assuming the usual character of under-clay, appears as a very hard compact silicious stone. It is sometimes four or five feet in thickness, and along Padiham Heights is in high requisition as a road material. It is in fact the hardest stone of the neighbourhood, only surpassed in this quality by the Carboniferous limestone of Clithero, and when struck by the hammer causes it to ring. Nevertheless it is a true under-clay or seat-stone; is penetrated in all directions by Stigmaria ficides (the roots of Sigillaria,) and has in all probability become indurated by the gradual infiltration of silex in solution. Indeed we may feel confident that it was not in its present state when the coal plants were rooted into it.

The area of the Burnley coal-basin is about twenty square miles. It contains a very large supply of mineral fuel for future use; probably enough to last for nearly 300 years at the present rate of production. Indeed it is only very recently that its true value has been thoroughly appreciated, as an impression had prevailed amongst the old miners that the coal-seams, along whose outcrop they and their forefathers had been working for generations, did not extend in the direction of the centre of the basin. A fuller light, the result partly of the spread of scientific knowledge, has dispelled this illusion, and the inferences which were fairly to be adduced from the structure of the strata and the margin of the basin, have been verified by actual experiment. It is now fully understood to be a true basin or trough, and the complete series of coal, as given above, has been satisfactorily ascertained by sinking at Burnley.

This district is also noteworthy as the residence of a goodly band of observers of natural phenomena, led and encouraged by Sir J. KayShuttleworth, of Gawthorpe Hall, and it is not the least pleasing fact in connection with the subject, that amongst them may be reckoned several miners and working men, whose knowledge of the paleontology of the Carboniferous rocks would do credit to many who have had the advantage of superior education.

On the Mexican Method of Amalgamation.
BY JAMES NAPIER, JUN., F.C.S.

Late Chemist and Assayer to the Guanaxuato Mint, Mexico.

§ II. GRINDING ORES AND SEPARATING THE GOLD.

In the introductory section I gave a brief sketch of the history of the Patio amalgamation process, and a few analyses shewing the general composition of some of the silver ores in certain of the Mexican mining districts. In the present section I propose describing the methods employed for grinding the ores and separating the gold; it being understood that my description refers particularly to the district of Guanaxuato. As the mode in which the sales of the ores are conducted is characteristic, I may be permitted to give a prefatory notice of it.

Sales of Ores.-Some of the poorer mines of Guanaxuato are worked by what are called "Buscones" (searchers). These are miners

answering to the Cornish "tributers," who work without any definite pay, but receive half of the value of the produce they extract from the mine. At stated times a sale takes place at each concern, when these buscones arrange in separate lots whatever they may have extracted since the previous sale, in the court-yard of the mine. These lots of ore are cunningly laid out, so as to expose the richest portions to the view, some shewing much taste in their arrangement; and each miner stands near his own lot and keeps it constantly sprinkled with water, which has the effect of making the mineral look darker and richer.

or

The various "Rescatadores" (purchasers) take from each lot a small portion of ore, as average a sample as possible, which they have finely ground, and tried or assayed on the spot. This is done by placing about a handful of the ground ore in а "Jicara *" small round bowl, washing away the whole of the earthy particles, and judging from the metallic portion remaining of the richness of the ore. It is really astonishing to mark how by this rude process those who constantly practise it are able to arrive at results so near the truth. However, many purchasers have assays now also made by fire for greater accuracy, and in this case the samples are obtained the day before the sale. These assays are either made by scorification, or by smelting the mineral with red lead, crude carbonate of soda, a little charcoal and salt, and cupelling the resulting buttern of lead.

At the regulated hour for the sale a bell strikes, as a signal that it is about to commence. The person intrusted with the sale takes his place successively at the foot of each lot, and every buyer in turn whispers into his ear what he wishes to give for that parcel. When all have given in a price the lot is called over to the highest bidder, the rate of his bid being named; where two buyers bid the same amount, the first bidder gets the preference. The person acting as auctioneer and the rescatadores thus move from lot to lot till the

whole of the ore in the yard has been disposed of. The weight of these lots of ore is not ascertained by weighing, but has to be judged of by eye. This, like the trying of the ore, is accomplished by those who have had long practice with remarkable accuracy; and, on the whole, one of these sales is rather exciting. The moment a lot of ore is called out to a buyer, the miner has nothing more to do with it ; the purchaser having to remove it from the ore yard to his reduction works, or Haciendas. This is usually done packed in sacks on mules' backs; the ore, if in very large lumps, as it often is, being previously

"There

* Of these vessels Sonneschmidt has the following interesting remarks. is a tree which is found in some parts of the coast of this kingdom, (New Spain), which produces a round or spherical fruit of various sizes; they clean out this in the middle, and take away the pulp which it contains, when two cups called "Jicaras" or "Chacules," are taken out. Those employed for the purpose of assaying are about 7 or 73 inches (Spanish) in diameter, and 3 or 3 in depth. In the village of " Olinalan," where the Indians occupy themselves in the manufac

ture of these vessels, they are coloured inside and out with varnish and colour, and at times they have figures on their exterior. Besides being used for the above purpose of assaying, they are also used for domestic purposes, being found in almost every habitation. For this purpose it matters not what size or colour they are; but for the purpose of assaying they should be black or blue inside, because the red and green deceive the eye."

broken into moderately sized pieces. Each sack holds about 175lbs., and two of them are loaded on each mule.

Rough Stamping.-The first operation to which these ores are subjected is a coarse stamping. This process is carried out in what are termed "morteros" or 66 molinos," which are very similar to the stamps used at the Cornish tin mines. The extent of these morteros, or the number of heads, naturally depends on the extent of the "Hacienda," and they are worked by mule power, excepting in a few districts, where steam or water is employed. The ore, as it is fed to the stamps, is in pieces about the size of the fist; as it is stamped, it falls into a piece of strong hide, perforated with small holes, and fixed in an inclined position. What does not pass through the holes is again returned to the stamps, whilst the finer portion, called "Granza," a very coarse sand, is conveyed to the "arrastres" or fine grinding mills. One stamping mill or battery, with ten stamp heads, worked by six mules which are changed every six hours, is capable of stamping from three o'clock in the morning till seven in the evening forty cargas of fourteen arrobas* each.

Fine Grinding: construction of "Arrastres."-Arrastres, or as they are sometimes also called, "Tahonas," are round, and vary somewhat in size in different places; but those mostly used in the large Haciendas of Guanaxuato have a diameter of 4 varas,t and are called "Arrastres de marca." The annexed drawing, fig. 1, is a

sectional elevation, and fig. 2, a ground plan: these will enable the reader to comprehend the construction of the mills. The bottoms are formed of hard stones, each stone being about from 28 to 30

*An arroba is 25lbs.

† A vara is equal to 33 English inches.

For these drawings I am much indebted to Mr. John Phillips, formerly etary to the Real del Monte Company.

inches long, and about 4 inches thick; they are placed vertically one against the other, as shown at a, the interstices between being filled up with "Cabecilla" (the residue remaining after washing a torta) moistened with water. In the centre of the circle there is a massive

stone, b, which rises somewhat above the floor of the mill. To this is attached, by means of a pivot, an upright shaft c, supported by a cross beam d at the top. The sides of the arrastres are formed of flag-stones e, or in some places planks of hard wood, which rise above the floor about 2 feet. To the upright shaft c are attached two cross bars ff, to which to harness the mules and secure the "Voladoras" or grinding stones h h, which in Guanaxuato are of porphyry; their size being a little less than the radius of the arrastre, and being about 16 inches thick, the same height, and about 2 varas long. In each of these stones two holes are bored, into which are fixed wooden pegs i, to which the stones h h are attached by strings of hide or pieces of chain, j, to the cross bar f. For an arrastre of the dimensions given above, four grinding stones or voladoras are required, and two mules are necessary to work them.

Porphyry appears to be the stone best suited for the purpose of forming grinding-stones, from the fact of its being coarse-grained, and presenting a rough surface to the last. Basalt, which is very plentiful in the district of Real del Monte, is there used for this purpose; but, although sufficiently hard, it very soon becomes smooth, and passes over the ore without having the same grinding effect as porphyry.

As soon as the bottom of an arrastre has been put in, a new "Voladora" is set to work to grind "Cabecilla" mixed with water. This ensures the whole of the interstices being filled up, and is continued for one day, when a second stone is attached. At the end of three days a third is added, when the grinding of poor ores may be gone on with, and after four or five days the fourth stone is added and the fine grinding gone on with.

Fine grinding in Arrastres and extraction of Gold.-In districts such as Guanaxuato, where the ores contain gold as well as silver, the