Grange. — Geology of Green Isleind Coalfield. 171 



The Abbotsford Tilery Company work a deposit of pure clay that 

 reaches a thickness of 35 ft. Towards Abbott's and Kaikorai Hills the 

 number of boulders in the formation increases. In a railway-cutting near 

 Freeman's mine large boulders are mixed with no order throughout its 

 depth of 15 ft. Park (1910b, p. 200) states that moa-bones have been 

 discovered at the bottom of the clay at Abbotsford. 



A difference of opinion exists as to the origin of the deposit. Beal 

 (1871, p. 276) and P. Thomson (1874, p. 312), mistaking structural marks 

 brought out by weathering on a basalt for glacial striae, conceived a 

 glacial origin for the clays containing boulders at Green Island and 

 Dunedin. Hutton and Ulrich (1875, pp. 69-70) considered that the deposit 

 had been formed by the ordinary weathering of the volcanic rocks. Park 

 (1910a, pp. 593-94) classed this formation as a boulder-clay. Marshall (1910, 

 p. 337) says, " In the clays about Dunedin the only recognizable mineral 

 grains that they contain are those of the most resistant minerals contained 

 in the underlying rock — a matter that at once suggests it has been formed 

 in situ. . . . There are a few places at relatively low levels where 

 there is a layer of well-rounded pebbles and boulders beneath the 

 clay. These mark old shore-lines. . . . The clay that covers the 

 boulders in the localities referred to has been washed down the hillside on 

 to them." 



The presence at Abbotsford of well-rounded boulders consisting of 

 dolerite, basalt, trachydolerite, and rarely phonolite rocks at the base, all 

 of local origin, is very damaging to the idea of classing the deposit as a 

 boulder-clay. Had the beds a glacial origin boulders of schist would 

 naturally occur, since that rock outcrops at no great distance from the 

 clay. The bed of gravel from 120 ft. to 160 ft. above sea-level, together 

 with the break in topography at about the 200 ft. contour, signify that 

 the clay accumulated when the land was about 100 ft. lower than it is at 

 present. The mud of the raised beach on the Saddle Hill Eailway con- 

 tains Chione stutchburyi (Gray), a common form on the present-day beach. 



The formation of the gravel forming the flood-plains of the creeks, and 

 the sand and mud of the lower part of the Kaikorai Stream, commenced 

 when the land was depressed. 



ECONOMIC GEOLOGY. 



Coal. 



History and Mining. 



The history of coal-mining in the Green Island coalfields may be found 

 in Hutton and Ulrich's Geology of Otago, Gordon's Handbook of New Z<:,_,_- 

 land Mines, the New Zealand Mines Reports, and Denniston's "Report on 

 the Green Island Collieries, Otago" (1877, pp. 143-73). 



Thickness of Seam, Faults, &c. 



The greatest thickness of coal is met with in the Saddle Hill mine, 

 where it averages 20ft. At Freeman's it averages 14ft., while at Brighton 

 and Green Island mines the general thickness is 10 ft. 



The main seam is practically free from fireclay. Lenses about 4 in. 

 long are in places met with in the upper part of the seam. Two bands of 

 pyrites f in. thick, one near the floor and the other 5 ft. 3 in. from the 

 floor, run through the coal of the Saddle Hill mine. 



