Fig. 6. This photomicrograph shows a remnant zoned plagioclase crystal in the massive, pink, Cape Ann granite several meters from the contact with the Salem diorite. The plagioclase is Carlsbad-twinned and has a weathered, sericitized calcic core. This crystal is similar in size and shape to Carlsbad-twinned plagioclase crystals in the diorite. In other places the Carlsbad-twinned plagioclase crystals are deformed and replaced progressively by microcline and wartlike myrmekite, and the microcline inherits its Carlsbad twinning from the former plagioclase.

In this particular photomicrograph, the microcline replaces one end (right side) of a Carlsbad-twinned plagioclase crystal (black, speckled, and light gray, left side). During replacement, the calcic core was truncated by the microcline (black and gray, right side) so that the rounded-part of the plagioclase core at the right end is missing, but the Carlsbad twin plane is continuous between the two feldspars. This is strong evidence that the microcline (and the granite) did not form by a crystallization from a magma but resulted from replacement of the former diorite.