BY W. N. BENSON. 685 



mosaic of prehnite-grains. It is evident that this change has 

 taken place after the felspar has been partly saussuritised, with 

 the development of dust-filled cracks; these now remain in the 

 prehnite. The diallage in this rock [M.B., 17] is being altered, 

 partly to tremolite, but chiefly to antigorite and pale pink garnet, 

 occurring as little irregular blebs, formed owing to reaction with 

 the felspar. 



In another example of the development of garnet, the case is 

 rather different. This rock [N.T., 417] occurred at Bowling 

 Alley Point, and, in some features, recalled the rocks of the 

 Paringu massif in Roumania, studied by G. M. Murgoci(Sl). The 

 original minerals were apparently diallage and plagioclase only. 

 The diallage is sometimes fresh, but usually only an outer shell 

 remains, the central portion having passed into antigorite, which 

 is bordered by numerous, small, colourless crystals of fassaite. 

 The plagioclase, where in contact with the ferromagnesian 

 minerals, has passed into a dusty aggregate of finely granular 

 garnet, and small strings of these grains are working into the 

 main mass of the felspar along the cleavage-cracks.* The de- 

 velopment of prehnite, from the felspar, is also in progress, and 

 this mineral, with the garnet and cloudy saussuritic products, 

 completely replaces the original plagioclase. The cleavage of 

 the felspar is preserved in the pseudomorph, even though it has 

 become merely a patchwork of brightly polarising, variously 

 oriented prehnite-grains. This mode of occurrence of prehnite, 

 is considered by Weinschenk ( Petrographic Methods, p. 299) to 

 be the same form described as lotrite by Murgoci. 



Another type of saussurite-gabbro is shown by M.B., 181, from 

 south of Gulf Creek. It is quite similar in appearance to the 

 other saussurite-gabbros, but differs in the presence of coarsely 

 crystallised clinozoisite. The rock, as a whole, is extensively 

 altered. The diallage is sometimes bent, but may remain other- 

 wise unaltered, or have passed into tremolite, and, locally, still 

 further into fibrous and platy serpentine. This last passes parallel 



* The saussurite-gabbro, described by Prof. Bonney from the Saasthal, 

 shows also this feature, of a garnet-border to the pyroxenes. See Phil. Mag. 

 1892, p. 243. 



