GEOLOGY OF THE PROVINCE OF AUCKLAND.

The first striking characteristic of the Geology of this Province—and probably of the whole of the Northern Island of New Zealand—is the absence of the primitive, plutonic, and metamorphic formations, as granite, gneiss, mica-slate, and the like. I have been informed by Mr. Heaphy, that these rocks are of wide-spread extent in the Middle Island, forming mountain ranges of great altitude, covered with perpetual snow, and reaching in Mount Cook probably to 13,000 feet. The rocks of these formations contain the principal metallic riches of the earth. Therefore we cannot hope to find these riches developed in the highest degree in the Northern Island; but as other formations also contain metalliferous veins, there may be found many mines worth working, in the rocks I am about to describe.

I.—PRIMARY FORMATION.

The oldest rock I have met with in the Province of Auckland belongs to the primary formation. It is of very variable character—sometimes being more argillaceous, of a dark blue colour, (when decomposed, yellowish brown, the colour generally presented on the surface,) and more or less distinctly stratified like clay-slate—(at Maratini on the Wai-temata); at other times the siliceous element preponderates, and, from the admixture of oxide of iron, the rock has a red, Jasper-like appearance—(at Waiheki, Mangonui Point.) In other localities it is more distinctly arenaceous, resembling the Old Sandstones of the Silurian and Devonian Systems, called Grauwacke—(at Tiritiri, on the Hauraki Gulf). As no fossils have yet been found in this formation in New Zealand, it is impossible to state the exact age: I am, however, of opinion, that these argillaceous siliceous rocks will be found to correspond to the oldest Silurian strata of Europe.

The existence and great extent of this formation are of considerable importance to this Province, as all the metalliferous veins hitherto discovered, or likely to be hereafter found, occur in rocks of this formation.

To these rocks belong the Copper-pyrites, which has been worked for some years at the Kawau and Great Barrier—the Manganese (Psilomelane) at Waiheki—and the Gold-bearing quartz at Coromandel.

The gold which is washed out from beds of quartz-gravel in the rivers and creeks flowing down from both sides of Coromandel range, is derived from quartz veins, of crystalline character and considerable thickness, running, in a general direction from North to South, through the primary rocks which form the foundation of the Coromandel range. In some places these veins stand up like a wall on the summit of the range to a height of eight or ten feet. The clay-slate rock itself is exposed only at the bottom of deep gorges which form the channels of the principal streams. In almost all places it is covered by large masses of trachytic tuff and breccia, of which the hills surrounding the Harbour of Coromandel are composed. The well-known “Castlo Hill”—which can be seen from Auckland—is a characteristic example of the Trachytic Breccia formation. The magnetic iron-sand which, in washing, is found with the gold, is derived from the same source as all the magnetic iron-sand of New Zealand, namely, from the decomposition of trachytic rocks. Small veins of quartz of amorphous character—that is, not crystallizing, but in the shape of chalcedony, cornelian, agate, and jasper—are found in numerous places on the shores of Coromandel. These veins occurring in trachytic rocks, are quite different from the auriferous quartz veins in the primary formation—a fact, I think, of much practical importance to state, to prevent the fruitless search for gold where gold does not exist. All the gold-bearing gravel in the creeks is derived, as I have already said, not from the veins in the trachytic breccia, but from the much thicker and crystalline veins in the primary rocks. The surface-deposit in those creeks is very rich, but, as compared with Australian and Californian gold-fields, of limited extent and depth. (I washed a few bucketfuls of surface earth, and gravel, at a creek, pointed out to me by Mr. Charles Heaphy, near King’s Mill, at the Kapanga. Every panful showed scales of thin gold, with small fragments of quartz streaked and studded with veins and spangles of gold. These “specimens,” as they are called by diggers, show no—or very little—sign of being water-worn, but are sharp and crisp fragments, as if they had been broken up on the spot, or in the immediate vicinity. I think the quartz-veins in the mountains should be thoroughly examined, and that, when once the day has come that the Coromandel gold-fields are worked, the attention of the “diggers” should be directed as well to the hills immediately above any rich deposits as to the alluvial workings below.)

The Coal Beds at Coromandel occurring between strata of trachytic breccia are too thin to be of any value, and, as the coal formation is absent, there is no ground for hoping that a workable seam may be found.

The primary formation occurs, to a more considerable extent, to the Eastward of Auckland, in ranges on both sides of the Wairoa river attaining an altitude of 1500 feet above the sea,—and striking, from thence Northwards, over Waiheki and Kawau, to the Bay of Islands. In a Southerly direction, they extend, through the Hangauroa and Taupiri ranges, across the Waikato, through the Hakari-mata and Hauuru range—parallel with the West Coast—to the Mokau district, where, at Waiare, the Mokau river falls in a magnificent cascade over a lofty precipice of that rock.

The same formation occurs again in the Rangitoto mountains on the Upper Waiapu, and West of Taupo lake in the Tuhua mountains. But the most extensive range of primary rocks is that which commences near Wellington under the name of Tararua and Ruawahine, and runs in a North-easterly direction to the East shore of Taupo lake, under the name of Kaimanawa, in which rises the principal source of the Waikato—there called Tongariro river. The range continues from the shores of Taupo lake, in a North-easterly direction, to the East