Professor Black in the Academy.

Thames Star, Volume XVII, Issue 5255, 20 November 1885, Page 2

Professor Black in the Academy.

QUARTZ FORMATION.

There was a rery numerous audience in the Academy last erening on the occaion of Professor Slack's second lecture, when he selected for his subject the formation of qnartz reefs, H. Kenrick, Esq., Warden, presiding. Before commencing the lecture, his class, consisting of about 90 pupils, received instruction from 6.30 until nearly 9 o'clock. The first hour and a half was occupied in blowpipe work under the direction of Mr Montgomery, who first explained the construction of the blowpipe, and then went on to explain the different action of the two parts of a candle flame, the inner or reducing flame and the outer or oxidising flame. The students were next shown bow to exag. gerate the effect of these flames by means of the blowpipe, as all blowpipe testing depends on the power of producing at will either a good oxidising or a good reducing flame ; the class was then set to work on exercises for gaining proficiency in this respect. At 8 o'clock, Dr Black took charge of the class, and explained, and caused his student! themselves to try the tests in the wet-way for the metals— silver, lead, mercury, copper, arsenic, antimony, and cadmium. In his opening remarks, Professor ! Black said the subject would be more geological than chemical; be would deal with the question as to how the reefs were formed, and afterwards how the gold got into them. Carbonic acid was the chief agent in bringing quartz into the reefs, and one of his students had told him that he could point out where such a thing was being done at the present time. The old theory as to quartz reefs ■being the result of upheavals in a state of fusion had been upset by chemists and geologists; it was simply an impossible one. The existence of pyrites Bnd other minerals in the quartz was one proof of the fallacy of such a theory. He had seen at the Thames a quartz crystal containing a globule of water, which was certainly conclusive evidence that the igneous theory was incorrect. Where reefs exist there were formerly crevices which had gradually become filled with quartz, it was difficult to say how these cavities were formed, but it was probably through shrinkage from the drying of wet matter, the damping of dry, or the cooling of heated substances; as a rule, these crevices passed in parallel lines, and one reason for this was that they were formed vertically, they always took a straight iine although sometimes in sexagonal or even octagonal form. Professor Hutton, of Christphurch, said that 20 or 30 miles below the sea tbe temperature was warm enough to melt even cast iron or platinum. At suoh a temperature the rooks would resemble a glassy or slaggy nature, and would form a rigid and strong wall of themselves, shrinkage would be the tendency, ■::nd a fall in migbt be expected. There were other means by which these crevices or fissures might be caused, such as pres» sura from the gases of the earth, the oieavage of slate formation, and volcanic upheavals—which had more than likely been tho case here—and the action of water, which hare to a certain extent a soluble effect on certain limestone formations, The fissures had been exis* test, and it was to be considered how they had been filled with quartz. The schistose formations had been formed by pressure, and ran in parallel lines. Carbonic acid was the chief agent in depositing quarlz; every gallon of rain which fell dissolved a gallon of carbonic acid gas in the atmosphere, and this was disposed of in three ways. A third ran off in streams, a third evaporated, and the remiiaJDg tbird Bank joto the eirtb }

that flowing over the earth to the tea dissolved in its course salts snd other materials, and that Sliding its way downwards carried its proportion of carbonic acid which required a base for combination. There were seven base* contained in the schists and slate rocks through which the Middle Island quartz ran; these were oxides of iron, alumina/lime, magnesia, manganese, potash and soda, while there was only one acid-silicic. About the Thames the rocks abounded in acid, but were poor in bases. It was generally found that gold bearing reefs passed through country where these bases existed, but here splendid reefs existed in their absence. Quartz is silicic acid and water. The water, charged with gat, which finds its way into the earth, seeks a base to combine with, and passiog through | the mica and slate schists and their seven bases, fights its way until it forms silicic acid, and as carbonic acid is stronger it assumes command and forms carbonates of lime, magnesia, pottassium, &c—at Te Aroha carbonates of alkalis reach the surface—and where these are formed quartz reefs are formed; the silicic acid is set free, the water, travels dissolving the jelly-like silicic acid, carrjing it uotnt comes to the wall of some crevice, and the water gradually dries up, but the contribution continues with successive streams of dropping, water, and so quarts is formed by the crystallisation of the acid. Sometimes the walls collapsed and " horses " and " saddle reefs "resulted; the leaders and stringer!] frequently seen show the track which the streams of quartz-bearing matter had followed, and they indicate the neighborhood of reefs. Miners had told him that they could show him evidence of the process of formation. In some cases fire clay or pug was dig* covered on the walls spoken of; this was silicate of alumina, which the oarbonie acid had not been able to separate from the silicic acid. On some fields dark stains were made on the surface of mica schists formed by small trickling streams of water. This gave evidence of the existence of oxide of manganese, which, was an important factor in depositing gold ; it was caused by carbonic acid ; it appeared in, red, brown, black, and blue colors, and it was very common here. Miners ,had told him they never found limestone in the neighborhood of reefs, but in all his analyses he had found from 2 to 9 per cent, of carbonate of lime; it was this substance which caused the incrustation shown him ia the big pump pipes. Although many freefs have been formed under different conditions, he held that water travelling for hundreds or thousands of years carrying particles of quartz in solution, these deposited, crystals formed, and a reef followed.: There were probably different ways of which geologists knew nothing, and all he that evening proposed to do was to show the chemistry of quartz reefs so far as it was known. The Professor then announced that he would this (Friday) evening lecture on the chemistry of gold, and how it, silver, copper, and .other minerals get into the quartz. A lecture will also be given in connection with gold saving, amalgamation, chlorination, and roasting quartz. •