BY T. W. EDGEWORTH DAVID. 423 



of contact. The oxidation of the laccolites of Manduramite, 

 where they have been brought by denudation within reach of the 

 action of surface-water and the atmosphere, has given rise to the 

 ferruginous masses of material (in places very siliceous, and 

 generally more or less porous from the decomposition of iron and 

 lime) which have been worked for gold for many years past, and 

 which yield from mere traces up to over an ounce of gold per ton. 

 The metalliferous portions of these deposits have probably been 

 derived from metallic solutions in the diorites, which are very 

 pyritous, especially those varieties which contain an abundance 

 of hornblende. A small quantity of the metals present in tlie 

 ore masses may have been derived perhaps from the replaced 

 limestone. 



(3) Next in order and youngest of the three formations are 

 certain dykes of later date than the laccolites, as there is distinct 

 evidence of the former having strongly intruded the latter. This 

 intrusion seems to have exerted a certain amount of segregative 

 influence on the metals present in the laccolites, as Mr. Stonier 

 and the author were informed by the manager, Mr. Hogue, that 

 the ore masses were richer in the vicinity of these dykes than at 

 a distance from them. The micro-petrological character of these 

 newer dykes has yet to be determined. 



Conclusions. — The manner in which the intrusive laccolites of 

 diorite have crept along the planes of bedding of the sedimentary 

 rock and absorbed some portions of the claystone, and probably 

 a large proportion of limestone into their substance, is very sug- 

 gestive as to the possible origin of certain remarkable varieties of 

 rock in New South Wales hitherto classed as metamorphic. It 

 has been asserted by Mr. C. S. Wilkinson, F.G.S., and Mr. E. F. 

 Pittman, Assoc. R.S.M.,that at Hill End and elsewhere there occur 

 metamorphic conglomerates containing undoubted water-worn 

 pebbles in a matrix showing freely crystallised felspar. The author 

 also has observed the same phenomena at Vegetable Creek, where 

 a bed of conglomerate can be traced passing by almost insensible 

 gradations into a true eruptive quartz-porphyry, the latter showing 



