A quantitative analysis of fracture-orientation data collected in Precambrian gneisses and overlying lower Paleozoic sedimentary rocks in three northern New Jersey areas yields the following results: (1) The data, as plotted on both rectangular co-ordinates and the Schmidt equal-area net, show definite clustering for all localities studied; the Poisson Exponential Binomial Limit is applied to test for significant departure of the diagrams from isotropism, to estimate mean orientations of fracture sets (mean fracture planes), and to aid in interpreting the rectangular point diagrams. (2) Fractures and foliations in the crystalline rocks do not appear to be genetically related. (3) Most localities have two principal joint sets, but there are proportionately more Precambrian than Paleozoic localities with three sets. (4) The Paleozoic localities show more concentrated clustering on the point diagrams. (5) The dihedral angle between mean fracture planes and bedding is typically near 90 degrees; the dihedral angle between mean fracture planes and foliation has no typical value. (6) The angle of fracture in both rock types appears to be independent of the nature of the rock. (7) Contrasts in rock type (metamorphic vs. overlying sedimentary) are usually accompanied by striking differences in point-diagram patterns. (8) The measurement of a few large fractures gives no better approximation of the mean fracture planes than the measurement of an equal number of fractures of any size. (9) The trends of mean fracture planes conform with linear features visible on air photographs. (10) A high proportion of fractures dip steeply; dips in the crystalline rocks are systematically smaller than those in the overlying sedimentary rocks.
The fractures in the gneisses are probably older than those in the sedimentary rocks.
The deformation of the gneisses seems to have taken place through mechanisms fundamentally different from those involved in deforming the overlying sedimentary rocks.