15

C.-9

crystals do cube faces appear. Taking one plate as typical of the whole, and considering one side first, we find that the basis of the form is a single rhombic dodecahedron, broken at one edge into two smaller distinct rhombic dodecahedra. The base form is surmounted by two more rhombic dodecahedral planes, and these again by a succession of octahedral faces with plane angles of 60° and interracial angles of about 110° (109° 28' ?), as nearly as could be estimated with the somewhat crude goniometer used. Seven, at least, of the octahedral faces are developed, being indicated by fine lines on the crystal. The reverse of the plate shows similar features, but here the octahedral planes are restricted to two, surmounting the uppermost of which is a dodecahedral face. In the centre of the plate, and with an edge parallel to the edge of an octahedron, is a cube corner. This cube corner has the apex bevelled inwards, forming a triangular pit or depression. It will therefore be seen that the specimen is probably unique in possessing a form composed of the cube, octahedron, and rhombic dodecahedron. No traces of salient edges appear. An apparently similar, but not nearly so perfect, form has been described by Mr. W. D. Campbell, F.G.S.,* as a pseudomorph after botryogen, the red-iron vitriol; but, as I have been unable to detect botryogen, and as pyrites in any form is rare in this particular locality, it seems to me improbable that in this case, at any rate, the gold has crystallized after botryogen, more especially as the majority of the angles approximate very closely to the 120° of the rhombic dodecahedron rather than to the 117° 24' demanded by botryogen. Again, with this theory the octahedral and cube faces are inexplicable, and, indeed, the only logical conclusion appears to me to be that the gold has crystallized in its own isometric system. Gold is common in a calcite matrix in both the Kapanga and Tokatea districts. In all cases the gold and the calcite appear to have been deposited contemporaneously. The calcite occurs as Iceland or double-refracting spar in clear, small rhombohedra. The gold is scattered promiscuously throughout the calcite, and does not appear to have been deposited with any reference to the cleavage-planes of the calcite, which, indeed, are crossed at all angles. It is not crystallized, and occurs as coarse scales, apparently characteristic of the gold when met with in a calcite matrix. The gold is also poorer in quality in the calcite. Where the gold is associated with native arsenic it appears to be generally massed in the hollow centre of the typical arsenic geode, and is rarely found in the divisions between the concentric spheres of growth. The gold is non-crystalMne or sub-crystalline, occurring in fine dendritic threads dichotomously branched, much like the dendritic markings of the manganeseoxides on rock-surfaces. In the rich Hauraki deposits the gold was, as a rule, found as a dense impregnation throughout a thin quartz vein. The value of the electrum or gold varies for different localities from £2 9s. 6d. per ounce to £3 55., but is fairly constant for any given locality. The following is the average value of bullion from the different districts, though at the same time it must be remembered that two adjacent reefs, or even parts of the same reef, may furnish bullion varying widely in value: Tokatea district, £2 145.; Kapanga district, £2 17s. 6d. ; Waikoromiko district, £2 17s. 6d. ; Waikoromiko district (sluicing), £3 55.; Manaia district, £3 Is. 6d.; Matawai district, £2 13s. 6d.; Pukewhau district, £2 165.; Success district, £2 19s. 6d.; Kennedy Bay district, £2 18s. 3d.; Blagrove's, £3 ; Hauraki district (beach side), £3 Is. 6d. ; Hauraki district (Wynyardton), £2 18s. From the foregoing figures it will be seen that the gold becomes poorer in quality as it approaches the slate area, averaging, in the Tokatea, Matawai, and Pukewhau districts, £2 14s. per ounce. All these districts rest directly on the slates. It is not, however, probable that the contiguity of the slates, per se, affects the quality of the bullion, but that the poorer quality in these districts is due rather to having been less exposed to weathering and solvent agents. For, owing to the greater solubility of silver, the bullion content of the Coromandel reefs decreases in value with increased depth. The associates of the gold are those minerals that have influenced its precipitation. They are native arsenic, found in reefs in the Tokatea, Kapanga, and Hauraki districts. In the Matawai district antimonite and mispickel are found with the gold, sometimes so plentifully that a specimen or rich auriferous rock containing 6 oz. gold to the pound of quartz showed to the naked eye no trace of gold. In the Kapanga Mine it has been noticed that a green discolouration of the country often precedes the discovery of rich quartz—a colour due probably to melanterite or iron-sulphate. The Coromandel reefs become poorer as they increase in width, the famous Hauraki No. 2 reef averaging only 12 in. in width. The auriferous quartz occurs in shoots in the reef, which are variable in width, depth, and dip. In the reef above mentioned the shoot of gold was 150 ft. wide, dipping north-east. The variability of these shoots will be readily understood when it is remembered that they represent the open water-channels through the reef at the time of the auriferous deposit; and, from the sinuous course of the original fissure, the passage of underground waters along the reef will be checked, both vertically and horizontally, at points where the hanging- and foot-walls are immediately in contact. Long experience has shown that the richest points in a reef are those where a cross-reef intersects, especially if the cross-reef be pyritous in character. Here the reef carries the auriferous solution and the cross-reef supplies the precipitant. From consideration of several auriferous shoots I am convinced that, in the majority of cases, the gold, after having been formed at a junction, is swept along the reef by the water current, falling at the same time, owing to the influence of gravity. (See Fig. 12.) Again, as illustrating this point, it is no uncommon occurrence to find the upper faces of horizontally disposed quartz crystals coated with gold, while the lower faces are devoid of the slightest trace of the metal. The source of the gold is probably the pyroxenes of the andesites. Experiments made by Mr. James Park on the Thames andesites gave a gold content of 1J grains per ton, and my own experi-

* Trans. N.Z, Inst,, Vol. XIV., pp. 457-458.