Boudins
Boudin is the French word for sausage, pronounced boo-dan. It is also a geologic term for a sausage-shaped body of rock that differs from the surrounding material. Typically, boudins are found in series of elongate bodies that may be detatched from each other (as if the original material was pulled apart, like toffee), or connected by narrow, pinched connections. They are usually interpreted to represent tectonic stretching of relatively rigid, competent rocks interlayered with rocks that are less resistant and capable of flowing into the "neck" portions of the boudins.
Igneous rocks, derived from magmas, may exhibit boudin-like structures, but their origin may be different than that described above. Sometimes flattened ovals of material resemble boudins on rock surfaces, but in the third dimension they may actually be long finger-like intrusions of igneous rock. This suggests that the igneous rock was emplaced as these long injections, and their geometry reflects the mode of emplacement rather than later deformation, as is indicated for the boudins described in the first paragraph. The little pegmatite bodies in this picture (from the Black Hills of South Dakota) are igneous in origin, but they show the boudin shape.
Geologists can use boudins and igneous boudin-like structures to decipher the history of rocks. This understanding helps in the search for economic resources.
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