CONCLUSION. 133 



4. The fourth view, and probably the correct one, is that the apatite was formed 

 in cavities in the pyroxenite, and that it became inclosed in the syenite during the 

 intrusion of the latter. The objection to this view, as in the preceding, lies in the 

 difficulty of accounting for the preservation of its crystallographic form. However, 

 as the ellipsoidal rock evidently represents an early stage in the development of 

 the gneissic structure, it is probable that the differential movement at this point 

 was not so great but that the crystal was able to withstand the strain. 



SIMILAR STRUCTURES IN OTHER ROCKS. 



Analogous structures in mica-gneisses are well known. A variety called 

 stengeliger-gneiss (wood-gneiss), according to Zirkel,* is characterized in 

 some cases by bands of mica winding about the stalk-shaped or wreath- 

 shaped feldspar-quartz masses, so that these are inclosed on all sides by 

 the mica layers. On cross-fracture, therefore, these will show discoidal, 

 ellipsoidal, elongated, roundish, trapezoidal figures formed by the mica 

 bands. Up to this point the description applies very well to the Ottawa 

 occurrences, except in the character of the mineral forming the bands. 

 Beyond this, however, the analogy fails, as further alteration produces 

 an asbestos-like structure which has no counterpart in the rocks under 

 consideration. 



Rothpletz has noted f a structure analogous to that here described in 

 the greenstone-schists (actinolite-schists) of Hainchen, and Williams % 

 from northern Michigan. Both of these observers ascribe it to breccia- 

 tion in situ, while the former explains the rounded character of the frag- 

 ments and the production of much of the interstitial material by the 

 rubbing together under the action of much orographic pressure of a mass 

 already finely subdivided by cracks. Lawson has described similar 

 occurrences in the Lake of the Woods region. § 



Conclusion. 



Reviewing now the different hypotheses in the light of all the evidence 

 available, it is apparent that no one of them seems to offer a full and 

 adequate explanation! The ellipsoidal and the gneissic structure in 

 these rocks are clearly closely related in origin, and any conclusion affect- 

 ing the one has a direct bearing upon the other. Our knowledge of the 

 processes by which the ancient gneisses were formed is extremely limited. 

 That they may be formed by dynamic processes has long been recog- 

 nized, and it is now well established that a laminated structure com- 

 parable to that of the gneisses may be produced in deep-seated igneous 



*Lehrbuch der Petrographie, band iii, p. 203. 



fZeitschrift der deutsch geology Gesell., vol. 31, pi. ix, x, 1879, pp. 374-397. 



X Bull. 62, U. S. Geol. Survey, 1890, pp. 166-177. 



§^Geol. and Nat. Hist. Survey of Cauada, Ana. Rep., 1885, Rep. CO, p..51. 



