tract of land shaped like the
letter "delta," especially when the land is alluvial, and enclosed
within two or more mouths of a river, as the Delta of the Ganges, of
the Nile, of the Mississippi" (fig. 19).

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.

[Illustration: 19]

[Illustration: 20]

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.

[Illustration: 21]

[Illustration: 22]

[Illustration: 23]

[Illustration: 24]

[Illustration: 25]

[Illustration: 26]

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 forma