a stratum was observed to maintain a substantially constant interval below. This lower stratum was but little eroded, and was used to obtain the theoretical position of the eroded portion of the coal-seam. A number of hills, high enough to pierce the seam thus located, were found to contain portions where its presence would not otherwise have been suspected. This indirect method of using parallel strata was also utilized where the seam lay too deep for observation direct, in this case the stratum being above the seam.

Fig. 57 illustrates how parallel strata may be used to corroborate and supplement direct data. This use of parallel strata has been described by M. R. Campbell.1

The direct data of the upper section, Fig. 57, give only three points on the seam, which is everywhere obscured or eroded. If the intervals between I. and II. and II. and III. in the columnar section be established, and if I. and III. be located at various outcrops, as shown in the lower section, then the intervals may be set off from these outcroppings, and will greatly increase our information on the surface to be contoured.

1 Trans., XXVI., 298 (1896).

I wish to acknowledge my indebtedness to Prof. John D. Irving, who was engaged on the work of the Masontown quadrangle, and who suggested this structural contour-method as one of the useful applications of descriptive geometry to miningwork.

CHAPTER X

APPLICATION OF GEOLOGICAL THEORY

THEORIES OF ORE DEPOSITION

Early in the development of the science of geology investigators were concerned more with the broad general problems which engaged their attention upon every side. As these became cleared other fields were entered for more detailed study, and of these perhaps the most attractive is that of the occurrence and origin of ores. This is more especially the case since the search for the metals has continued since the earliest days of the race and has frequently been a powerful motive in the spread of civilization. And again the control of the sources of metallic wealth is recognized as one of the factors necessary to industrial supremacy.

The science of economic geology has therefore developed along practical lines and the geologist engaged in such work occupies the middle ground between the scientist and the practical man. Too frequently he is nearer the latter than the former; but from the work of the Government surveys and the various private surveys it has become quite apparent that the application of geological knowledge to mining problems is one of increasing importance. It would be impossible to take up all the phases of the matter in a book of this kind, but it may be well to point out the lines along which geological considerations may be of great value.

To do this it will first be necessary to review the present theories of ore deposition. In outlining the general principles of the origin of ore bodies it has been necessary to draw upon the beliefs of two more or less opposed schools of thought; naturally

nothing strikingly new or profound is proposed, since the idea is rather to indicate the practical application of existing theories.

Generally speaking, geological history falls into two great groups of processes. Upon the one hand are the forces which tend to build up the land areas and on the other those which tend to erode or destroy them. The formation of ore bodies is a phase of natural activity attendant upon these general forces, and we therefore might expect to find it accompanying the action of those of both types. Thus the discussion falls naturally into two lines of thought each of which has been followed up closely by its adherents. Where there have been disturbances of the earth's outer mass resulting in folding, fracturing and the manifestation of volcanic action we have the chief instances of the upbuilding forces, and the association of ore deposits with areas which have been so affected, was noted by the earliest observers. More closely, regions where igneous rocks are abundant seem to be the more favored localities, and in the great majority of cases we find a relatively clear connection between ore bodies and igneous masses. It has further become apparent that the metals seem to be associated with the acidic and basic extremes of rock types.

Going further into the matter two questions are to be answered. Where did the metals come from? What was the transporting agency? It is upon these questions that the two lines of argument have been developed. In the igneous rocks we find such minerals as magnetite, ilmenite, pyrite, and other sulphides closely associated with the rock minerals and of the same age or period of crystallization. Metallic minerals sometimes segregate to such an extent as to form ore bodies. Hence the adherents of the "magmatic theory" hold that the igneous masses in the depths of the earth are the source of the ores. Both schools agree upon water as the great transporting agency and the supporters of the magmatic theory believe that the water is derived mainly from the same source as the ores, namely the "magmas" or solutions of rock minerals. In confirmation of this view they note the great clouds of steam observed in volcanic eruptions,

and call attention to the fact that deep mining penetrates a dry zone. (Refer.: J. F. Kemp, Transactions A. I. M. E., Vol. XXXI "The Rôle of the Igneous Rocks in the Formation of Veins.")

A rock magma may be regarded as a solution from which the rock-forming mineral compounds crystallize successively as their points of saturation are reached, under varying conditions of temperature and pressure. If such a solution were homogeneous at all stages of its cooling, we would expect to find rocks of the same type always resulting from its solidification. As a matter of fact this is not the case, since lavas of widely different character are derived from the same volcanic vent. This might be explained by assuming the existence of different reservoirs each contributing its particular rock type to the main vent, but it does not seem so reasonable in view of further facts. In the field within comparatively limited areas we sometimes find great variations in the rock which shade into each other by gradual stages; the extreme types would not be recognized as belonging to the same mass were it not for the field relations.

Thus the idea of magmatic "differentiation" or splitting up has been developed. How this takes place has been a subject. for considerable speculation; some theorists have attributed it to differences in specific gravity of the component elements, or differences resulting from partial solidification of the molten mass in the great subterranean reservoirs. Convection currents and molecular flow have been supposed to play a large part in this arrangement of the rock material. Again it has been suggested that the behavior of a rock magma is analogous to the formation of a complex alloy. (Refer.: T. T. Read, Economic Geology, Vol. I, No. 2-"The Phase Rule and Conceptions of Igneous Magmas; Their Bearing on Ore Deposition.")

However this may be, the chief things to bear in mind are that there are successive stages in crystallization, and that toward the end of the process certain elements or minerals may be found in excess of the requirements for the rock of the solidified mass. Associated with this residual material we would expect to find the more volatile compounds-much water vapor, chlorine, fluorine,