REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 13 



Wiegand 1 in the bronzite of Starkenbacli. They seldom exceed 0*008 mm. in breadth, 

 and may reach 0"04 mm. in length. The presence of nearly similar enclosures, having 

 the same arrangement, has already been observed by Zirkel, 2 in the hornblende of some 

 syenites and diorites. I find that actinolite is most abundant in those slides wherein 

 the ground-mass is fine-grained, and poor in large sections of olivine. 



The large ellipsoidal sections mentioned before as more abundant in the slides with 

 band structure, and to whose presence the undulations in the bands are due, should for 

 the most part be classed as enstatite. 3 They never present a regular crystallographic 

 outline. They are colourless, or of a slightly greenish-yellow tint ; and are lamello- 

 fibrous in structure, the fibres being about 0"02 mm. thick, and extending from side to 

 side so as to preserve their parallelism throughout the section. They are distinguished 

 from hypersthene by the absence in them of these enclosures which have come to be 

 considered as distinctive of the latter mineral, 4 and by the absence of strong dichroism. 

 Their lamellar structure is easily seen in ordinary light ; but in polarised light it becomes 

 very conspicuous, and shows that the crystals are not simple, but polysynthetic. These 

 bear a marked resemblance to sections of triclinic felspar. If the lamellae are placed 

 parallel to the spider lines of the microscope, a series of alternate lamellae is extinguished, 

 whilst other alternating lamellae, less numerous but broader, are often extinguished at an 

 angle of about 40°. Hence these polysynthetic crystals are composed of lamellae of a 

 rhombic pyroxene, between which are intercalated other lamellae of a clinorhombic 

 pyroxene. The twin face of the two minerals is the macropinacoid of the enstatite, or 

 the orthopinacoid of the clinorhombic pyroxene. The lamella? of the augitic mineral are 

 generally thinner ; but, apart from this difference, the two kinds cannot be distinguished 

 in ordinary light : they are of the same colour, and possess the same intensity of 

 dichroism. Association of diallage with a rhombic pyroxene, similar to that just 

 described, has been observed in the rocks of Volpersdorf, Hausdorf, and Elfdalen. 5 



1 0. Wiegand, Tchermak, Min. Mitth., 1875, iii. p. 193. 



2 Zirkel, Mikroskopische Besehaff, &c, p. 401. Bosenbusch (Massige Gesteine, p. 261) has made the same observa- 

 tions regarding some diorites where the hornblende is present in large crystals. He observed that brown transparent 

 grains, or minute prisms, disposed like those above described, were often embedded in the hornblende crystals. He 

 observed others arranged in lines parallel to the base of the crystal. He does not believe their formation to have been 

 contemporaneous with that of the encasing crystals, but thinks the grains and prisms are due to some subsecpient 

 changes. He accepted this interpretation the more readily because the hornblende containing enclosures was more 

 fibrous and brighter in colour than that without them. 



3 I do not attach a sjiecific value to the term enstatite used in this passage. Bronzite and enstatite show more 

 difference macroscopically than they do in thin slices where a crystal of bronzite may become colourless, owing to the 

 thickness having been reduced by polishing. 



4 Perhaps less importance will lie attached henceforth to the presence of these enclosures for the purpose of 

 determining hypersthene and diallage. Kosman maintained that these enclosures were of secondary formation (Neues 

 Jalrrbuch, &c, 1869, p. 533). Trippke, in a recent paper (Neues Jahrbuch, 1878, p. 676), takes up the same view. 

 He considers them to be formed of opal. Should this interpretation prove correct, the microscopical diagnosis of 

 augite and diallage will become more and more difficult. 



5 Bosenbusch, Massige Gesteine, p. 463. 



