ON CUPRIFEROUS TUFFS. 287 



majority of them are lapilli of basaltic lava, but those surrounded 

 with the opaque whitish zone and showing a rough hexagonal 

 shape, are probably ilmenite fringed with leucoxene. Black 

 magnetic iron and ilmenite are plentifully sprinkled through the 

 rock, as tine dust, as well as in crystals and crystalline aggregates. 

 A purple amethystine colour prevails in tlie ground-mass surround- 

 ing some of the ilmenite, the colour being probably due to a small 

 quantity of manganese, or possibly to the titaniferous iron. 



ECONOMIC VALUE. 



These shales, apart from their great scientific interest, may at 

 some future period be of value for two reasons :— 



1. For the metals contained in them. 



2. For the indication which they afford as to the depth of the 



underlying Bulli coal-seams. 



1. At Holt-Sutherland the total thickness of the coppei'-bearing 

 portions of the shales is at least three feet. This thickness, 

 however, is made up of a number of difierent layers, several of 

 \vhich are separated from one another by bands of shale which do 

 not show native copper freely, though they probably contain a 

 small proportion of the same metals which occur in the more 

 productive bands. The tliickness of the metalliferous bands is 

 respectively one inch, two and a half inches, one foot, seven and 

 a half inches, five inches, two and three quarter inches, and six 

 inches. The highest ore band is about twenty -four feet above the 

 lowest, and the beds are separated from one another by from one 

 foot to nine feet of comparatively barren shale. 



At Heathcote there are two thin beds shewing native copper, 

 the upper six inches thick, and seven feet above the lower, which 

 is only one inch thick. 



Assuming, therefore, that a continuous bed of cupriferous shale 

 extends from Heathcote to Holt-Sutherland, a distance of eight 

 and a half miles, having an average thickness of one foot, there 

 would evidently be a large body of metalliferous strata. 



The average specific gravity being taken as 2.6, every acre of 

 shale would contain 3,160 tons of copper-bearing rock which 

 would yield on the average (the average yield of copper being 

 assumed to be 0.15%) 3 lbs. 5| ozs. of metallic copper and 7 dwts, 

 7 grs. of silver per ton. If, therefore, the value of the gold be 

 left out of consideration, and, as already stated, gold was only 

 found in appreciable quantity in one doubtful assay, the value of 

 copper and silver in the aggregate would be only 3s. 9d. (?) per 

 ton, so that the shale is far too poor to work at present for these 

 metals. 



2. As regards the question whether the copper shales form a 

 reliable horizon from which to calculate the probable depth of the 

 underlying Bulli seams, a very interesting question suggests itself. 



