133 



Forellenateiti (Troctolite). Is a local name applied to a variety of gabbro 

 occurring at Neurode, in Silesia. It has been extended to similar rocks in 

 other localities. It is coarse-grained and granitic in texture, being composed 

 essentially of plagioclase and olivine or oli vine- serpentine. The serpentinised 

 oli vines appear as dark spots on a grey groundmass ; hence the local name 

 which means trout-stone. The felspar has been described by Von RATH (1) as 

 an altered labradorite, and by STRENG (2) as an altered anorthite. Augite 

 (diallage) occurs as an accessory constituent, so that the rock is merely a 

 gabbro extremely rich in olivine. 



Noritc. This name was originally applied by ESMARK (3) to many 

 Norwegian rocks which we should now separate from each other under such 

 heads as diorite and gabbro. SHEERER (4) limited its signification to rocks of 

 the gabbro-type, such as those occurring in the island of Hitteroe. These 

 appear to embrace typical gabbros, and also rocks in which a rhombic 

 pyroxene takes the place of augite or diallage. Professor ROSENBUSCH 

 proposes to limit the term norite to pre-tertiary rocks composed essentially 

 of plagioclase and a rhombic pyroxene. The simultaneous occurrence of 

 rhombic and monoclinic pyroxenes in rocks of the gabbro- type < 5 ' shows that 

 there is no hard and fast line between the gabbros and the norites 

 (ROSENBUSCH). In this work we shall use the term norite in ROSENBUSCH'S 

 sense but without the restriction as to geological age. 



Hypei'fit/ienitc and Hype rite. These are synonyms of ROSENBUSCH'S norite. 

 Many rocks formerly termed hypersthenites in this country are now known 

 to be gabbros. The present writer is not acquainted with any British 

 norite. 



Diorite. This term was introduced by HAUY @) for a rock composed of 

 white felspar and hornblende. It was distinguished from syenite (WERNER) 

 by the greater abundance of the latter mineral. It was employed by 

 ZIRKEL (7) for a crystalline granular compound of oligoclase and hornblende, 

 often associated with quartz. ROSENBUSCH uses it for a similar rock but 

 without reference to the particular species of plagioclase, and divides the diorites 

 into two groups according to the presence or absence of quartz. The quartz- 

 diorites are rocks of intermediate composition and do not, therefore, belong to 

 the group now under consideration. The quartz-free diorites form a very 

 unsatisfactory group so far as at present known. Some of them, including 

 the epidiorites of GUMBEL, are plagioclase-augite rocks which have been 

 affected by contact and regional metamorphism ; others have probably always 

 been plagioclase-hornblende rocks. To the latter belong the diorites of War- 

 wickshire described by Mr. ALLPORT (8) some of which contain olivine, and 



(1) P.A. XCV., p. 551. 



(2) N.J. 1864. p. 257. 



(3) Magazin fur Naturvidenskaberne I, p. 207. 



(4) N.J. 1843, p. 668. 



(o) See JUDD Q.J.G.S. Vol. XLIL, 1886, p. 64. 



(6) Traite de Mineralogie. 2nd. edition. Paris, 1822. p. 541. 



(7) Lehrbuch der Petrographie, 1866. Vol. II., p. 1. 



(8) Q.J.G.S. Vol. XXXV., p. 637. 



