298 ON THE TRANSMUTATION OF ROCKS, &c., 



derived from the trachyte, or from a source common to both, have 

 resulted since, or during the formation of the Hawkesbury beds. 

 It is possible, that the basalt may have had something to do with 

 the iron, for, on descending about six weeks ago from the summit 

 of the basaltic portion, near the head of the Gibraltar Creek, I 

 found the sandstone in contact with the basalt hardened into a 

 ferruginous conglomerate like " cement " of the gold-fields, and 

 traces of hydrated oxide of iron exactly resembling that which 

 forms the Fitzroy mine. Nodular lumps of carbonate of iron 

 occur in the Wianamatta beds and in the Hawkesbury rocks, 

 with patches of coal ; but the great masses of hydrated ore occur 

 in beds as in the places named, and at Soldier's Pinch near 

 Mount Yictoria, and in other localities in that region. 



Returning now to the fissures near Sydney. About a year 

 and a-half ago, I found at the end of a peninsulated mass of sand- 

 stone, on the North Shore between Greenwich Bay and Ball's 

 Head, a dyke of brownish rock, at the sea level, which was 

 highly ferruginous and clayey, and on each side of which for some 

 distance the beds of sandstone were tilted away from it and 

 extremely hardened. Breaking it up, I found the interior on one 

 side of the cliff putting on the appearance of an amygdaloidal 

 trap, and this being in connection with the hardening of the 

 sandstones, there is good ground for believing that the supposition 

 of its origin is not chimerical. Yet this dyke appears from the 

 analysis of Professor Smith to be principally a silicate of alumina, 

 though highly ferruginous. 



Having the impression on my mind that this is a true igneous 

 dyke, I began to trace out its further relations, and I have now 

 traced it distinctly from the mouth of Lane Cove, through the 

 Greenwich isthmus, on the one side, through Ball's Head isthmus 

 on the other side, and so into a dyke of similar character at 

 Point Piper and into the sea very near to the occurrence of the 

 Meriberi columnar sandstone and dyke of basalt, a distance of 

 six geographical miles. 



Of course continuous tracing is impossible, for the waters of 

 the several bays of the main harbour intervene between the 

 points where the dyke is visible ; but as the general bearing E. 

 20 S. is persistent, I have no doubt that it is a dyke formed when 



