On the north side of the outcrop of the Chalk in the Isle o Purbeck, there is an outlier of Tertiary beds the exact age of which has given rise to some discussion. There must be some reason for the form of Creechbarrow Hill, and the outline suggests that it is due to some hard bed near the top, which we find on close examination to be a tough limestone, a considerable portion of which is hard and porcellaneous. The upper beds of this conical hill, which rises to a height of 637 feet above sea-level, are cut off from the adjoining masses, and therefore no continuous section can be traced to the well-known formations on the south or on the north. The section is further complicated by the occurrence of faults on both the north and south sides. We have, therefore, only the lithological character and fossil contents to depend upon for the correlation of the beds occurring in this outlier.

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.