ON CUPRIFEROUS TUFFS, 289 



Some green fragments, much like malachite, may be seen in the 

 microscopic section of tlie tuti'aceous grit, in Slide No. 4a. This 

 evidence is confirmatory of the existence of two horizons of 

 volcanic tufts, and suggests that the iron oxides which impart the 

 reddish colours to the chocolate shales and the purple shales, may 

 have been derived from the magnetic iron in the volcanic tufts. 



PROBABLE ORIGIN. 



The cupriferous tufts and tuftaceous shales are evidently partly 

 of ^•olcanic and partly of sedimentary origin, as proved by the 

 presence in them of both basalt lapilli and rounded quartz grains. 

 Two facts indicate Kiama, or the line of volcanic country between 

 Kiama and Mittagong, as the probable source of the volcanic 

 particles ; first, the presence of minute segregated veins of 

 metallic copper in the basalt at Kiama ; second, the resemblance 

 of the Kiama and Mittagong lavas to those of which the frag- 

 ments in the tuft" at Holt-Sutherland are composed. About four 

 years ago a Mr. — Cameron brought from Kiama, a sample of 

 basalt which contained native copper. Under the microscope the 

 Kiama basalt is seen to consist of a holocrystalline base in which 

 are micro-porphyritic foreign crystals of triclinic felspar and 

 straw-coloured augite. The base is composed of the lath-shaped, 

 felted, triclinic felspars, with a large proportion of bottle-green 

 chlorite, and a considerable quantity of magnetic iron and 

 ilmenite. Minute segregated veins of metallic copper are visible 

 in three slides exhibited. These veins are about one-fortieth inch 

 thick, and their surfaces shew well formed crystals of copper. 

 Their secondaiy origin is evident from the fact that they intersect 

 all the minerals composing the base of the lava. The genesis of 

 these veins of copper in the basalt is difticult of explanation. 

 Metallic copper may have been present in a finely divided state in 

 the basaltic magma, and have segregated along the shrinkage cracks 

 formed in the lava on cooling ; or it may have been precipitated 

 in such cracks from some solution of copper salts. Its occurrence 

 in veins proves that it was not precipitated by metallic iron in 

 the basalt from a solution of sulphate of copper. In the tuffs, 

 however, at Holt-Sutherland, there can be little doubt that the 

 copper was chemically precipitated. 



Tlie above facts point to the following conclusions : — 



1. That at some period, intermediate between the close of the 



Permo-Carbonif erous and the commencement of the Triassic 

 (if, indeed, the Hawkesbury Sei'ies be Triassic), volcanic 

 explosions, accompanied by showers of volcanic dust, 

 occurred probably in the neighbourhood of Kiama and 

 Mittagong. 



2. That this dust fell into the waters of the lagoons or estuaries 



in which the lower part of the Estheria Shales had already 

 been deposited. 



