Mining.

Otago Witness, Issue 1342, 18 August 1877, Page 3

Mining.

INVESTIGATIONS OF MINING IN CALIFORNIA. .

Mr G. Thureaui who went from Victoria to California for the purpose of obtaining information on mining subjects, for the Sandhurst School of Mines, has forwarded the following notes of his researches :—: — In my endeavours to give a concise and clear account of my investigations amongst the mines of Calif ornia, and, later on, of Nevada, I would take this opportunity of stating that the work actually performed by me will not be adequately represented in these monthly reports, especially as they are unaccompanied "by the many diagrams I am making, both of geological features observed above and under ground, and also of the machinery, and I shall, therefore, reserve to myself the issue of a more comprehensive report, with such diagrams, shortly after my return to old Bendigo. Owing to the introductions I was favoured with, I have experienced not the slightest trouble in my mission, and I have not been, so far, obliged to offer inducements of any kind to anybody in order to obtain information, after the nature of my mission became known through the Press. I have been treated as a brother mining engineer frem Australia by the American superintendents in charge of mines in this State. My time being limited — in fact, I cannot see how I can fulfil or carry out the conditions imposed in the short time allowed, and to meet my engagements already entered upon for the next three or four weeka to come— l shall confine myself to the description of "a blue gravel mine," and modus operandi adopted by means of which its superintendent obtains not less than 7^ per cent, profit every month upon the capital stock of the company. This mine is situated about 25 miles north-east from this place—Grass Valley — and we reached it at noon in a pair of two-horse buggies engaged for the purpose. There I was left with Mr. Perkins, the superintendent, whilst the judge and the other gentlemen went further on, to start some drift mining on the old Ballarat plan.

HYDRATTLTC SLTJICING.

The North Bloomfield Gravel-mining Company's mine is located aboub 1200 ft above the sea level, upon the well proved line of an ancient (diluvial) river bed filled in by two distinct deposits of gravel. The lower of "blue" is richer than the more recent, which is also of a lighter colour, and it is likewise distinctly separated by a tough bed of pipeclay. In some places this bed is covered with flows of basalt almost identical with that over our deep leads in Victoria, but contrary to our experience, this river bed is even richer at its intersection with the Sacramento River, 40 miles below its source in the Sierras than at any place higher up. The working of this immense gold-bear-ing deposit — for I saw myself not less than 340 ft of a vertical bank of this gravel — is gradually brought to such a perfection that close upon 90 per cent, of the total contents of the gravel in gold areobtained In view of the approaching completion of the Coliban scheme for similar purposes, the comparison of what a few individuals here have done and are still doing should be interesting if not valuable, for, with water, there appears no obstacle whatever on Bendigo, (Jastlemaine, and elsewhere to be able to achieve similar results from our deposits far less in height. This company constructed a reservoir near where the perennial snow clothes the Sierras, which affords a good and continuous supply of water some 40 miles above the works. After repeated failures with dam banks constructed of earth and puddle gutters, their dam was built on a new principle, and has stood now for over eight years. It is 97ft high and 400 ft ia length, and

consists of large rocks closely piled upon each other, the interstices being filled with smaller rubble, and in this manner the bank has been carried up, so as to leave a narrow gangway on its apex. To make it watertight, a series of 6in square sticks, placed vertically, were laid upon the water side, secured to the inner wall, and upon these a 2in planking, tongued and grooved, was fixed horizontally, and this completed the work. It is claimed, and certainly with good reason, that these kind of dam tanks do not shrink, as is the case with earth tanks, ia which the clay gutter shrinks unequally. They are watertight, and if a leak occurs, the water percolates through the crevices of this stone bank without endangering as it would otherwise, the whole structure. There is a tunnel in the dam, and a gangway constructed above the water issuing from the valve inside, which permits the outlet to be controlled as required. This reservoir holds, when full (within 2ft of the top) 920,000,000 cubic feet of water, and it costs to build but 35,000d01. The ditches or races in connection therewith have such a capacity as to deliver 44 million gallons of water daily a distance of forty miles, which is equal to 3200 Calif ornian "miners inches, "into three main distributingpipes, each of which commences with a diameter of 30in decreasing, however, to 15in diameter near the works, and these again supply each three or four "nozzles," as need, be, which latter measure from four to nine inches at their delivery. As regards these large pipes, I think we could advantageously adopt the Californian style as the outcome of many years' study and experience, because by so doing we would obtain the same results as now, and that at very much less cost. Hitherto all conduit pipes outside our cities, and at the inverted syphon near Taradale, have been made from cast-iron in 9ft lengths, the metal being nearly 2in think, I believe, and consequently large amounts of money have been spent to obtain such pipe 3 outside the colony. Now, I don't recollect the exact height of pressure in all those Victorian cases, but assume a pressure of 375 ft as a very good test for such cast-iron pipes. Now, the hydraulic miner on these slopes avoids, as much as possible, cast-iron in any shape or form, except for valve seats, &c. The pipes, bends, junctions, are all made in the vicinity of sheet iron of the best Swedish description. These pipes are rivetted together in 16ft lengths ; they have no flanges, but are simply telescoped into each other, and three-sixtenths of an inch iron is being used for pipes over 32 inches diameter, which have been tested up to a pressure of 4001b per square inch. Two men can easily handle a length of these pipes, whereas the 9ft length cast-iron ones in the colony require strong tackle to place them into their position whilst being joined together. The sheet-iron is imported. Rolling mills are constructed in the mining centres, and the rivetting of all pipea above 6in diameter made to order, thus saving large sums of money which otherwise would have to be sent abroad. I give an instance. At Cherokee flat a pipe made of one-tenth of an inch sheet-iron, 30in in diameter, 13,000 ft in length, receiving a head of water 180f fc in height, crosses a valley 900 ft deep, and was put in place for 75,000 dollars, and it delivers 4,395,000 cubic feet of water in 24 hours, thus resembling somewhat our Back Creek syphon, at much less cost and more effective. As will be seen further on, the ditches of the Coliban 3cheme will not deliver sufficient water at low rates, with the one cast-iron pipe there. The experiment might be tried to use a sheet-iron one in the place already provided, under certain precautions. In these ditches the grade of descent varies from 12 to 20 per cent., and the banks were not worn down as expected, whilst a lesser grade would not deliver enough water. What about the Coliban race ? Is it less than 8 per cent, on average 1 The water having acquired in its descent from the main pipes such a velocity and so great a pressure, issues with great force from the nozzles, which latter resemble those used by the firemen, only they are much longer and larger. These nozzles are erected in the lowest portion of the river bed, so as to command the gravel banks around, and they can be directed at almost any angle towards the former. By means of ingeniously constructed joints and balance weights I could lift and direct a 9in nozzle anywhere, and what required seven or eight men formerly is done by one miner at present. The jet of water, 9in thick, strikes the opposing bank, 340 ft in height, at a distance of 400 ft. The effect upon the gravel is remarkable, for large rounded quartz boulders, over half a ton in weight, are trundled along by the stream just the same as balls in a bowling alley ; and others, from 301b to 1001b, are tossed into the air for 30f fc high or more, thus removing obstacles that interfere with this hydraulic sluicing. Where there are from four to six such like jets in play, the torrent caused seeks an outlet, and this is provided generally by a tunnel driven below the bed of the ancient river, and through the "rim country," i.e., the reef or bed rock or bank at each side. In this case such a tunnel was driven (from an adjacent " canon " gorge, or river, eroded since the formation of the ancient wash), through the bed-rock for the length of one and a-half miles ; eight shafts, with machinery for pumping and winding, assisted much in the project, and on completion it was found to have cost $490,000. This tunnel, whilst providing an outlet both for the water and the displaced debris, is made also to serve the purposes, in the first instance, of saving the gold retained, or rather deposited on the bed rock primarily, but subsequently washed

down the chasm — some 40ft deep — which forms the entrance to the tunnel from the gravel beds. This blue gravel especially is interesting, on account of the many fragments of various rocks it contains, besides trunks of trees totally mineralised, and the year rings of which are composed of iron pyrites, thus in their concentrate forms, giving a very peculiar appearance to these remnants of those ancient forests. Boulders from half a hundred-weight to 20 tons abound. They are all rounded off at their edges, and they present quartz, granite, syenite, marble, metamorphic schists, and others, thus presenting a fine field for the geologist. The means adopted to save the gold are of two kinds, viz., by means of peculiarly constructed " ripples," and by means of artificial " under. currents." As regards the first method, the tunnel, as already stated, is made a receptacle for the coarser gold, and this is accomplished through placing upon a prepared incline, having a grade of 4£ per cent., and in a tunnel of 6 feet wide, a series of square blocks of soft pine wood for a height of 18 inches, and as these blocks are not permitted to join close, a continuous series of rectangular and lateral crevices is the result, in which primarily the quicksilver, used largely in the operations, finds its way, and subsequently the gold, which comes down in the commingled mass of water, sand, and gravel, with boulders the size of whim buckets. These ripples are continued for a length of 1500 feet, and every morning they are carefully examined and repaired, because the displacement of one block frequently causes the destruction of many feefc in length. The gravel, &c, having travelled at its headlong rate through the remaining portions of the tunnel, at last reaches — as quite a homogeneous mass, for all the cemented portions of the gravel have by this time become totally disintegrated from friction — the last resource for saving the fine gold still retained. The " under currents," as they are termed, comprise three distinct operations, viz., the saving of gold in the main or ground sluices, the bottom of which is paved with the hardest quartz or syenite boulders obtainable to act as ripples, and some idea of the force and action of the current may be formed when it is considered that every three months or less these beds of boulders are worn down, ten to fifteen inches, necessitating a renewal. Up to this point the operations are of considerable danger to the employes, because, as the superintendent quaintly observed— " With the jets from the nozzles, a man struck by either of them wouldn't know what hit him ! " and at the main sluices it appeared a Chinaman recently lost his footing and tumbled into thesurging torrent of water and boulders, and after a long search not a " shred " of his remains were ever recovered below. The second method of catching fine gold consists in the "sizeing" of the gravel, which operation is effected by means of strong, flat iron bars being fixed on their edges across the sluices. These bars having been fixed so as to leave a short; space between them, separate the finer gravel from the coarser, which former is made to fall on another stone bed, i constructed upon the same principle aa the main or "ground" sluices, whereupon they pass over a kind of "tye," having, however, from 5 to 20 per cent, grade of fall. The coarser gravel, not passing through, rushes on to the next " under-current," of which this company have six beneath each other, and they intend to increase that number to 20, aa they save about 12 per cent, of their total yield of gold. This closes the operation ; but there are some processes, closely allied to the removal of the gravel whilst in situ, that should be noticed, viz., the blue gravel is often very much bound together with iron pyrite, or its resultant after decomposition, ferruginous cement, so well-known in Victoria, and in order to bring this down drives are made, half in the bed-rock and half in the gravel, in the form of aT. The cross arms are filled with as many kegs— from 80 to 1200 501b. kegs — of black powder as are requisite ; then the main drive is rammed as hard as possible, and the wires connected with an electric battery, the result being simply tremendous, for the gravel is shattered thoroughly, and the harder cement ground up fine. Besides this the gravel contains many large boulders, too heavy to be moved by the jets; these are drilled into (by the force of white miners and their subservient Chinamen employed at the mine), and charged with powder in the ordinary way previous to the diurnal firing. During our journey from Nevada city we were frequently much exercised in our minds as to the cause of the occasional roaring noises we heard, which were re-echoed in the mountain recesses, but afterwards it became clear to us that these blasting operations of from 100 to 300 holes together caused these disturbances. As regards the roads, they were well kept after we left Nevada city, but instead of the county or shire councils constructing them, they were built by permission of the county, by private individuals, who charge toll for a certain period. They must have co3t a great deal of money, for on approaching the South Yuba River we had to descend its precipitous banks on zig-zag, fully 1500 ft. vertical, before we could cross the bridge below, and the same on the other side, the points of descent aud ascent being but five miles apart. Whilst describing the proceeds of hydraulic sluicing, as pursued at North Bloomfield and other mines, I mentioned, amongst other things, that close upon 90 per cent, of the contents of gold in the gravel were

obtained in the first-mentioned mine, and since then I have ascertained that this yield fluctuates in accordance with the gravel, as it is removed by the jets from its location either close to the bedrock or above the pipeclay, which separates the upper from the true blue gravel beneath. The upper gravel beds carry a lighter description of gold, which is, therefore, more liable to be lost in the sluicing as it floats ia the lighter sands carried alon:* by the overpowering torrent. The addition of more under-current, is therefore imperative, because by them it is only possible to gather this "float" gold. Comparative calculations have been made, and at Bloomfield it ha* been ascertained that every foot of ancient river bed or gutter, in its sectional area from bank to bank, and from the surface to the bottom, is worth from §1200 to to §1500. And as each nozzle can remove in the ordinary w?,y, from 2 to 2| acres of gravel per annum, there remain many years of work in these Galifornian gravel mines.

Daring the month now ended I have been occupied, besides the usual inspections of the Ohlorination Works near Grass Valley, and machinery employed for the hoisting and reduction of quartz in that neighbourhood, with an extensive tour through Mariposa, Tuolumne, and Calaveras Counties, in order to become thoroughly acquainted with the formation of the "mother lode " of California, together with its numerous side lines, all of •which crop out at the surface, and extend from north to south for hundreds of miles. Doubtless this lode has led to the deposition of gold in the "placers," which surround it on all sides, and this quartz is now receiving increased attention because of the exhaustion of the more limited auriferous deposits. We began our work of investigation at Coulterville, a pretty little towa in Mariposa county, and besides the assistance afforded by Mr F. M'Orellish. proprietor of the Alta CaliiO'rnia — at whose invitation I formed one i>f a party of assayers, superintendents, &o. — the local mining men accorded me Bpecial attention. We reached our destination on the Merced River, some five miles from Coulterville, late at night on the 12th of May, and very early next morning mounted our horses, which were furnished with that peculiar kind of Mexican saddle so comfortable to those used to them, and so thoroughly otherwise to those accustomed to the English saddle, became of the rider in the former case having ali his weight in the wooden stirrups— the lasao-horn alone affording a good hold. A great part of the day was lost through an accident happening to me on the road to the Crown Peak mine, doubtless caused by the ostler supplying me with a stubborn animal, and besides, not tightening the girths properly, which latter circumstance was not noticed in the hurry of our departure, as we had a great deal to do during our stay. lam happy to say that my health in body and mind ia, comparatively speaking, nearly restored, after having received a very severe shock and bruises.