Anderson, Alfred T., Jr. Department of Geophysical Sciences, University of Chicago, Chicago, Illinois.
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A rock fragment enclosed in another rock, and of varying degrees of foreigness. Cognate xenoliths, for example, are pieces of rock that are genetically related to the host rock that contains them, such as pieces of a border zone in the interior of the same body. Included blocks of unrelated rocks are more deserving of the xenolith label. Such foreign rocks help establish the once fluid and hence molten condition of invading magma capable of incorporating and mixing an assemblage of unrelated rock inclusions, as at Hutton's Rock near Edinburgh, Scotland. Dark inclusions, commony called enclaves, up to many feet in length can be found in granitic plutons and silicic lava domes and flows. In many cases, these have fine-grained borders against the enclosing rock and are interpreted to be lenses of formerly fluid basaltic magma which crystallized as a result of contact with cooler surrounding silicic magma.
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