THE MATRIX OF THE DIAMOND                 15 Another kind of alteration, rare in the Kimberley rock, but very common in the Kentucky variety, is into a peculiar finely fibrous substance of dark blue colour. The olivine is never wholly changed into this material, which appears only at the two extremities of a crystal or along cracks. The fibres always stand parallel to the cleavage planes of the olivine, and seem to be due to a finer splitting up of these cleavage planes until they become fibrous. This fibrous substance is faintly pleo-
	chroic like bastite, and seems to be a new variety of that mineral. (It is seen only in the Kentucky rock.) Dr. Eosenbusch suggested to me that it resembled the 'aerinite' of Lasaulx,1 also a product of alteration. It will be more fully treated under the description of bastite. The un­usually perfect cleavage in the olivine is particularly well developed near the edges, where the fibrous substance is formed. This alteration is at the edges of otherwise 1 Neues Jahrb. 1876, p. 352; Bull. Soc. Min, de France, i. 1878, p. 125.