G.—B

56

In his report on the Geology of the Thames Goldfield,* Mr. Cox considers that the soft yellowish-grey gold-bearing rocks are still in their original condition, and are not the decomposed products of any other rock. He bases his opinion on the presence of undecomposed pyrites throughout their mass. This seems to me to afford but a slender basis for so important an argument, for it is a well-known fact that pyrites is always a secondary product, and in clastic rocks it is a common occurrence to find organic remains entirely replaced by this metallic sulphide. With regard to the pyrites disseminated in the Thames rocks, it is found in all but the more solid lavas, and it is pretty certr.in that it appeared at an early stage in the process of decomposition. To the three stages distinguished by Professor Hutton may be added a fourth, in which the pyrites undergoes change first by alteration into the black ferrous sulphide, and then into the soluble ferrous sulphate. Large quantities of the latter salt are being deposited in the old dry workings in the Kuranui and Caledonia Mines, and in some of the levels the accumulation has been so rapid and extensive as to completely block up the underground passages. In the places where the waters charged with the ferrous salts issue into wet workings or drives, rapid oxidation takes place, and considerable deposits of the hydrated oxide are formed. The third and fourth stages of decomposition to which the once solid andesites are still subject may be seen actually in operation in many of the mines where the harder bars are rendered accessible. The cold, acidulous waters from the surface are continually acting upon the carbonates of lime, magnesia, and iron, which were segregated in veins during the second stage of decomposition, with the result that they become highly charged with soluble salts of these substances, while large volumes of carbon dioxide are being continually liberated. This CO 2 , in its turn, in conjunction with water, further reacts on the freshly-exposed surfaces of the feldspars and secondary chlorite in the already partially-decomposed cores of rock. Thus the progress of alteration goes round in a cycle, and in time will result in the complete decomposition, by leaching and oxidation, of the existing bars of solid andesites and tuffs. In the mines in the vicinity of hard bars, the country is saturated with CO 2 gas, which often interferes with the mining operations, especially during periods of low barometric pressure. It is probable that after the volcanic forces which originated these rocks had spent themselves, sofatara action took their place, and continued in operation for a long time ; and, in support of this, we still have the thermal mineral springs at Te Aroha issuing from the same class cf rocks. The leaching-action of such thermal waters, no doubt accompanied by steam and acid vapours, would be more rapid and deep-reaching than the action of the surface-waters just described. The former existence of solfatara action would satisfactorily explain the presence of the grey, gold-bearing rocks, at depths far below the reach of surface-decomposing agents. At the present lowest level of the old Queen of the May shaft, situated in Waiokaraka Gully, the grey decomposed andesite, at a depth of about 400 ft., is identical with that found at the surface. The same fact is seen in the lowlevel tunnel of the Occidental Mine at Una Hill, where the grey " kindly country," highly charged with pyrites and veins of pearlspar, is found at a point in the centre of the hill at least 700 ft. from the surface of the nearest point. Numerous examples of the same fact can also be obtained in the Queen of Beauty, New Prince Imperial, Waiotahi, and Cambria Mines, where the grey, gold-bearing country has been carried down from the surface to depths varying from 300 ft. to 748 ft. below sea-level. The soft, gold-bearing rock, or " kindly country," is now acknowledged by all the leading European and American petrologists to be a decomposed andesite ; but for many years there was much discussion and controversy as to its true character and proper designation. In 1868 Yon Richthofen adopted the distinctive term "propylite" for the grey, decomposed, orebearing rocks of the Comstock Lode, a name which was subsequently adopted by the officers of the United States Geological Survey. In 1876, the results of Dr. Zirkel's microscopic examination of the North American rocks was published, and in this work he endeavoured to justify the retention of the term " propylite " as a distinct type of rocks, although he went so far as to admit their association with the Tertiary lavas. In the following year Professor Bosenbusch refused to accept the name " propylite " as a distinctive group, but classed many rocks known as such with the andesites. In 1879 Dr. Doeltor showed that the Hungarian rocks, which possessed the peculiar features held by Yon Eichthofen and Zirkel to be characteristic of the propylites, could be seen to pass insensibly, by a gradual process of alteration, into ordinary andesites. Shortly after this Doelter's view was strongly upheld by Eosenbusch; and in the same year Dr. Wadsworth very forcibly insisted that_the distinction between the propylites and andesites could not be maintained in the case of the North American rocks. In his " Geology of the Comstock Lode and Washoe District," 1882, Mr. G. P. Becker states that the grey decomposed ore-bearing rock which corresponds with the " kindly country " of the Hauraki goldfields could not be regarded as a distinct group of rocks, but only as a distinct fades or habitus of the andesitic lavas. The microscopic examination of a large number of the rock-specimens obtained in the deepest workings, and during the construction of the Sutro Tunnel, enabled Mr. Becker to show that by the gradual alteration of their constituent minerals the hornblende- and the augite-andesites gradually acquired those characteristics which had been held to be peculiar to the propylites. In 1886 Professor Eosenbusch accepted the term " propylite," not, however, as indicating a distinctive group of rocks, as originally contended by Richthofen and Zirkel, but only as a convenient name serving to distinguish a well-marked and interesting pathological variety of the andesitic type of rock. The term "decomposed andesite "is inconvenient and clumsy, and I think the name "propylite " might with propriety be adopted by all New Zealand geologists in the restricted sense proposed by Eosenbusch. I shall hereafter in this paper speak of the soft yellowish-grey decomposed andesites as propylites.

* Geological Reports, 1882, p. 4.