to note the various minerals, the succession in which they occur, and associations which may be of economic value.

It would seem to be a simple matter to tell whether the contact between an igneous and a sedimentary formation is intrusive or a case of the latter simply overlying the former. But in

FIG. 23.-Irregular boundaries are usually the rule for intrusive bodies. This is particularly true of the porphyries and finer textured intrusives.

+

Boundary showing Contact Metamorphism.
Ls Limestone.

Di-Diorite.

M.Di Metamorphosed Diorite.

M.Ls =

Metamorphosed Limestone.

FIG. 24. Contacts of this type offer many interesting problems to geologists, and are usually worthy of the closest mapping and study.

the field relations are obscured in many ways; also small intrusive bodies are hard to find unless they are of a resistant character, or on the other hand, make grooves, or depressions as they weather. This may sometimes be the only evidence of a dike.

A fault boundary is usually characterized by its regularity so that when two formations meet along a relatively straight line, faulting should be expected. Again the absence of dikes along an igneous contact may point to faulting, though not necessarily so.

Work within Areas of the Same Formation.-"Traversing," or "meandering," is simply covering an area in more or less detail by trips along lines which would seem to offer the best chances for additional information. Such trips are made within the boundaries of the same formation for the purpose of observing the various characteristics of the rock and to look for unusual features such as veins and dikes. It is best to work across the strike of the formation, or of the vein system, which is usually mapped by itself and will be taken up later.

In traversing a sedimentary area it is necessary to observe the following things:

Strike and dip of beds.

Character and thickness of beds.

Minerals composing the rock and nature of grains or fragments.

Areas of alteration.
Fossils.

Structural relations as a whole-systems of folds, minor folding, direction and pitch of the axes of folds, relation of folding to faulting, the general distribution of the faults or fractures, etc.

Relations of topography to the geologic features.

Within areas of igneous rocks the geologist should look particularly for the following:

The mineralogical composition of the rock.
The rock texture and variations of it.

Variations in phase of the mass as a whole.
Segregations.

Dip and strike of schistosity or gneissoid structure.

It should be noted whether there is any variation in texture from the margin inward, and whether there is any alteration in certain portions of the rock.

Variations in phase are due to the fact that rock magmas are not homogeneous and that accordingly the chemical and mineralogical composition may vary considerably between different portions of the same mass. In field work it is well to have this always in mind, since it is generally recognized that in many cases the ores are associated with the more acid rocks and the more acid phases of the same rock, so that there may be a decided economic value to such observations.

A segregation is the local occurrence of some mineral or group of minerals in preponderating quantities. Thus there will be portions of the acid rocks that are nearly all quartz; or in the basic types horneblende sometimes develops in quite noticeable masses. Magnetite gathers in this way to such an extent as to form workable ore bodies.

Schistosity results from folding, shearing, and compression, so that by taking strikes and dips of the planes of schistosity some idea may be gained as to the general nature and extent of the forces involved.

In areas of metamorphic rocks the chief point to be determined is the origin of the metamorphosed types. This is quite simple in some cases, such as sandstones altered to quartzites; with recrystallized limestone the determination of age becomes difficult or impossible owing to the destruction of fossils.

With the schists and gneisses the problem of origin becomes difficult, and requires the closest examination in the field, especially if it seems in any way connected with the origin of the ore bodies. It is necessary to examine carefully the distinguishable minerals, both as to their probable original character and the alteration in composition and form. Structural features, such as the strike and dip of schistosity, or gneissoid structure should be carefully noted.

Sometimes silicification and the development of metamorphic minerals are encountered in limestone areas some distance from

any igneous contact. Such occurrences should be examined carefully, the minerals noted, and the general distribution of the silicate or altered patches indicated on the map.

Boundaries are shown on the map as fine unbroken lines when their location is certain; broken lines are used when there is doubt as to the accuracy of the representation; dotted lines are used to indicate possibilities. The strike of a formation is shown by a short line plotted accurately or with the direction of strike given in writing; the direction of dip is indicated by a small arrow drawn at right angles to the strike line and with the degree of dip shown near it. Thus the symbol for bedding striking N30°E and dipping 60 degrees to the east would be N30°E-60°. Formations should be distinguished on the map by small symbols such as "Gr" for granite, "Ls" for limestone, and so on. The various areas should also be colored to bring out the relations more graphically. In general, for purposes of uniformity, it is well to follow the usage of the U. S. Geological Survey in the matter of cartographic and geologic symbols; these may be found by reference to almost any of the Survey publications, such as the Professional Papers or Folios.

CHAPTER V

GEOLOGIC MAPPING (Continued)

In Chapter IV I described the methods employed in mapping the general geological features of an area. There remains the discussion of the details which must be observed in the examination of the more important features from an economic standpoint. This mapping of faults, the outcrops of ore bodies, and underground workings requires much more geological knowledge and close attention to detail than the other part of the work; it is not so generally understood and proficiency comes only with practice. The general remarks on field notes, office work, etc., may prove of value to anyone engaging in such work.

In mapping faults enough setups should be made to accurately give the course and to take frequent readings on the dip. The best evidence of faulting to be found in the field, or that which makes the clearest case, is obtained when formations of different type or age are placed in contact as a result of the fault movement. When later formations are laid upon eroded surfaces of older areas the contacts sometimes appear to have been made by faulting, but in most cases the true condition is easily recognized. Straight line boundaries should be examined closely for further evidence of faulting. Further, the geologist should examine marked topographic features, especially those of a more or less straight line character, such as escarpments, and in some cases the water-courses which may have developed along lines of easier erosion made by the crushing of the fault. More closely, he should look for polished or "slickensided" surfaces which usually show the scratching or grooving made by the rock surfaces or fragments as the movements progressed; these marks are the "fault striæ" and indicate the direction of relative displacement. The angle which these striæ make with the horizontal