When the use of the word "delta" in physical geography is fully grasped, its fitness as applied in fingerprint work will become evident. Rivers wear away their banks and carry them along in their waters in the form of a fine sediment. As the rivers unite with seas or lakes, the onward sweep of the water is lessened, and the sediment, becoming comparatively still, sinks to the bottom where there is formed a shoal which gradually grows, as more and more is precipitated, until at length a portion of the shoal becomes higher than the ordinary level of the stream. There is a similarity between the use of the word "delta" in physical geography and in fingerprints.

The island formed in front of the diverging sides of the banks where the stream empties at its mouth corresponds to the delta in fingerprints, which is the first obstruction of any nature at the point of divergence of the type lines in front of or nearest the center of the divergence.

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In figure 20, the dot marked "delta" is considered as the delta because it is the first ridge or part of a ridge nearest the point of divergence of the two type lines. If the dot were not present, point B on ridge C, as shown in the figure, would be considered as the delta.

This would be equally true whether the ridges were connected with one of the type lines, both type lines, or disconnected altogether. In figure 20, with the dot as the delta, the first ridge count is ridge C. If the dot were not present, point B on ridge C would be considered as the delta and the first count would be ridge D. The lines X--X and Y--Y are the type lines, not X--A and Y--Z.

In figures 21 to 24, the heavy lines A--A and B--B are type lines with the delta at point D.

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Figure 25 shows ridge A bifurcating from the lower type line inside the pattern area. Bifurcations are also present within this pattern at points B and C. The bifurcation at the point marked "delta" is the only one which fulfills all conditions necessary for its location. It should be understood that the diverging type lines must be present in all delta formations and that wherever one of the formations mentioned in the definition of a delta may be, it must be located midway between two diverging type lines at or just in front of where they diverge in order to satisfy the definition and qualify as a delta.

When there is a choice between two or more possible deltas, the following rules govern:

- _The delta may not be located at a bifurcation which does not open toward the core._

In figure 26, the bifurcation at E is closer to the core than the bifurcation at D. However, E is not immediately in front of the divergence of the type lines and it _does not_ open toward the core.

A--A and B--B are the only possible type lines in this sketch and it follows, therefore, that the bifurcation at D must be called the delta. The first ridge count would be ridge C.

- _When there is a choice between a bifurcation and another type of delta, the bifurcation is selected._

A problem of this type is shown in figure 27. The dot, A, and the bifurcation are equally close to the divergence of the type lines, but the bifurcation is selected as the delta. The ridges marked "T" are the type lines.

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- _When there are two or more possible deltas which conform to the definition, the one nearest the core is chosen._

Prints are sometimes found wherein a single ridge enters the pattern area with two or more bifurcations opening toward the core. Figure 28 is an example of this. Ridge A enters the pattern area and bifurcates at points X and D. The bifurcation at D, which is the closer to the core, is the delta and conforms to the rule for deltas. A--A and B--B are the type lines. A bifurcation which does not conform to the definition should not be considered as a delta irrespective of its distance from the core.

- _The delta may not be located in the middle of a ridge running between the type lines toward the core, but at the nearer end only._

The location of the delta in this case depends entirely upon the point of origin of the ridge running between the type lines toward the core.

If the ridge is entirely within the pattern area, the delta is located at the end nearer the point of divergence of the type lines. Figure 29 is an example of this kind.

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If the ridge enters the pattern area from a point below the divergence of the type lines, however, the delta must be located at the end nearer the core. Ridge A in figure 30 is of this type.

In figure 31, A--A and B--B are the type lines, with the dot as the delta. The bifurcations cannot be considered as they do not open toward the core.

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In figure 32, the dot cannot be the delta because line D cannot be considered as a type line. It does not run parallel to type line A--A at any point. The same reason prevents line E from being a type line.

The end of ridge E is the only possible delta as it is a point on the ridge nearest to the center of divergence of the type lines. The other type line is, of course, B--B.

The delta is the point from which to start in ridge counting. In the loop type pattern the ridges intervening between the delta and the core are counted. The core is the second of the two focal points.

_The core_, as the name implies, is the approximate center of the finger impression. It will be necessary to concern ourselves with the core of the loop type only. The following rules govern the selection of the core of a loop:

- _The core is placed upon or within the innermost sufficient recurve._

- _When the innermost sufficient recurve contains no ending ridge or rod rising as high as the shoulders of the loop, the core is placed on the shoulder of the loop farther from the delta._

- _When the innermost sufficient recurve contains an uneven number of rods rising as high as the shoulders, the core is placed upon the end of the center rod whether it touches the looping ridge or not._

- _When the innermost sufficient recurve contains an even number of rods rising as high as the shoulders, the core is placed upon the end of the farther one of the two center rods, the two center rods being treated as though they were connected by a recurving ridge._

_The shoulders of a loop are the points at which the recurving ridge definitely turns inward or curves._

Figures 33 to 38 reflect the focal points of a series of loops. In figure 39, there are two rods, but the rod marked "A" does not rise as high as the shoulder line X, so the core is at B.

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Figures 40 to 45 ill.u.s.trate the rule that a recurve must have no appendage ab.u.t.ting upon it at a right angle between the shoulders and on the outside. If such an appendage is present between the shoulders of a loop, that loop is considered spoiled and the next loop outside will be considered to locate the core. In each of the figures, the point C indicates the core. Appendages will be further explained in the section concerning loops.

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Figures 46 to 48 reflect interlocking loops at the center, while figure 49 has two loops side by side at the center. In all these cases the two loops are considered as one. In figure 46, when the shoulder line X--X is drawn it is found to cross exactly at the point of intersection of the two loops. The two loops are considered one, with one rod, the core being placed at C. In figure 47, the shoulder line X--X is above the point of intersection of the two loops. The two are considered as one, with two rods, the core being at C. In figure 48, the shoulder line X--X is below the point of intersection of the loops. Again the two are treated as one, with two rods, the core being placed at C. In figure 49, the two are treated as one, with two rods, the core being placed at C.