Sill
Photo by Marli Miller, University of Oregon. Used by permission. (fair use policy)
A sill is an intrusion of magma that spreads underground between the layers of another kind of rock. The darker layer in the flanks of Mount Gould, in Glacier National Park in Montana, is such a prominent and widespread sill that it has its own name, the Purcell Sill. The rocks of this sill are diorite. Both it and the limestone it invaded (the Helena Formation) are extremely old, some 1300 milllion years old. Read more about it at Warren Lane's "Montana Earth Science Picture of the Week" site. The "bleaching" above and below the sill is an example of contact metamorphism.
An intrusion that cuts across the layers of its host rock is called a dike (or "dyke" in British spelling). Dikes tend to be vertical and sills tend to be horizontal, but later tilting can change that. The important thing is how the intrusion is oriented relative to the bedding.
The term "sill" is borrowed, like many words in geology, from architecture, where a sill is the horizontal beam at the base of a windowframe or doorway.
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