The lower limit of visible traces of penetrative movement in quartz-rich rocks can be defined by studies of orientations and mutual relations of grains. Intensity of deformation in the central Appalachians decreases gradually from the crystalline Piedmont toward the north, northwest, and west perpendicular to the regional trend. The author has attempted to determine the limits of visible deformation geographically and stratigraphically.
Five types of quartz have been distinguished on the basis of texture: original grains, peripheral growth quartz, crush quartz, needle quartz, and recrystallized quartz. Each type indicates a particular degree of deformation grading from original grains showing little or no deformation to completely reoriented and recrystallized SiO2.
If granulation affects 22 per cent of the quartz present a lineation may become visible megascopically. It is then perpendicular to the fold axis b, contained within the flow cleavage plane, and parallel to a. Arrangement of grains into physical parallelism precedes the development of preferred optical orientations. Lattice parallelism develops either through fracturing or through coalescence of reorganized crushed quartz particles.
A study of the regional distribution of recrystallization and flowage on a quantitative basis permits the definition of a transitional zone in which the nontectonites on the northwest merge with the tectonites on the southeast. Fabrics of rocks in the tectonite area are readily deciphered, while those northwest of the demarcation belt are increasingly indistinct.