they are not always 'planar' in the strictest sense and may violate the rule of being perpendicular to the regional stress field, due to local influences. This is a megascopic version of what may occur around porphyroblasts. Often, fine observation of foliations on outcrop, hand specimen and on the microscopic scale complements observations on a map or regional scale. Description When describing a foliation it is useful to note the mineralogy of the folia; this can provide information on the conditions of formation the mineralogy in intrafolial areas foliation spacing any porphyroblasts or minerals associated with the foliation and whether they overprint it or are cut by it whether it is planar, undulose, vague or well developed its orientation in space, as strike and dip, or dip and dip direction its relationship to other foliations, to bedding and any folding measure intersection lineations Following such a methodology allows eventual correlations in style, metamorphic grade, and intensity throughout a region, relationship to faults, shears, structures and mineral assemblages. Engineering considerations In geotechnical engineering a foliation plane may form a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel, foundation, or slope construction. See also Cleavage (geology) Discontinuity (geotechnical engineering) Exfoliating granite Fissility (geology) Fold (geology) List of rock textures List of rock types Rock microstructure Shear (geology) References Blatt, Harvey and Tracy, Robert J.; 1996, Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed., p. 359-360, W. H. Freeman, Vernon, Ron H., 2004, A Practical Guide to Rock Microstructure, Oxford University Press, Oxford. Metamorphic petrology Metamorphic rocks Petrology Structural geology