It frequently happens that micas exhibit small basal cleavage rifts of roughly circular shape, and these may be so numerous as to render opaque or feebly translucent a plate which would be otherwise quite transparent. The so-called silver-amber mica of the mica trade, a variety of phlogopite which is highly prized for its low conductivity, apparently owes its silvery sheen to the presence of these cleavage rifts, which overlap each other to a considerable extent, and reflect the light copiously. The presence of these basal cleavage rifts in silvery phlogopite is rendered quite obvious when a plate is examined under the microscope in ordinary light, by the interference rings seen round the borders of the rifts. Such a plate of silvery phlogopite exhibits a peculiar phenomenon in consequence of the presence of these rifts.