With the low efficiency of stripping observed in some localities, profitable quarrying under such conditions would be almost impossible. As the rock is removed, the area worked out is available for disposal of overburden. The equipment, as shown in Plate I, consists of an inclined bridgelike structure of steel mounted on wheels. On this incline, two independent cable cars on separate tracks are operated with an electric hoist. The cars are loaded with the steam shovel, hauled up the incline, and dumped. The total length of the conveyor is 175 feet, and it places the material 150 feet from the steam shovel. The shovel has a 24-yard dipper, and each car is of 5-yard capacity. The whole machine may be moved forward as the shovel is advanced. The company claims that stripping can be done by this method at an actual working cost of 2 cents a cubic yard, not including overhead charges, depreciation, or interest on the investment, and that with a gang of 9 men 2,000 yards can be handled per day. The equipment is well adapted for stripping back into pits from which all serviceable stone has been removed.

HYDRAULIC STRIPPING.

Hydraulic stripping is practicable only where there is an abundant. water supply and good drainage. If the conditions are favorable it is one of the cheapest methods that can be used.

The usual method of hydraulic stripping is to pump the water through pipes and force it at high pressure through a nozzle directed against the bank. If the water has to be pumped some distance, it has been found advisable, in order to reduce the loss of power by friction, to use pipe of 4 or 5 inch diameter, close to the pumping station, and to reduce the size at successive intervals. The jet from the nozzle is usually so directed as to undermine the material and let it fall in a loose mass. It is then easily carried away in the stream. It is obvious that bowlders too large to be washed away retard the process, and if the material contains many bowlders the use of a steam shovel may be preferable.

In a quarry in northeastern Missouri where this method is employed a ravine affords a means of drainage back from the quarry face. As the material is rather hard holes 8 to 10 feet deep are bored with a soil auger, and the ground is loosened with small charges of explosive. The cost of stripping is said to be about 7 cents a cubic yard.

In an Indiana quarry stripping with steam shovels is supplemented with hydraulic stripping during seasons when the water supply is adequate. The material is washed down a channel, which is nearly parallel with the quarry face, and into a creek. A settling basin has been provided, but complaints are sometimes made of the turbidity

of the stream below the basin. The effects of hydraulic stripping may be far reaching, and the operator must carefully consider a possible conflict with other interests.

In one quarry in eastern Missouri the conditions are unusual. The quarry is situated on a bluff facing the river. The water supply is therefore abundant, but the drainage is toward the quarry face, whereas in most localities where hydraulic stripping is practiced the water may be drained away from the quarry pit. Owing to this condition the soil-laden water must be conveyed through an aqueduct from the quarry face to the river bank. However, the operators claim that the cost of hydraulic stripping, including the construction and maintenance of the aqueduct, is considerably lower than with steam shovels.

At one Virginia plant, where the hydraulic method is employed successfully, the quarry faces a river. The overburden consists of red clay and sand, varying in thickness from 2 to 15 feet. Clay seams and clay pockets are abundant. Stripping is conducted during the winter months, when the resulting turbidity of the stream causes the least inconvenience to other industries. Work on rock quarrying is suspended during the stripping. The rock surface dips toward the river, and the overlying material is washed down onto the quarry floor. A row of barrels filled with earth placed near the river bank serves as a dam or weir to hold back the bulk of the stripped material, which would otherwise obstruct the stream. This material, after it becomes comparatively dry, is loaded into cars with a steam shovel and removed to a dump.

To those unfamiliar with the conditions in this quarry, it would seem more reasonable to remove the overburden with a steam shovel in the first place. The rock surface is, however, so deeply eroded that steam-shovel work would be impossible. Plate II, A, illustrates the extreme irregularity of the stripped surface. This view is an excellent example of the effectiveness of the hydraulic method in removing soil from erosion cavities.

ROCK WASHING.

In some localities quarrying is made difficult by open seams or pockets that extend many feet down into the rock. The material with which these erosion cavities are filled may be undesirable as a cement constituent. The hydraulic method of cleaning out such cavities is very successful where it can be employed. The usual method, however, is to clean the seams as well as possible by slow and laborious hand methods. The material that can not be thus removed is shot down with the rock. The subsequent sorting which is necessary makes it undesirable to use a steam shovel, and hand-loading methods are

employed. When the good rock is removed the fine débris is loaded by hand into cars and removed to a dumping ground. The high cost of loading both rock and waste by hand would seem to justify the introduction of a more economical method.

The writer is of the opinion that in many quarries where such conditions are found a much better method would be to load with a steam shovel, taking rock and soil indiscriminately, and after crushing the material, pass it through washing screens. A sufficient water supply, of course, would be essential. The cost of installing a washing equipment would not be high. With sandy or other friable material the success of such a method would seem to be assured. It would not, however, work as successfully with sticky clay, which would be difficult to wash out. Also the crushing of rock mixed with clay may involve great loss of time in wet weather.

UTILIZATION OF STRIPPING.

As a rule the overburden of limestone deposits is of clay and can be used as one of the necessary constituents of Portland cement. Therefore in many localities part or all of the overburden is utilized for this purpose.

The material removed may also be used to fill in swamps, ravines, or other low places and thus render such areas available for agriculture or building. In one instance the reclaiming of swamp land for city lots brought in a return to the quarry company more than sufficient to pay the cost of stripping. The quarry operator may also utilize stripped material for making dams, roadways, or railway embankments on his property.

DRILLING.

RELATION OF DRILLING TO BLASTING.

Only certain phases of the subject of drilling can be considered independently of blasting, for the depth, size, spacing, and arrangement of drill holes are all related more or less closely to the character of the explosive and the method of blasting employed. In this chapter, therefore, the types of drills used and facts relating to their actual operation will be considered, other matters being reserved for subsequent consideration under "Blasting" (pp. 46 to 97).

PRIMARY AND SECONDARY DRILLING.

Two distinct processes of drilling are employed in practically all quarries where rock is obtained for cement manufacture. One, two, or more rows of holes are first drilled in the undisturbed bedrock,