14 THE VOYAGE OF H.M.S. CHALLENGER. 



The enstatite of Harzburg and the hypersthene of the norites l present the same poly- 

 synthetic structure when examined with crossed Nicols. Mr. Trippke 2 has lately dis- 

 covered similar polysynthetic associations of diallage and enstatite in the olivine rock of 

 Groditzberg. A section of a crystal of enstatite from the latter locality, cut following the 

 brachypinacoid, shows parallel lamellae, some of which are extinguished between crossed 

 Nicols when their principal axis is parallel to the threads of the reticle, while the others 

 are extinguished at an angle of about 38. Sections cut parallel to the macropinacoid 

 give extinctions parallel and perpendicular to the vertical axis. From what I have been 

 able to observe, the serpentinous peridotic rocks of Eubae, described by F. Becke, present 

 polysynthetic associations similar to those here described. 



In the slides containing sections of enstatite I have also found rather large grains of 

 olivine, similar to those described above. They are elliptical in form, like the pyroxenic 

 mineral, from which, however, they differ in being entirely devoid of traces of lamellar 

 structure. They contain, moreover, numerous enclosures, apparently liquid, occurring 

 sometimes in lines, and sometimes scattered irregularly through the sections. I have 

 never noticed any such enclosures in the enstatite of the rock under description. 



After having described the general distinctive characters of the constituent minerals 

 of the rock, I propose to examine in detail certain features of the olivine, and more 

 especially the enstatite sections deserving of attention. We shall thus arrive at the 

 question of the origin of the rock, as far as this problem can be elucidated, solely from 

 the standpoint of microstructure. We have to ascertain whether certain deformations 

 shown by sections of olivine and enstatite should be interpreted as having been produced 

 by the action of a moving igneous magma (fluxion-structure), or should be considered 

 more or less abnormal forms assumed by these sections, to which analogies may be found 

 among the crystalline schists. If the explanation of fluxion-structure could be adopted, 

 it would undoubtedly furnish the strongest argument for the igneous origin of the Eocks 

 of St. Paul. So obvious, indeed, is the resemblance between the structure about to be 

 described and that of true fluxion-structure, that at first sight they may readily be 

 regarded as identical. In certain preparations of the Rocks of St. Paul, especially in those 

 with more or less banded structure, the large sections of olivine and enstatite are disposed 

 on a line with their vertical axis according to the direction of the bands. It looks as if 

 this disposition had been brought about by the motion of a mass in fusion, in which 

 crystals of large dimensions were floated away by the flow of the magma. What tends 

 to confirm this interpretation of the facts is, that on the edge of the sections of these 

 crystals something like traces of fusion may be observed, the olivine and enstatite 

 seeming to dissolve insensibly into the surrounding mass. Still more worthy of atten- 

 tion are the modifications of the section represented in fig. 2. Besides its fracture and 

 the displacement of its fragments in the direction of the bands, the crystal, as I have 



1 Rosenbusch, Massige Gesteine, p. 478. 2 Trippke, Neues Jahrbuch, &c., 1878, p. 673. 



