II. Economic Geology.

We cannot, in the immediate vicinity of Dunedin, boast of any great natural mineral riches; still the geological features of the district are not of an unfavourable character for the development of a city. There are, it is true, no deposits of true coal, but close to our doors is the excellent fuel from Green Island, known as brown coal, and existing in very large quantities; and on either side the Kaitangata and Shag Point coalfields (see appendix) are ready to supply us with pitch-coal of the highest quality. The coal at Green Island is hydrous brown coal, containing, as will be seen by the following analysis, about 16 per cent of water:—

Fixed Carbon	44-87
Hydro-carbon	35-21
Water	16-47
Ash	3-46
	100-01

(Mean of analyses of Green Island coal recently made in Colonial Laboratory for Railway Department.)

It occurs in a seam 19ft. thick, and has when first got a lustrous appearance and dark brown colour, but on account of the large proportion of water contained, dessicates and crumbles on exposure to the air. It burns freely, with a slightly unpleasant smell, and leaves a bulky incandescent ash; as a fuel for locomotion it does moderately well; and the slack and small coal are largely used for stationary engines. When left below ground the slack is very liable to spontaneous combustion, and many mines have been lost from this cause.

In the coal measures are first-class clays and fire-clays; indeed in the matter of materials for making bricks, the locality is thoroughly well supplied.

An excellent sand for building exists in large quantities in the neighbourhood.

The soil of Dunedin and the environs may be said to be generally of an excellent quality; with the exception of the coal-page 88measures, and some few parts where the rachitic roots have unfavourably influenced it, we have usually a rich decomposed basaltic soil, mingled in some places with the debris of limestone, and thus forming a most excellent material for dairy produce. The swampy flats, now drained, of the lower parts of Dunedin, and of the Taieri plains, are wonderfully fertile: the latter being formed in great part of decomposed schist rock, brought down from the interior of the Province, and enriched by the decay of many generations of plants.

The building stones surrounding Dunedin are principally in the volcanic rocks; certainly the Caversham sandstone has been used for this purpose, but without success, as it is incapable of resisting the action of the weather. The following analysis indicates its composition, which is mainly carbonates of lime and magnesia, with a small proportion of silica:—

Silica	24.4
Carbonates of lime and magnesia	53.0
Alumina	17.6
Soluble clay	1.5
Oxide of iron	1.4
Water and loss	2.1
	100-00

On the Peninsula, near the camp, is avery good siliceous sandstone, which has been utilised by the Hon. Mr Larnach in building portions of his residence. From the volcanic series can be obtained not only the hard bluestone or basalt, but also rachitic breccias of considerable beauty and great durability; both are largely used in the public buildings of the city. For purposes of cement manufacture, very good lime stones exist in several parts of Otago; and there is a large deposit of valuable hydraulic limestone in a line extending from Seal Point on the southern side of the Peninsula to Dowling Bay on the northern shore of the lower harbour. This yields an excellent material, but, unfortunately, the cost of cartage is too great to allow of its being extensively utilised.

The necessary mud for the manufacture of Portland cement is obtained from the Dunedin harbour

page 89

Although gold-bearing rocks exist in more than one locality near Dunedin, there has never been that conspicuous success in working them, which the investor has a right to expect.

As has been stated, an instance of an exceedingly interesting auriferous rock occurs at Harbour Cone, near Portobello, on the Peninsula. Although frequently spoken of as a quartz reef, this is in reality a dioritic rock, richly impregnated with iron pyrites, and apparently occurring in very large quantities. Several attempts have been made to work it, but all have, so far, proved ineffectual. Some years ago trial crushings were made in Victoria, which yielded 7 ½ dwts. to half an ounce per ton, and other samples from a portion of the deposit a little lower down the hill gave 8 dwts., 1 oz., 2 dwts., and 6 dwts. per ton respectively. Some doubt was felt as to the probability of finding gold in such a matrix, and other samples were tried in Dunedin, all of which yielded the precious metal. Inlateyears, about 1886, another attempt was made to develope the field, but without success. Professor Ulrich, who has examined the locality, describes the deposit as a peculiar, hard, diorite-like rock, the structure being holo-crystalline and medium coarse-grained; the composition triclinic felspar, hornblende, and some quartz with iron-pyrites finely and uniformly impregnated, more especially through the hornblende part. He considers that a whitish mass overlying the crystalline rock is either a decomposition product or more probably a decomposed rock of genuine rachitic type, which has flowed over the other. This is indicated, he states, by the fact that on its line of continuation, only a few feet distant from the shaft, there exists on the hill-side a massive outcrop of a hard rock, which, according to all appearance, is of rachitic character, consisting of a very fine-grained greyish or yellowish-white base, with impregnated crystals of sanidine-like felspar, hornblende not being observable.

Presuming that the rock exists in quantity, and contains a general average of gold, such as has been mentioned, it seems a matter for regret that steps are not taken to thoroughly determine its value; the prospect of finding rich veins of auriferous quartz, such, as are usual in the Thames district, would lend an additional inducement. In addition to the above instance, there is at Green Island a series of quartz reefs, known as the Saddle page 90Hill Reefs. The stone is of considerable thickness, up to 14 feet, and strikes S. 76 deg. E. It dips northward at an angle of 55deg., and is enclosed in soft phyllite. Several of the reefs carry gold, and some attempts have been made to work, but so far no success has attended the efforts, and the place is now let to a party of miners who are working surface stone.