Geological and other Reports
I have carefully examined the specimens of rocks brought from Wellington by the Honorable G. S. Evans. I have also read the printed reports of excursions made by Mr. Crowford in that Province, and the MSS. accompanying the specimens.
I have had some difficulty in fixing the several localities (not being provided with a good map) and I have not been able to gather any information relative to the extent of the areas occupied by the several rock formations. Under these circumstauces it is necessary to speak cautiously and to draw conclusions only when the evidence is clear.
Amongst the more important specimens of rock submitted, I recognise mudstones, similar in lithological character to the rocks occurring in the basin of the river Yarra, greyish blue mudstones with iron-pyrites and thin veins of alumite containing more or less silica, and quartz.
I also observe a very fine grained granite rook, and a rock, evidently derived wholly from the degradation of this, consisting of quartz, felspar and mica, with included pebbles of quartz.
Much of the granite rock contains veins of quartz.
Judging from the specimens submitted, it would appear that part of the valley of the Upper Hutt is occupied by plutonic rocks, but how far these extend, or whether the quartz veins intersecting the primary rocks also run into the granite, it is impossible at present to say.
In Mr. Crawford's report, dated the 24th October, 1861, it is stated that the metamorphic rocks fill a large area and compose the Rimutaka and Tararua ranges, but I do not recognise amongst the specimens any gneis, clay slates, or mica schists.
I regret that Dr. Evans' hurried visit did not admit of Mr. Crawford's giving more information respecting the quartz veins of the Province. I observe specimens of ferruginous quartz, milky quartz, and much chalcedonic quartz, but how the veins occur, or whether they are thick or thin is not stated, without this information it is impossible to give any useful opinion respecting them; for hand specimens may be obtained abundantly where the veins are thin, and, comparatively, few and unimportant.
One very interesting specimen of basalt, with glassy crystals of felspar, a fragment of a boulder from the bed of the Hutt, would indicate that igneous as well as plutonic rocks may be looked for in that part of the basin.
A fragment of a purple slate rock, marked “Hawtrey, Johusonville” appears to me to be important in connexion with the specimen of quartz No. 11, from the same locality.
All the rocks, including the quartz and excepting the granites and conglomerates contain more or less iron pyrites, and the specimens taken from Barraud's well, which contain alumite, resemble very much the mudstones of the Heathcote District, where we find gold, sulphide of antimony, chrome iron, and a mineral resembling bournonite.
The other specimens are devoid of fossils, but the mudstones may, I think, be safely set down as palæozoic.
Tertiaries, age unknown.
Carboniferous rocks of the oolitic age.
Mudstones, probably Silurian.
Granites and other plutonic rocks.
If the suite of specimens forwarded by Mr. Crawford fairly represent the rock formations of the Province it may safely be asserted that the search for gold may be prosecuted with every prospect of success. I have not yet had time to analyse any of the specimens, or indeed to apply any but the slightest chemical tests to one or two, and therefore I cannot say whether or not they contain gold, except as regard the fragments of rock and clay taken from Barraud's well. In these I detected a very small grain of gold.
Whether the gold found in the well is derived from small veins of quartz penetrating the mud stones, or whether there is a thin surface stratum overlying the older rocks which is auriferous, is well worth investigating, and I would recommend that the locality should be carefully examined. What is the prevailing rock in this neighbourhood? and has it been ascertained whether or not the surface soil is auriferous? Is there any reason to believe that gold dust has accidentally fallen into the well and become mixed with the debris there? These questions would be answered by a not very laborious investigation.
Wherever we meet with silurian (or older) rocks intersected by veins of quartz, we may look with confidence for the discovery of metaliferous and mineral veins of more or less importance. It appears that we have all the conditions in the Province of Wellington. If I were to submit many of the rocks collected by Mr. Crawford to any gold miner in Victoria, he would recognise them at once as precisely similar to those occurring on some of the Victorian gold fields; and while a few specimens of the milky and ferruginious quartz are quite like our own, the chalcedonic quartz exactly resembles that found in the Northern gold fields of New South Wales. Without attaching too much importance to these resemblances (for quartz veins are not always auriferous) they yet serve to show the necessity for earnest and careful exploration of the country.
Looking to the physical geography of the North Island of New Zealand, one observes a main chain of mountains, running generally in a Northerly direction with Easterly and Westerly spurs of considerable extent. The position of this main chain (broken as the other lands may be) has determined the course of the principal river basins, the more important of which lie on the West side of the main range, If a geological map were constructed of this country it would not probably differ, in its main features, from many parts of Eastern Australia. We should find, I apprehend, the central axis composed of rocks of the primary age, with masses of intrusive granite, and many of the spurs of basalts and lavas of different ages. We should also find areas occupied by carboniferous rocks, and large tracts covered by tertiaries of greater or lesser thickness.
Though the information I have received is not very complete, it would appear that on the Eastern sea board rather thick tertiaries prevail (probably of the same age as the Murray tertiaries) and I would therefore recommend that the rivers flowing to the Westward should be first explored. There, I am led to believe, the quite recent accumulations of drift, similar to these occurring on our Victorian gold fields, are prevalent, and no very costly examinations would be required to ascertain whether or not these recent accumulations contain gold, or tin ore, or the ores of other metals, in remunerative quantities. There is no reason to believe that gold is more likely to occur on the Western side of the main chain than on the Eastern. But it is probable, owing to the local distribution of the rocks, that it will be found more easily on the Western side. The rivers in Victor'a on the Eastern side are all auriferous.
Unless careful attention be directed to the conditions under which gold occurs, a long period may elapse before any important discoveries are made in this part of New Zealand. Looking alone to the modes of occurrence of gold in Victoria, people in other countries may utterly neglect the most important localities, page 20 because the conditions there are not the same as there. In Victoria we find the silurian and metamorphic rocks every where intersected by veins of quartz, varying in thickness from a line to fifty feet. These quartz veins, nowhere, it is believed, penetrate the great masses of plutonic rocks which every where have broken through the sedimentary strata. They stop short at the granite boundaries, and the gold miner, accordingly, as a general role, neglects the granite country. This peculiarity is due probably to the relative ages of the granites and the sedimentary rocks. The force which rent asunder and left wide fissures in the clay slate and schist formation was perhaps exerted before the granites came to occupy their present place, or perhaps the force was insufficient, or exerted so as not to affect the denser, tougher and harder plutonic rocks. It is a local peculiarity, not a condition universally occurring. Elsewhere gold is found in granite, in gneiss, in mica schist, in syenitic porphyry, in green stone, as well as in quartz veins intersecting rocks of the silurian age.
It may be looked for, and yet may be profitably worked in conglomerates much older than tertiaries.
Gold is found in quite modern drifts overlying granite rocks—not derived from auriferous quartz veins penetrating the granite but from veins in slate rocks which have wholly disappeared. The slate rocks have been denuded and their rich stores of gold have been left in holes and “pockets” of the granite, the bed on which the slate rocks were originally reposing. A granite country therefore (more especially if in the neighbourhood of schists) should not be neglected, but explored carefully and anxiously. The gold said to have been found in Greaves' Gulley, may have been derived from auriferous veins penetrating sedimentary rocks, or it may have been derived directly from veins intersecting the granite.
If I might venture to offer a suggestion, I would say that instead of a hurried examination of a great extent of country, it would be more satisfactory to make a careful examination of a comparatively small area,—say the basin of the Hutt, the Otaki, or the Manawatu. I would recommend that the upper and lateral branches of these rivers (at some distance from the coast) should be carefully explored, and rough sketch maps made showing the various rock formations,—rough maps which might hereafter be amended, revised, and in some parts wholly altered as discoveries would from time to time, be made. The recent drifts filling the valleys of the small tributaries, should be carefully examined down to the bed rock, and the whole depth of each section washed very carefully. One or two careful examinations of this kind would in fact be the very best kind of “Prospecting.” I would suggest just such examinations as Mr. Stutchbury made in New South Wales, and exactly such maps as he constructed.
I do not think explorations very near the coast will be productive of useful results. If we look to the rivers in Victoria we gather some useful hints for the gold prospector. Near the coast the sands of these rivers are almost invariably so poor in gold that they may be said practically to be non-auriferous For many miles the Yarrowee is non-auriferous, and quite at its sources we have the Ballaarat Gold Field. The Coliban and Campaspe flow into the Murray through a pastoral country, but their tributaries, miles away from their embouchures, are now supporting large numbers of gold miners. The Loddon is only auriferous near its sources, and the same may be said of the Hopkins, the Wimmera, the Werribee, and every river in the country. A very slight consideration of the facts counected with the occurrence of gold in drifts and recent accumulations will show that the sources of streams, rather than the streams themselves, are most likely to yield the metal in remunerative quantities.
It is probably true that gold is widely disseminated in rocks of the oolitic age, but, according to our present knowledge in such a state of minute subdivision as to be wholly unavailable to the miner. The sludge from a puddling machine contains gold, but the greatest quantity is caught and retained in the box. And so we may consider this operation of nature by which great masses of rock have been worn down and washed as but another kind of puddling, and we must reach the source of that grand agency before we find the gold.
I have not alluded to the modifications likely to be produced in the strata of the drifts and recent tertiaries by glacial action. These should not be overlooked by the gold prospector. Where this force has been exerted gold may be distributed very unequally through the strata, and such rocks may quite puzzle even the experienced gold miner. Though it is true that the specific gravity of gold is sufficient under ordinary circumstances to determine its position on the surface page 21 of the bed rock, yet when the gold is imbedded in large fragments of quartz it may be found at all depths from the surface downwards, and in conducting explorations in drifts of this kind all the debris should be carefully examined.
As there are undoubtedly coal bearing rooks in the Province of Wellington no opportunity should be lost of shewing the extent, thickness, dip and general character of these.