tioned in a building where they can be surrounded, or in such a position as would place them in a state of siege by the mob.

Case II. When the troops are required to go to a distant place, a remote city, controlled by a mob, the movements would be somewhat similar upon arriving on the ground. The following points should, however, be carefully considered :

The troops would, in all probability, be transported by rail. They should be thoroughly armed and equipped as in the previous case. The men should have their overcoats and blankets, and be supplied with rations and ammunition. Transportation should be furnished for the horses of the cavalry, artillery, and mounted officers, and in general it may be said that the command should be prepared for a campaign, and be able to rely upon its own resources. This would make the men independent, comfortable, and capable of enduring privation. The experience of all wars demonstrates this fact, that the efficiency of troops is very greatly increased by their being properly clothed and fed.

In moving troops by rail through a country likely to be hostile, great care and extraordinary precautions should be taken. The possibility of accident to the trains containing the main body of the troops and the horses and baggage should be carefully guarded against.

A special train consisting of a locomotive and one or two cars should be sent in advance. A company of infantry under the command of an experienced officer, and a strong gang of workmen, provided with tools, should be sent with this train. The bridges should be carefully examined, and when one is crossed the advanced guard should halt and wait until the trains containing the main body of the troops come up. This plan of action will prevent the possibility of the bridge being burnt or destroyed by enemies lying in ambush, and who may allow the advance-guard to pass by in order to slip in between it and the main columns.

The several trains containing the main body of the troops and the horses and baggage, etc., should keep as close to each other as safety from accident will permit. In passing through towns where danger is apprehended, a strong advance-guard should be sent in front of the trains. It might also be desirable to have a line of troops march on either side of the cars, and to make dispositions to force a passage or repel an attack. The trains should close up to each other as they pass through the towns on the route, and the men should not be permitted to have any communication with the inhabitants. It is hardly necessary to state that there should be a strong guard with the horses and baggage. A rear-guard is also required.

In passing through tunnels and defiles the utmost precaution is necessary. A body of troops, if taken unawares, in such a position, is in great danger of being destroyed. Before passing through a cut or defile, the General should assure himself that the surrounding heights are not occupied ; if they are, the enemy must be driven from them before the trains are permitted to enter the pass.

A tunnel should not be entered until it is found to be entirely clear, and

Thayer.]

[Dec. 20,

after the passage of the advance-guard, one train only should pass through at a time.

If, in passing through the country, it should be found that the entire population is alarmed and opposed to the passage of the troops, one brigade should not attempt to penetrate any farther, lest a general uprising of the population might occur, and the "line of communication" of the troops from their "base of supplies" being cut, the entire command might be surrounded and captured.

Such precautions as these every capable General will observe. Their neglect has at times caused disaster and ruin.

Under no circumstances should the trains be run directly into the city which is under control of the mob; such action would be in the highest degree imprudent, as the mob would, in all probability, be waiting for the troops at the depots, and by attacking them while in the cars, and unprepared for an assult, great confusion and loss would result, if indeed the entire command should be fortunate enough to escape rout.

Upon nearing the city, the trains should close upon each other and proceed with the utmost caution. Upon arriving within a short march of the city, near the suburbs, and if possible where convenient roads lead into the town, the trains should be halted, the troops including the artillery and cavalry, should be disembarked from the cars, and the several commands be formed.

If three parallel roads or streets lead into the city, a formation similar to that pursued in Case I. can be followed with advantage. That is to say, the Brigade can be formed in three columns and enter the city by three parallel streets, the columns being within easy supporting distance of each other. If this plan is not practicable (and the General can always decide this point, as he will have with him a plan of the city, showing the location of the several streets, etc.), he will be obliged to move in one or two columns. In either case his command must be preceded by an advanceguard, and strong gangs of workmen, capable of leveling any obstructions that may be met with. If possible "flankers," consisting of small bodies of men, should be thrown out upon both flanks, their commanding officer being instructed to notify the General as soon as the position of the mob in the city is found. He will thus be enabled to make his dispositions intelligently, and prepare for the attack.

In entering into the thickly built up portion of the city, it may be found that the houses on either side of the streets through which the troops must pass are occupied by the mob, who begin firing on the troops. If such a state of affairs should be found, the General must immediately halt his command, and detail a certain portion of it to clear the houses on either side of his way. Infantry only is serviceable for this purpose, and if the mob is determined in its resistance, severe fighting will have to be done. If the houses are detached and standing alone, they should be captured by surrounding them; if contiguous, and vigorously defended, a passage may be made from one to the other by breaking through the separating walls, meeting the enemy hand to hand, and compelling his submission.

1878.]

[Thayer. In no case should the General move his command forward while he is exposed to a flank fire from the houses on either side of the street. In this case the same rule is applicable as is prescribed for the passage of defiles, viz. first, clear the enemy from the surrounding heights before entering the pass. A violation of this rule may lead to serious results.

The houses on either side of the streets being cleared, the General can make the same dispositions as were applicable in Case I. If the mob should resist his progress in front, the skirmishers that were deployed in front of the line of battle should immediately open fire upon the enemy's position, and protecting themselves by cover as much as possible, endeavor to silence his fire. If this is successful, a charge may be made upon him in force. A strong reserve should also be kept in the rear, which can be moved to any point that may be threatened.

After the mob has been dispersed the troops should boldly take possession of a commanding position in the town and await further developments. Under no circumstances should the troops be shut up in a building where they can be besieged and their "base of supplies" be cut off. Experience has shown the folly of such action. Troops without water and food are quickly overcome, and they should not be placed in a position where such a misfortune can occur.

If it is possible that the mob may reassemble in great numbers and return to attack the troops in their position, with the intention of driving them from the place, the position should be at once fortified by throwing up earthworks and barricades. In the construction of these defenses the workmen before referred to will be found of great service. Barricades can be constructed of anything that may be at hand. Paving-stones, wagons, carts, furniture, bedding, etc., can be used. The artillery should be placed where it will sweep the ground in front of the defenses. The troops should then calmly await the approach of the mob, and upon its arrival within about one hundred yards, simultaneously pour upon it a fire that will destroy it and prevent the possibility of another attack.

This fact should be remembered, that as a general rule in these cases a display of weakness or hesitation on the part of the troops or their commanding officers will proportionately augment the courage and numbers of the mob and incite it to acts of violence. Bold and resolute action, when action is necessary, will in the end save much bloodshed and prevent great destruction of property.

It should be observed that in the consideration of this subject, one brigade of troops only has been considered. Should it be found necessary to employ more than one brigade, a division may be used advantageously. The general movements of the troops, and the plan of action to be followed, will be substantially the same whether a brigade or a division be employed, although in the latter case the movements will be on a more extended scale. A strong display of a well-disciplined and skillfully-handled force will in most instances be sufficient in itself to suppress the mob.

Philadelphia, Dec., 1878.

Contribution to the Lithology of Pennsylvania.

On the Physical and Chemical Characteristics of a Trap occurring at Williamson's Point. By Persifor Frazer, Jr. (1 colored plate.)

(Read before the American Philosophical Society, Dec. 20th, 1878.)

A thin vein of trap intersects the chloritic rocks at Williamson's Point, on the Susquehanna River in Lancaster Co. Pennsylvania, and near the Maryland line.

This trap dyke which cuts through the hard quartzose and chloritic rock at Williamson's Point is peculiar in its isolation from known rocks of igneous origin; in the manner in which it is foliated transversely to its contact planes; and in its disappearing on its under side in a feather edge. Its upper continuation is now obscure from the denudation of the rocks which it intersects, but as far as it can be followed it widens in an upward direction, and the uneven facade of rock against which it appears gives it the semblance of being dislocated in places, but this is a deception of the judgment.

The rocks are here twisted in a most extraordinary manner, and this twisting is more remarkable just south of the position of the dyke. A very fine specimen of a portion of this vein, with both walls distinct and attached on one side to the rock which it intersected, is No. 1760 in the collection of the Geological Survey.

An examination of this specimen will reveal the fact that the fissure has not been exactly along planes of lamination, but truncates the tops of several small waves into which the strata have been forced.

A specimen of this trap was obtained and reduced to a thin section, of which a representation as seen under a power of 400 diameters and in polarized light has been very faithfully made by Mr. Faber.

It was not found expedient in this drawing to imitate exactly all the details in any one field of view, but the more characteristic exponents of the minute crystals were brought together from all parts of the slide and substituted for those less perfectly formed; due regard being had always to the proportions in which the several constituents of the mass manifested themselves.

In the centre of the field is a large double, or multiple, columnar crystal of labradorite to the bottom, and to the middle of which other smaller crystals are attached-whether accidentally in contact or an off shoot in the former case is not certain.

In the upper left hand portion of the field a curious instance of the splitting of labradorite may be observed. It was at first thought that the apparent divergent curvature of the two branches of this crystal might be an optical delusion, and that in reality two independent individuals were thus accidentally in contact at one extremity. Under higher powers than that here given, however, it proved to be an actual ramification of the mass from one common stock like the growth of twigs from the same branch. The other labradorite crystals will be easily distinguished by the eye,