observed. Figs. 18 and 19 will perhaps make the foregoing description clearer.

There should be no steel nails or screws in the plane table, and in making the setup care should be taken that the head of a hammer or axe carried in the belt does not deflect the needle. Where there is local attraction due to magnetic ore, slag, scrap iron, or electric wires, the compass cannot be used; if there is much work to be done under such conditions, a sundial compass may be used, or the plane table oriented by a series of trials sighting at known points.

Strike

FIG. 19.

The alidade used for this work has a base or double straightedge 12 in. or 14 in. in length, graduated to tenths of inches on one side and to millimeters on the other. The line of sight parallel to the edges of the base is made by two sight vanes 6 in. high, which fold down to the base for convenience in carrying.

The most common method of locating a point on the map is to set up the table, level and orient it by means of the compass, making the true north line of the map correspond with the meridian as indicated by the compass. The edge of the alidade is then placed upon a known point on the map and the point itself (usually a flag, claim monument, or building corner), is sighted. A line is then drawn along the straightedge of the alidade; this is repeated at least twice and the intersection of the various lines is taken as the location of the new point on the map. This is called "making a three pointer." A full discussion of this and other methods of plane table location will be found on pages 4 and 30.

The note-book should be of convenient pocket size and of

the loose leaf type, though ring binders are not desirable for field use. A cover with eyelets for the passage of binding cords is more satisfactory. Cross-section paper is very convenient for sketching and plotting to scale.

Hard pencils, 6H to 8H, are best since they do not smudge the map. A sand-paper strip fastened to the under side of the plane table or in the note-book is very handy for giving the fine point required in detailed sketching. Colored pencils are often carried in the field and the different areas are colored as the work proceeds. A small protractor is used in plotting on the map though the compass may be used in the field. The edges. of the alidade serve as scales, or if the divisions are not suitable flat wooden scales may be used.

The geologist should always have with him his lens (the Bausch and Lomb Aplanatic Triplet, 3/4 in., is a good one) which may be carried with a soft eraser on a cord or chain; a knife with a magnetized blade, a geological hammer, a specimen sack, and labels complete the outfit.

As to clothes, much will depend on the climate; light kahki is very good for work in hot climates, corduroy more serviceable in the cooler countries where there is more vegetation. Shoes should be heavy and hobnailed, but not waterproofed, unless the climate is very wet. A good substitute for the ordinary hobnail is a small wood screw which may be put in in the field and will not come out. A shooting vest with large pockets is convenient for carrying note-books, and specimens, etc.

Geological Mapping in the Field.—The first step in examining an area is to make a general reconnoissance. This gives an idea of the formations and the problems to be encountered; it is well to collect specimens of questionable or unfamiliar rocks at this stage of the work, for comparison and to furnish thin sections for microscopic study. If the area is large this trip will give ideas as to the location of roads, trails, water and camping places, which will be of assistance in planning the work. The information so obtained should also serve as the basis for estimates of the probable time and cost of the undertaking.

Detailed Field Work.-For purposes of description observations in the field may be regarded as falling into two broad classes. Those which have to do with the tracing of boundaries, the determination of the relations between formations, and the structural features of the area as a whole, may be taken as one group under the title of

General Geology

The boundary or line of division between two formations exists either as the rocks were originally formed, or is the result of faulting, or possibly metamorphism, at some subsequent period.

The fault boundary, Fig. 20, is the simplest type since it is usually a relatively straight line or has a fairly regular curve;

FIG. 20.-Example of a relatively simple boundary between an igneous and a sedimentary rock, where the limestone has been "down thrown" by faulting against granite.

sometimes, however, faults are quite erratic and must be carefully followed. Boundaries between two conformable sedimentary formations, Fig. 21, or volcanic flows, are usually easy to follow unless made very irregular in outcrop by erosion. This is usually the case when the beds are flat-dipping (Fig. 22). A discussion of the effects of erosion and faulting upon outcrops

is hardly within the scope of such a book; references covering the subject are given below.1

In mapping an easily followed boundary the plane table need not be set up so frequently and often much sketching may be done by the topography if it has been accurately mapped. It is always desirable to have dips along a boundary but they are not

The

FIG. 21.-Limestone-shale boundary strongly marked by the massive beds of the former standing as a vertical wall above the softer shales. top of the cliff shows another shale bed.

always obtainable. When a boundary comes on a hillside and is badly obscured by débris, the exact location becomes an impossibility; in such a case it is not necessary to be too painstaking in the setups, fragments of the lower formation may be followed up hill and the highest points at which they are found will determine the approximate location of the boundary. Sometimes a noticeable feature of the topography such as an escarpment, or a change in slope, will give a clue as to where to look for a covered boundary, but this cannot always be relied

upon.

166 Geology Applied to Mining," J. E. Spurr, pp. 139-142, for migration of outcrop; pp. 149-176, measurement of folds and faults. "Geology," Vol. I, Chamberlain and Salisbury, pp. 500-525.

Complex boundaries are likely to be found when igneous rocks intrude limestones and other sedimentary rocks, or other igneous masses. Arms and dikes go from the main intrusive body for varying distances, Fig. 23, and are often accompanied by alteration and metamorphism which may take place either in the intruded formation or in the intrusive, or both. The commonest examples of this are contacts between limestones and

intrusives of the more acid type; varions ferro-magnesian, limesilicate, and sometimes metallic minerals develop in both the formations until there is every gradation between fresh and completely altered rock of each type. The location of the boundary in such cases becomes a matter of difficulty and it is usually desirable to give the general relations rather than the minute details. This may be accomplished by setting up the table and making short traverses in the vicinity, measuring the distances by pacing. Fig. 24 is an example of such a boundary. The important thing in mapping and studying such a contact is