Galena, which is sometimes so rich in silver as to be worth working for that metal, may often be followed by surface indications; namely, a white limy track with detached fragments of float ore in the surface soil. The blowpipe or fire assay quickly determines silver ore.
Tin in lode, stream, or alluvial deposits occurs only as an oxide, but its appearance is varied. It may be almost any color and shape. It is always near granite, containing white mica known as muscovite.
The minerals for which it is most easily mistaken are:
| Sp. gravity. | Streak. | |
| Wolfram | 7 to 7½ | Red, brown or black. |
| Rutile | 4.2 | Light brown. |
| Tourmaline | 3.2 | Whitish. |
| Black Jack | 4.3 | Yellow, white. |
The magnetic or dipping needle is used in New Jersey, as follows, according to the State Geologist, W. H. Scranton, M.E.: "An attraction which is confined to a very small spot and is lost in passing a few feet from it, is most likely to be caused by a boulder of ore or particles of magnetite with rock. An attraction which continues on steadily in the direction of the strike of the rock for a distance of many feet or rods, indicates a vein of ore; and if it is positive and strongest towards the southwest, it is reasonable to conclude that the vein begins with the attraction there. If the attraction diminishes in going northwest, and finally dies out without becoming negative, it indicates that the vein has continued on without break or ending until too far off to move the compass needle. If, in passing towards the northwest, along the line of attraction, the south pole is drawn down, it indicates the end of the vein or an offset. If, on continuing further, still in the same direction, positive attraction is found, it shows that the vein is not ended, but if no attraction is shown, there is no indication as to the continuance of the ore.
"In crossing veins of ore from southwest to northwest, when the dip of the rock and ore is as usual to the southeast, positive attraction is first observed to come on gradually, and the northwest edge of the vein is indicated by the needle suddenly showing negative attraction just at the point of passing off it. This change of attraction will be less marked as the depth of the vein is greater, or as the strike is nearer north and south. The steadiness and continuance of the attraction is a much better indication of ore than the strength or amount of the attraction. The ore may vary in its susceptibility to the magnetic influence from impurities in its substance; it does vary according to the position in which it lies, that is according to its dip and strike; and it also varies very much according to its distance beneath the surface."
Further instructions are given in the paper from which the foregoing extract was taken, some of which follow:
"It is sufficient to say that the first examinations are made by passing over the ground with the compass in a northwest and southwest direction, at intervals of a few rods, until indications of ore are found. Then the ground should be examined more carefully by crossing the line of attraction at intervals of a few feet, and marking the points upon which observations have been made, and recording the amount of attraction. Observations with the ordinary compass should be made, and the variation of the horizontal needle be noted. In this way materials may soon be accumulated for staking out the line of attraction, or for constructing a map for study or reference.
"After sufficient exploration with the magnetic needle, it still remains to prove the value of the vein by uncovering the ore, examining its quality, measuring the size of the vein, and estimating the cost of mining and marketing it. Uncovering should first be done in trenches dug across the line of attraction, and carried quite down to the rock. When the ore is in this way proved to be of value regular mining may begin. In places where there are offsets in the ore, or where it has been subject to bends, folds, or other irregularities, so that the miner is at fault in what direction to proceed, explorations may be made with the diamond drill."
[1] Dislocation of the strata.