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The examination of a prospectis a very different undertaking from the examination of a mine;prospects are not expected to show ore reserves as a basis forpurchase, and in the last analysis the recommendation of aprospect rests on an opinion rather than demonstrable facts.The examination of a prospect requires that all significantsamples shall be taken and a thorough geological investigation bemade, which need not, however, be put in formal shape unless ityields a favorable result. The question to be answered inexamining a prospect is: "What chance has it to make a mine?"

Preliminary examinations areprecisely what the term implies, and are undertaken to determinethe advisability of making a formal examination. There isalways a reason why a mine is offered for sale, which may or maynot be known from the start; frequently the chief object of thepreliminary examination is to determine this reason for selling.

In the opening of any property two distinct objects should be kept in mind—the development of known ore-bodies, and the exploration for further deposits.

The surface affords a complete section of the geological features of any district, and in all but the simplest occurrences a geological map of the surface and a few vertical sections are invaluable guides in the examination or exploration of any district. All significant outcrops of rocks, dikes, beds, veins, faults, shear zones and so forth should be determined and their strikes, dips, and elevations recorded on a large scale map, from which the data may be referred for study to a horizontal plane.

Gouge-filled fissures appear to be unfavorable to ore deposition, probably chiefly because the pasty gouge does not permit the ready passage of mineralizing solutions, and also because the continual movement along such fissures tends to close any minute channels that are permitted to form. Such fissures once formed probably persist as lines of weakness and movement for long periods, and the occurrence of a gouge-filled fissure in connection with an ore-deposit is no proof that it is of later formation than the ore.

A line of weakness once established is likely to continue as such through long periods of time, the original gouge seam or zone of attrition material being preserved and offering a plane of easy relief to subsequent stresses; where a fissure has been completely healed by mineralization, the resulting vein of brittle quartz or other minerals is likely to be less resistant to fracture than the tougher enclosing rock. It is common, therefore, to find evidence of post-mineral movement along veins.

Where an ore-body has been cut off by a fault the discovery of its continuation beyond the fault is a problem of the greatest importance.

Numerous rules have been formulated to aid in a search for the continuation of a faulted ore-body, but it is safe to say that the great variety and complexity of the results of faulting render such rules valueless in most cases.

Where faulting dislocates a non-tabular ore-body that is contained in a homogeneous rock, the problem is insoluble, unless the fault carries a drag or trail of ore mixed with the fault filling.

A fault is a fracture through a rock mass the opposite walls of which have moved past each other, the word indicating lack of correspondence between opposite walls.

The strike of a fault is the direction of a horizontal line within the plane of the fault; the dip of a fault is the angle between the plane of the fault and a horizontal plane.

Parallel planes of fracture, developed by compressive stresses, of relatively great continuity as compared with j omts, but of only incipient displacement, are called sheeting planes. Closely set and well-developed sheeting planes often afford channels for the circulation of solutions, and, when mineralized, form sheeted lodes. Not infrequently systems of sheeting planes occur in pairs, parallel in strike, but intersecting in dip, such fracturing being the typical result of compressive or torsional strains; these interdependent systems of sheeting planes are known as conjugate systems.

Planes of division through rocks that are the result of stresses insufl&cient to produce more than microscopic movement are called joints. Fragments of broken rock, both sedimentar}' and igneous, are commonly bounded in part by plane surfaces, which are joint planes. Joints are of several kinds, according to the nature of the stresses that produced them; they vary in expression from cleavage, or incipient jointing, to the welldeveloped planes that boimd the prismatic columns of certain basalts.