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The dip thus found is very probably only an apparent dip, and is less than the true dip, which runs in some other direction. Two or more observations taken near to one another will settle this point. Thus where there are two dips seen on different walls of the same quarry, or in closely adjoining quarries, and where these are evidently not due to mere local slippings or to the very common creep of the higher beds down the slope of a hill-side, then the direction and amount of the true dip can be found by the simple geometrical method of Mr. W. H. Dalton.* The directions of the walls, or rock-faces, on which the dips are seen are determined with the compass, and two lines are drawn to represent them on paper, giving the angle rab (fig. 1). Should one dip in the actual quarry-sections incline towards a and the other away from a, one of the lines drawn must be produced, so that the dips represented in direction by the lines a b and ar both either incline towards or away from a.

t

Fig. 1.

d

Draw a c perpendicular to a b, and of any convenient length, say, for greater accuracy, about 3 inches; and draw as perpendicular to ar and equal to a c. From c and s draw lines making with ac and a s respectively angles equal to the complements of the observed angles of dip, and cutting a b and a r in d and t. Then the angles a dc and a ts represent the angles of observed dip along the directions ab and ar respectively.

a e,

Join dt; this line represents the strike of the beds. drawn from a perpendicularly to it, gives us the direction of true dip. Draw af perpendicular to a e and equal to a c or a s; join fe. The angle a ef, when measured with a protractor, gives the amount of the true dip.

The matter is clear if the three triangles a st, a cd, and a fe are imagined as bent up so as to stand perpendicularly to the plane at d, which remains horizontal. The points s, c, and f coincide, and a plane laid upon the dipping lines s t, fe, and c d will represent truly a surface of one of the strata observed in the field, when both the apparent dips were inclined away from a.

*Geol. Mag., 1873, p. 333.

dt is a horizontal line in this surface, and is therefore the strike; the line fe, now perpendicular to it, and also in the same surface, represents the true dip both in compass-bearing and in inclination to the horizontal.*

If both the observed dips are inclined towards the point a, it is clear that the same construction suffices, only the arrow set down upon the map to indicate direction must point along a e towards a and not away from it.

Graphic methods like the foregoing serve the geologist far better than any system of elaborate tables. Provided the scale of the drawing is sufficiently large, the errors of observation in the field, owing to the small exposures studied, will be greater than any that can be introduced afterwards by measurement from a carefully constructed drawing.

To find the relation of the point where observations are being made to features marked npon the map, and thus in one's notes to localise the observation, is often difficult in a wide and open country. Even the map on the scale of 6 inches to a mile cannot represent every rock and projecting boss, and measurements must be made extending from some recognisable point to

the place of observation. The tape-measure, so important in determining the thicknesses of beds on faces of a quarry, is often of use in direct measurement on the surface of the ground, for which purpose it should be at least 40 feet in length. Mere pacing over the interval is sufficiently accurate in many cases; but where the position in azimuth can be found, and it is a

Though dip and strike are commonly considered together, the Dip may be defined independently as the line of greatest inclination to the horizon, and the Strike as the direction of a horizontal line, in the plane surface of any bed.

question of how high above or how far below some known point we are standing, the little instrument known as Abney's level (fig. 2) is of constant service. This level is a combination of a contouring-glass with a graduated arc and rotating spirit-level, so that fairly accurate altitude-readings can be taken. Within

a horizontal tube, square in cross-section, is an inclined metallic mirror, which extends half-way across the tube, its lower edge being straight and horizontal. Through an aperture in the upper side of the tube this mirror reflects the bubble of a little spirit-level, which is attached to an index-arm and can be rotated by the finger in a vertical plane. The index-arm, bearing a vernier reading to 10 seconds of arc, moves over a semi-circle graduated in degrees.

For contouring-i.6., determining a chain of points at the same level above the sea as that from which we start the index is adjusted to the zero-point, and the tube containing the spiritlevel thus becomes accurately parallel to the eye-tube. On looking through the latter, and shifting it in altitude till the image of the bubble is exactly bisected by the horizontal edge of the mirror, any object seen through the tube to coincide with that edge is on the same horizon or contour as the eye of the observer. By proceeding nearer to that point and levelling on from it to one more distant, the chain of points may be established.

By this means, even when only scattered heights and not contour lines are set down upon the map, the height above sealevel of some point near the place of observation can finally be determined. Such a point being known, the use of the level will determine the height of any other, provided it is reasonably accessible. Keeping the index still at zero, the observer, if the point to be determined is above that already ascertained, stands upright at the known point and levels through the tube at the slope above him. Selecting some prominent stone or grass-tuft that appears on the edge of the mirror when the bubble is bisected, he walks to this point and repeats the observation. By a succession of such observations, which may be made along the direct line between him and his goal, or along as zigzag a course as the nature of the ground may dictate, he finally arrives at the point the height of which is to be found. The number of times the observation has been repeated, multiplied by the height of his eye above the ground, as measured with a tape *Made by Messrs. Troughton & Simms and other opticians; price about 40s. Messrs. Troughton & Simms also make a cheaper instrument of the same kind.

when standing upright, gives the total height that must be added to that originally ascertained.

Similarly, if the unknown point is below the known one, the levelling starts from the former and rises to the latter.

So far, a simple level, about 1 foot long, with sights at each end and a little folding mirror just above it to show the bubble when it is being held level in the hand, will serve for these and similar contouring observations. But the convenience of Abney's level is that it can be also used for reasonably accurate determinations of the heights of cliffs, and the thicknesses of great divisions of strata displayed on them. Thus, if a level shore can be obtained, two observations of altitude taken at a measured distance from one another will determine the height of any part of a rock-face. The tube of the instrument is directed towards the point in question, and the spirit-level is rotated until the bubble appears bisected. Removing the instrument from the eye, the index will indicate the angle which the eye-tube made with the horizontal during the observation. drawing out the results graphically, the height of the eye from the ground will again have to be taken into account.

In

Moreover, this level works excellently as a clinometer, and thus enables one to dispense in practice with any other instrument for measuring angles. Since, in the forms made, the bubble is not always visible when the graduated arc faces the observer, the instrument should be turned round so that the spirit-level is nearest to the eye. The edge of the eye-tube, which for this purpose might easily be made a little longer, serves as the straight edge of the clinometer. When it is adjusted so as to appear coincident with the line of dip, rotate the spirit-level until the bubble is seen to lie centrally in its tube. The angle at which the main tube has been inclined to the horizontal will be accurately shown by the position at which the index now stands. After the spirit-level has been rotated so as to become horizontal, the coincidence of the straight edge and the dip must be carefully checked, lest the hand supporting the instrument should have shifted its position.

A common triplet pocket-lens, or any useful form that will bear rough usage, must always be carried in the field, as indeed it should be carried by the geological observer every day of his life, whether in town or country. A note-book without ruled pages, so that outline-sketches may be added to the ordinary notes, can be kept ready in the side-pocket.

Seeing that the page of a field note-book is necessarily small, it is a good plan to carry sheets of the size of writing-paper in

one's pocket-book, folded in quarters. Such paper can be unfolded to suit the nature of the sketch.

Facility in drawing the outlines of scenery comes with practice, and a sketch should be made of every critical area or section, the points being lettered to facilitate reference in the notes or on the labels of specimens.

Lastly, the geologist who knows how to treat and use a fountain - - pen will never be without it in the field. For precision of line in sketching, for writing on loose pieces of wrapping-paper, which may be waving in the wind, or on smooth surfaces of the specimens themselves, there is no com parison between the utility of an ink-pen and a pencil. Specimens may undergo various hardships during travel, and may remain packed up for months; but their labels, if written in ink, will be always black and legible.

CHAPTER II.

ON THE COLLECTION AND PACKING OF SPECIMENS.

EXCEPT in the case of brilliant examples of minerals, or of fossils exhibiting characters in an unusually fine state of preservation, specimens are of little utility or interest to the geologist unless gathered actually in situ. A talus-heap, still worse a road-heap, the materials of which may have come from anywhere, affords very tempting but very misleading material. Some "specimens" seen in their true position are, however, far too large to be carried away. In such cases a sketch giving dimensions, or a photograph, must suffice, and chips from various parts may serve subsequently as illustrations of the whole.

Hints are scattered through the following pages as to the points to be regarded in selecting specimens of various kinds. We need only note here that soils are best collected in artificial cuts or on the banks of streams, some 2 feet or so below the ordinary cultivated and altered surface. Well developed crystals of minerals are to be hoped for only in cavities and on the walls of open joints; while rock-specimens should be broken out from larger masses, so as to secure fresh unweathered surfaces. It is often useful. however, to show the amount of resistance of

* Von Richthofen, Führer für Forschungsreisende (1886), p. 28.