneous action may be 

 made visible, and that as we ascend to the Hawkesbury and 

 Wianamatta beds, which are so much above the coal beds, the 

 igneous products may themselves be found to have been altered 

 and so decomposed into these silicated ferruginous clay dykes. 



