132 C. H. GORDON — SYENITE-GNEISS (lEOPARD ROCK) FROM CANADA. 



up to many inches in diameter. Similar masses of pyroxene as well as crystals 

 are also sometimes imbedded in the apatite." 



It is also worthy of note that the form of the deposits in both gneiss and pyrox- 

 enite is strongly suggestive of crushing and squeezing. This w^as remarked by 

 Harrington,* who says: "The apatite masses look as if they had been driven or 

 squeezed into the curious forms which they now present during the folding or 

 crumpling of the enclosing rock," 



The explanation of the large apatite crystal (pages 103, 127) in the 

 leopard rock is not without difficulty. One or two instances only were 

 observed and we have no information that they ever occur in the streaked 

 gneiss. From the manner in which the ellipsoids and bands of the gneiss 

 arrange themselves concentrically about the apatite, it is clear that the 

 crystallization of the apatite took place prior to the development of the 

 gneissic structure. 



Four views may be suggested to account for the apatite crystal in its 

 present position : 



1. That it crystallized out of the original magma before the solidification of the 

 latter. From the relations of both the crystallized and granular deposits of apatite 

 occurring in the syenite-gneiss, it is evident that an explanation that will account 

 for the presence of one must apply also to the other. We have considered already 

 the objections to the view of the formation of these deposits in the original magma 

 (page 127). These are the large size of the crystals, their inclusions and rounded 

 outlines, and the extent and more or less crushed condition of the deposits. 



2. That it is due to segregatiou and crystallization after the solidification of the 

 rock. The belief is expressed l)y Harrington in the report above cited that in 

 many cases there has been a segregation of apatite and other minerals which ac- 

 company it from the surrounding rock into irregular or lenticular masses without 

 any true cavity or crevice having ever existed. The growth of crystals by replace- 

 ment in situ has been noted by various observers. f 



Indications of sucli development appear in the small hornblende crystals occur- 

 ring in the streaked gneiss, as described in another part of this paper (page 118). 

 The development of large crystals by this process, however, has not been demon- 

 strated and is considered improbable. 



3. The third view is that they were deposited in a cavity in the syenite prior to tiie 

 development of the gneissic structure. The objection to this hypothesis lies in the 

 difficulty of accounting for the obliteration of the cavity without crushing the apa- 

 tite. It may be supposed that the cavity was large in proportion to the size of the 

 crystal and did not become closed at once but gradually, and that by the time the 

 walls had closed in, the surrounding rock had become sufficiently plastic to adjust 

 itself about the crystal without breaking it. The objections to the view that the 

 rock had reached such a condition of plasticity have been considered (i)age 130). 

 Moreover, the former presence of a cavity should be indicated by an irregularity 

 in the arrangement of the ellipsoids about the apatite, which does not appear to 

 be the case, though the observations upon this point were not conclusive. 



* B, J. Harrington : Geol. Survey of Canada, Report G. p. 7. 

 t C. R. Van Hise : Am. Jour. Sci., 3(1 ser., vol. 33, 1887, p. 385. 



