Page:The Journal of geology (IA journalofgeology11893univers).pdf/282

of the rim. It is impossible to determine the character of the fibres in the finest rims, but in those in which the structure is coarser, it is learned that two components are present. One is possessed of a high index of refraction, and strong double refraction, and this appears to be continuous with the diallage of the narrow zones interposed between the fibrous growth and the surrounded olivine. The other component penetrates between the pyroxene fibres, and has club-shaped ends. Occasionally the twinning bars of plagioclase may be detected in it, and hence it is assumed to be a triclinic feldspar. The fibrous rim is thus an intergrowth of plagioclase and augite, both of which minerals are normal constituents of the gabbro. In the fibrous rims they have evidently crystallized contemporaneously, whereas in the main body of the rock the main portion of the diallage preceded the plagioclase in its separation from the magma. There is no necessity for regarding the intergrowths as in any way connected with reactionary processes, while there is abundant reason for believing them to be due solely to the tendency of simultaneously crystallizing minerals to mutually interpenetrate each other. This tendency is well recognized as existing to a marked degree between quartz and orthoclase, whereby granophyre is formed, and to a less extent between various other minerals. Micropegmatitic intergrowths between hornblende and feldspar, for instance, have been described by Lévy,^[1] Camerlander^[2] and Lacroix,^[3] between hornblende and quartz by Kalkowsky,^[4] between garnet and feldspar by Becke,^[5] and between garnet and quartz by Lacroix (l. c., p. 317,) between diopside and quartz by Lévy,^[6] and between various monoclinic pyroxenes and plagioclase by Becke (l. c.), Camerlander (l. c.), Lacroix (l. c., pp. 316 and 318), and Lévy.^[7] In the Minnesota rock the diallage in many