Fig. 5. Diagram showing theoretical stages in the formation of aplite and pegmatite dikes that replace diorite-gabbro wall rock. Areas between A-A', B-B', C-C', and D-D' represent the original massive diorite-gabbro with randomly oriented crystals. Area between 1-1' represents flexed rock in which shearing and cataclasis have oriented the crystals into parallel or subparallel alignment. Here the ferromagnesian silicates are partially replaced by quartz. Area between 2-2' shows differential movements parallel to foliation, which produce relatively low-pressure sites into which silica can move to replace the ferromagnesian silicates by quartz and into which K can move to replace zoned plagioclase to form microcline, myrmekite, and unzoned plagioclase. Migmatite can occur on either side of the 'dike,' but a central zone, where replacement is complete, is aplite or pegmatite. Remnant dark minerals show traces of the former flexed foliation. Area between 3-3' shows an aplite-pegmatite dike cutting across massive diorite-gabbro wall rock. A faint foliation is truncated by the granitic dike; layer b-b' appears to match up, however, a-a' actually connects equivalent layers. Island x is a remnant block of unsheared massive diorite-gabbro which remained as an enclave in the dike.