for a long while a subject of conjecture what it could mean. Finally, some few hundred feet from this point of change in grade a dyke was opened which carries arsenical copper in considerable quantities, and upon examining the course of it I found it to intersect this point in the terrace.

This change of grade is not noticeable on the northern terrace, from the fact that it is too much cut up by the ravines to be traceable at the point where the course of the dyke would intersect it. The dyke may not have extended so far, but the fact that some arsenical ore was once found in explorations near the top of the north hill, would lead us to conjecture that it did extend for a long distance in that direction, and if so, it would probably pass between the Franklin and Mesnard mines.

The great main fracture of the range, which exists under the bed of the lake, would most likely be accompanied by minor ones, near at hand, and these we find on the hillsides, cutting through the Quincy, Hancock, and Sheldon-Columbian mines. These belong undoubtedly also to the most recent class of veins, and have so far proved worthless.

You will thus perceive that the study of the topography of these terraces becomes a valuable element to the engineer in this district. After proceeding to the southwest from Portage Lake about seven or eight miles, we enter into a comparatively unexplored region, which extends a distance of many miles before we reach the real Ontonagon district. This unexplored section, if we may judge from the irregularity of the hills-only here and there appearing to form range lines-is very much disturbed beneath, and much broken by heavy faults.

In the Ontonagon district, there are a number of wide and deep passes in the range, through which flow the rivers Fire Steel, Flint Steel, and Ontonagon, unwatering the country lying to the south. An examination of these mountain ridge blocks-if I may so designate them-between the passes, will show that at the northeast end of the blocks the strata have a much flatter dip than at the southwest end. Therefore, if we trace a belt towards the southwest, we will, in descending the southwest end of the hill, have to make an offset to the left to find the continuation of the belt on the southwest side of the gap.

This, as it were, oblique elevation, has apparently been the cause of the mixed character of the deposits of this district, since both strata and fissure lodes have been found workable. The fissures,

however, do not, as in the Point district, intersect the strata at right angles, but obliquely, and seem more the result of the warping of the strata in the blocks into which the ridge has been broken. The most conspicuous and best known example is that of the section. between the Flint Steel and Ontonagon River gaps.

At the north or Flint Steel end of the mountain the rocks dip at an angle of 20° to 25° to the north, and increase gradually until, at the south or National mine, they have attained to 50° to 60°.

Any sliding motion between the strata which would take place during the elevation, would be apt to be along the smooth surface of the sedimentary or conglomerate and sandstone beds; hence the great contact or conglomerate lode which produced the famous Minnesota mine.

The process of warping of the more rigid beds of trap was most likely the cause of the oblique fracture known as the North Minnesota lode.

We can easily illustrate this whole case by taking hold of the leaves of an open book by one corner and raising them slightly, the lenticular openings between the leaves will then represent the receptacle or lodes. Viewing the subject in this light caused me, in the year 1859, after having made a close examination of the mine, to express the opinion that they were fast approaching the limits of the deposit, although the mine was then making its largest returns. The end came even sooner than I had expected.

I have now given the leading features, in the topography above and below ground, of the three sections or districts, and have endeavored to show the cause of the variety in the nature of the deposits. In the first we have a series of fissure veins, resulting from the elevating force, being apparently a central and local one; in the second, it seems to have been a uniform raising of the range as a whole, therefore causing no fractures; and in the third, to have been a moving force traversing the range obliquely.

These speculations may be vague and very imperfect, but at the same time may form a sufficient basis upon which to build others when the region has been better explored, and affords more data with which to supply the deficiency. I might enlarge upon the subject by pointing out some interesting local features of the drift deposits, and the elevations and dislocations which took place before and after these deposits; but that would lead me away from my subject, which was to point out the value to the mining engineer of neglect

ing nothing whilst doing the preliminary work in new ground, namely, making the surveys.

THE USE AND ADVANTAGES OF THE PROP SCREW-JACK.

BY E. GAUJOT, M.E.

(WITH FIGURES I-IV, PLATE I.)

IN connection with the question of coal waste and economy in mining, we would call the attention of those interested to an apparatus invented by M. Dernencourt, Superintendent of the Anzin Division of the Anzin Coal Company, North of France.

This apparatus is known in France and Belgium under the name of prop screw-jack (vis botte), and has been used with good results in the above-named countries for some years past.

M. Ponson gives, in his "Traité de l'exploitation des Mines," a description of it (page 533), but as the description given is old, the work not widely known in this country, and as some valuable improvements have been made since, I thought it might be useful to give a description of it, having seen it in operation in 1867, while in Europe to attend the great Exposition, and on a scientific tour to the mining districts of France, Germany, Belgium, and Italy.

The prop screw-jack is composed of the following pieces:

I. The body or column B, Fig. 4 [Plate (1)], of oak or yellow pine, sawed square, the bark taken off, and any length, proportionate to the vein. The smallest are 10 inches diameter at the base, and 9 inches at the top, the height being from 12 to 15 inches less than the size of the vein. In the axis of this column, and at the top end, is a cylindrical opening or hole, C, 23 inches diameter, 10 inches deep, for the screw, S.

II. Two wrought-iron rings, R R, to strengthen the two extremities of the column; the rings must be put on hot.

III. A cast-iron washer, E, with hole in centre a little larger than the screw, to be put on the top of the column, and on which the nut is to rest; the top side of this washer must be faced on the lathe, also the bottom of the nut, to prevent friction.

IV. The screw, S, having a total length of 12 inches and a diameter of 2 inches (for very high veins this diameter may be increased to 3 inches), and has on its top a wrought-iron plate, T,

5 by 10 inches, and 1 inch thick; the screw must be made of firstclass hammered charcoal-iron.

V. The nut, N, also of the same iron.

VI. A plank, P, about six feet long, 1 foot wide, 2 inches thick. VII. A piece of plank, P', 12 by 12, and 2 inches thick. VIII. The independent ring, O, 2 by & inch iron; the inside diameter must be inch larger than the base of the column. IX. The wrench, W, to work the nut.

To bring the apparatus in use you work as follows:

The column being placed perpendicularly to the roof, receives the washer, E; the screw with the nut on it is placed in the opening, C, the nut resting on the washer, E; the miner turns by hand the nut while holding firm the screw, which brings the screw towards the roof, a space being left to receive the planks P' and the plank P, which is firmly tightened to the roof with the wrench. See Fig. 1.

The screw-jack, once in place, holds the roof more firmly than the usual way of timbering.

The loose ring, O, is used to take down the apparatus.

The pressure of the roof is so great that it required three or four men to unscrew the jack; to overcome this, M. Dernencourt puts a dirt mattress or cushion under the column; the loose ring is slipped down, and holds the dirt under the base of the timber. When the jack is to be taken down, the ring being lifted up, the dirt will run out, or can easily be removed, and the apparatus is loosened without any screwing, jerking, hammering, etc., etc.

It is understood that this apparatus cannot be used in gangways or permanent openings, nor in a vein pitching more than 40°.

Figure 2 shows how they are placed in a flat vein and long wall. Figure 3 shows how they are placed in a vein pitching at 30°. The advantages are incontestable for all workings where they can be used, particularly in long-wall work, where you fill up behind you; the filling or stowage, where the jacks are used, is safer, better, and done quicker than with the common timbering, as it leaves the whole space open, having no props in the way.

M. Fayes, at the mines of Bernissart (Belgium), after having used them for several months, gives the following account: "The economy realized during the month of April is 1 franc 25 centimes per miner per day. In the ordinary way of timbering, the miner spent one-fifth of his day in preparing the timber and putting it in

place; by this new method, it is reduced to one-tenth; therefore one-tenth more of his time can be employed in cutting coal."

M. Cornet, the well-known director of the mines of Sars Longchamps (Belgium), read the following before the Société des Ingenieurs du Hainaut: "The use of the prop screw-jacks does away with timbering in all veins below 40° pitch. The economy varies according to the quantity of timber used in the ordinary way, and increases with the opening of the vein. At Sars Longchamps it is 1 franc per day per miner; but, with the direct results, we must not forget the indirect ones, which are of some importance, as the filling up can be done better, closer, more compact than when props are left. From this will result more safety for the galleries, less room for the accumulation of gases, and therefore a better ventilation. With it the miner furnishes more coal per day and enjoys more safety; as the prop screw-jacks are put in place and taken down quicker, without any hammering, falls are nearly impossible. It does away with the hauling of timber in the gangways and shafts, by which considerable time is lost. At Sars Longchamps (where both methods of timbering are in use in different gangways) the results have been to furnish the coal from the chambers with prop screw-jacks, one-half hour sooner at the bottom of the shaft than the coal from chambers in which the old way of timbering is in use. To conclude, I would say that experience has shown, in the apparatus invented by M. Dernencourt, the following advantages: economy in timber, greater security for the miner, to lessen the quantity of rubbish to be extracted, to give the miner more time for cutting coal."