ARRANGEMENT OF THE IGNEOUS ROCKS 395 



basin, together with syenite of the " Cove type," though the relations 

 between the two are not quite clear. 



Surrounding the ijolite zone, and lying above it, is a somewhat irreg- 

 ular zone of metamorphosed rock (hornstone) which occupies the sum- 

 mit of the ridge, as already mentioned. Surrounding this hornstone 

 zone, all round the Cove, except at the west where it lies inside this, is 

 a broad zone of leucite-porphyry which is fine grained and porphyritic. 

 This is apt to be mingled in places in a confusing way with tinguaites 

 and the nepheline-syenites, especially the " fine grained variety." The 

 tinguaites, however, are quite subordinate in amount, and scarcely in- 

 terfere with the general arrangement. 



Along the edges of the igneous area at various points are found small 

 patches and large strips of nepheline-syenite. This is either a rather 

 coarse grained foyaite* (Diamond. Jo type) or a fine grained shonkinitic 

 syenite. The small patches at the extreme periphery are of foyaite, and 

 a strip of this is also seen at the border of the western side, while patches 

 are noticed in the surrounding shales. The shonkinitic syenite forms a 

 large strip on the west, with the Diamond Jo type be}^ond (outside) it ; 

 is also met with along Cove creek and apparently above the leucite- 

 syenite at the northeast, and also as a rather large patch in the south- 

 eastern part. In Diamond Jo quarry the foyaite shows a distinct system 

 of joint planes dipping about 80 degrees southwest and splitting it into 

 thick plates. It is true that, as Williams says, the relationship of these 

 types to the leucite-porphyry is most confusing, and that the two tend to 

 commingle, but the evidence is clear that the foyaite (Diamond Jo type) 

 occupies the extreme peripheral position along a good part of the border, 

 while the relations of the " fine grained " (shonkinitic) syenite to the 

 leucite-porphyry are uncertain, though the former lies apparently out- 

 side or above the latter. 



Transition forms. — In general it is impossible to say whether these 

 various types are sharply separated from each other or whether they 

 grade into each other through transition phases. This is especially true 

 of the first three, the contacts between them being quite hidden, though 

 the ^probabilities are that the divisions are not sharp. The two syenites 

 certainly, as Williams says,f " pass into one another without showing 

 any line of separation whatever." 



He also remarks on a sort of flow structure seen in the foyaite, arguing 

 therefrom that the leucite-porphyry cooled first. Judging from the way 

 in which the two commingle and from the confused manner in which 



*I use foyaite throughout this paper as a general term for nepheline-syenite, with traehytoid 

 structure. Cf. Brogger, Zeit. Kryst., vol. xvi, 1890, p. 39. 

 * Williams : Op. cit., p. 277. 



