VIIVI  .. 7554.20 54.46 1219.96.65 1.53 2.34 3.33 0.61 2.12 8.68 2.76 5.20 0.28 0.26 0.60 4.14 5.67 0.12 I., Elvan or

granite porphyry (with pinite after cordierite)-Prah sands, Cornwall . II., Granophyre-Armboth, Cumberland . III., Granophyre-Carrock Fell, Cumberland . IV., Rhomben-porphyry -Tonsterg, Norway . V., Elaeolite porphyry-Beemerville, New Jersey . VI., Tinguaite-Kola . VII., Grorudite-Assynt, Scotland . Porphyrites.-The porphyrites as above mentioned are intrusive or hypabyssal rocks of porphyritic texture, with phenocryste of plagioclase felspar and hornblende, biotite or augite (sometimes also. quartz) in a fine ground-mass . The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous age and older . Both the red porphyry and the green porphyry of the ancients are more properly classified in this group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or no primary quartz . Porphyrites occur as dikes which accompany masses of diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure .

The phenocrysts are plagioclase, often much zoned with central kernels of

bytownite or labradorite and margins of oligoclase or even orthoclase . In a special group there are corroded blebs or porphyritic quartz: these rocks are called quartz-porphyrites, and are distinguished from the granite-porphyries by the scarcity or absence of orthoclase . The hornblende of the porphyrites is often green but sometimes brown, resembling that of the lamprophyres, a group from which the porphyrites are separated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres . Augite, when present, is nearly always pale green; it is not so abundant as hornblende . Dark brown biotite is very common in large hexagonal plates . Muscovite and olivine are not represented in these rocks . The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely absent . The Alpine dike rocks known as ortlerites and suldenites are porphyrites containing much green or brown hornblende and augite; these, however, hardly require a distinctive designation . Diorite-porphyrites have almost as wide a distribution as granite-porphyries, and occur in all parts of the world where intrusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks . To gabbros and norites certain types of porphyrite correspond which have the same mineral and chemical, composition as the parent rocks but with a porphyritic instead of granitic structure . Gabbro-porphyrites are not numerous; or rather most of these rocks are described as porphyritic basalts and dolcrites . The beerbachites are finely granular, dike rocks resembling gabbros SiO2 Al20a Fe20a FeO CaO MgO K2O Na2O H2O I .

64.94 1 17.50 0.69 3.94 2.59 2.83 3.11 3.44 I.36 II . 61.58 18.84 4.68 - 6.59 2.04 1.49 4.27 I.61 I , Quartz-porphyrite-Lippenhof, Schwarzwald . II., Porphyrite-Esterel,

France . III., Norite-porohyrite-Klausen, Tirol . (J . S .