Definitions and images to illustrate geological terms, links to images and website articles


Phacoliths are concordant plutonic bodies that lie parallel to the bedding plane or foliation of folded country rock. They occur along the crests of anticlines or the troughs of synclines in folded sedimentary strata.

Rarely, the body of a phacolith may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that it has an S shape in cross section. The hinge of folds in intensely folded terrains are areas of reduced pressure, so are potential sites for magma migration and emplacement.

[links: images: close-up: autolith of biotite-rich, enclave bearing, porphyritic granite in another more phenocrystic granite, Wolf Mountain Intrusion (Phacolith), Proterozoic Llano Uplift, central Texas]

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