area of the two after etching. The upper part of the photograph consists of chalcocite, which is composed of grains each with its own orientation, while the lower part is bornite with its peculiar decomposition cracks. The pyrite was the first mineral to form and was afterwards fractured and eroded and then around it the bornite was deposited. The last to form was chalcocite, which is seen to cut the bornite in veinlets and stringers.

A recent paper on the "Paragenesis of the Cobalt-Nickel Arsenides and Silver Deposits of Temiskaming" works out the order of deposition by metallographic means. First of all came the smaltite, then the niccolite. These were fractured, after which followed the deposition of calcite. Next came argentite, and lastly native silver.

The above examples are given merely to illustrate the results obtained in this method of work, which ought to find a wide application, especially in those cases where the order of formation cannot be distinguished with certainty in the hand specimen.

In conclusion I have to express my indebtedness to the Carnegie Institution of Washington whose grant for apparatus made it possible for work to be done along this line.

* Campbell and Knight, Eng. and Min. Jour., June 9, 1906.

STRESSES IN THE COUNTRY ROCK AS THE CAUSE OF AIR BLASTS IN THE MINES AT

PRIBRAM, BOHEMIA.*

BY HUGO STEFAN,

GENERAL MANAGER.

In many coal mines air blasts are a phenomenon of not uncommon occurrence and more exact knowledge as to their geological relations is desirable in order to secure greater safety in the working conditions underground. They are by no means confined to collieries, however, and have long been known in metalliferous mines as well. Their nature and mode of occurrence can only be fully elucidated by correlating as many observations as possible from various places and persons.

DYNAMIC PROCESSES.

Stresses in the country rock may develop from chemical as well as from mechanical causes, although in the case under discussion the latter alone are of importance.

. Assuming that there be at any point in the earth's crust a strain such as would tend to buckle the rocks in which it occurs, it will vary in its effects according to the physical characteristics of the different strata; thus, mild and plastic clays will behave very differently under tension than harder and more brittle coal. Conditions prevailing near the surface, moreover, are quite different from those at great depths. If the buckling force is brougth to bear on a rock stratum at great depth, the direction of the movement taking place in the folding of such a layer will be opposite to that in which the force of gravity acts on the rock. Near the surface of the earth equilibrium of these stresses has long since been established, partly because the original superincumbent strata have been removed by denudation or else because deep weathering or disintegration has loosened the ground and so allowed it to yield readily. As long as any bed in a formation maintains its solid relationship to others, it is in a position to withstand much

* Translated for the School of Mines Quarterly by C. R. Corning from Oesterreichische Zeitschrift, 1906, p. 256.

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greater forces than after having been disturbed or loosened. These conditions, which are so frequently found in mines, are similar to those prevailing in a rod or column under longitudinal compression. Such a rod, fixed only at its two ends, will be bent in proportion to a force:

If it be firmly fixed at 1, 2 or 3 equidistant points along its length, 4, 9 or 16 times the above force will be necessary to produce the same result.

When the equilibrium of layers of rock is disturbed by underground workings and sudden changes of strain produced, breaks may occur with irresistable force, frequently resulting in the phenomena termed "air blasts." In support of this statement the following paragraph may be cited from F. R. von Hauer: Geology of the Austro-Hungarian Monarchy.

"Interesting observations on what may be termed a latent pressure to which beds of rock in their natural position in the earth's crust are sometimes subjected, and concerning the sudden breaking apart of such beds, have been published by Professor Niles of Boston. He noticed in a number of quarries in the United States that on loosening large sheets of rock from their original position, they underwent considerable expansion. Actual measurements showed an increase in length of at least one in a thousand. It appears that this expansion occurred in a north and south, not in an east and west direction. In a limestone quarry at Lamont, twenty miles south of Chicago, on stripping off the surface, an underlying stratum of rock was exposed, which afterward buckled slightly, forming a wave-shaped crest or anticlinal, the ridge of which extended east and west for over 800 feet. The height of the anticlinal was 6 to 8 inches and its width about 18 feet. Later on a longitudinal fissure formed with a loud explosion along the crest of this fold. The buckling was evidently the result of a horizontal pressure, in a north and south direction, to which the strata yielded as soon as their over-burden had been removed."

Aside from chemical processes, therefore, the cause of air blasts may be assumed to be mainly a combination, in varying proportion, of several factors: (1) The composition, structure and cohesion of the rocky strata. These present varying resistance to pressure, together with conditions permitting the equalizing of internal strains. (2) Depth below the surface. Here the weight of the over-burden of rock tends to prevent the stress from being relieved by buckling. (3) The advance of surface weathering, if very extensive, produces elasticity in the over-burden, which is then easily forced aside or upwards by any tendency on the part of underlying formations to form anticlines.

LOCAL GEOLOGY.

At Pribram a number of silver and lead veins are worked. They are frequently associated with greenstone and diabase dikes of various widths and have many stringers. These veins dip from 70° to 90° east or west, and run approximately north and south in a syncline of Cambrian sandstone from 2 to 34 kilometers in width. The northwesterly slope of this syncline dips 80°, while the southwesterly side is quite flat. The Maria shaft is 350 meters west of the bottom of the syncline. At a depth of 1,009 meters (the thirtieth level) the shaft is within 40 meters of this syncline and cuts it at the thirty-second level at a depth of 1,109 meters, where a vertical fault fissure traverses the horizontal strata so that east of this fissure the beds are 25 centimeters below their position on the western side. The course of the fault fissure is nearly at right angles to the sandstone strata. In the easterly and lower strata there are a number of calcite stringers up to I centimeter in thickness, traversing the strata and starting from, but younger than, the fault fissure itself. The individual beds, which often pinch out, vary greatly in thickness as well as in structure and composition. Very hard silicious grey-wackes alternate with clays, both fine- and coarse-grained, in dense more or less schistose layers.

The diabases, which are an important element in the structure of the formation, have, when fresh, much of the hardness of feldspar; when much altered, they are very soft. It is quite evident that the squeezing of the rocks incidental to the intrusion of the greenstones in the grey-wackes was the cause of much stressing.

The metalliferous veins which occur in the diabase and some

times also in the sandstone, are accompanied by fissures of calcite, and these again by barren fissures showing many slickensides. All of this indicates that considerable movement has taken place. Although the average strike of the fissures themselves is fairly constant, they are in individual cases very crooked and much split up. It is noteworthy that the number of metalliferous veins is much smaller than that of greenstone dikes.

The realization to which these facts lead of the enormous forces which have come into play in this district is greatly strengthened by a study of the so-called Lettenkluft. This fissure cuts off the sandstone syncline to the northwest and is one of the largest fault fissures ever yet opened up by mining operations. It dips 70° to the northwest and adjacent to it are dark, much distorted and compressed slates. The fissure is older than the greenstone, but the mineral veins of the district, even if they do pass through into the slates, very soon become barren and at the same time are deflected to the northeast.

Such relations between stratification, country rock and metalliferous fissures give the entire complex an unusually varied character, so that the various forces to which it has been subjected have produced unequal stresses in its heterogeneous members. This condition of unequal stressing is to be considered one of the causes of air blasts.

PHENOMENA OF AIR BLASTS.

At the Maria shaft there are two distinct conditions producing air blasts:

1. On driving drifts and cross-cuts in the hard greenstone, a crackling sound is frequently noticed. This is attributable to the formation of small fissures and is a warning to the miner, because angular fragments of rock are often thrown off with loud explosions from the face of the drift. Fortunately, these pieces are rarely large, but they are sometimes of sufficient size to cause painful wounds. The cause is probably to be sought in the pressure which the hanging and foot-walls exert in many places on the dike or vein formation. In most cases this pressure is produced at points where the vein changes its strike or dip (Fig. 1). A movement of the hanging or of the foot wall, or of both simultaneously, may then readily bring about a compression of the vein matter. Such movements, as well as irregularity of the fissuring, are