18 ROYAL .SOCIETY OF CANADA 



weather. It has a su])vitreous and somewhat greasy lustre. It is un- 

 doubtedly an alteration product of the nepheline, the cleavage planes, 

 in coutigiious masses or areas, being eomniou to both minerals while 

 boundaries between the two aie rarely, if ever, sharp or distinct.^ 



Feldspar. — Plagioclase varying in composition from alhite through 

 oli^goclase to andesine, is the prevailing feldspar in all of these syenites. 

 Albite, with a small percentage of lime, seems to be the most common 

 variety. Phe specfic gravity of the rock near York river is 2.6207 and 

 2.625, while in a separation of the rock from lot 25, con. XIV of Dun- 

 gannon, it was found to be not greater than 2.623. The specific gravity 

 of the fresh oligoclase from the syenite was about 2.64, although some of 

 it which had undergone partial alteration was considerably lighter. The 

 andesine which is the feldspathic constituent of the nepheline syenite 

 from lot 12, concession XV of Dungannon, was determined by heavy 

 solution on fine fragments to be 2.668. An analysis of this feldspar is 

 given on page CT. 



A noteworthy feature in connection with the development of the 

 feldspar is the frequent occurrence of a thin mantle of plagioclase (al- 

 bite) more or less completely surrounding individuals and even aggre- 

 gates of hornblende and separating those from the surrounding and more 

 abundant nepheline. It has also been noticed as a border surrounding 

 calcite and between this mineral and the nepheline. This bordering zone 

 of plagioclase is rather variable in width, but shows very marked, 

 optical continuit}' over long distances, in this respect also being in close 

 agreement with similar feldspathic material which occurs filling up tlio. 

 various inequalities in the hornblende individuals formed as a result of 

 this mineral's imperfect crystallographic development — and also with in- 

 clusions of feldspar in the midst of the hornblende. In some respects 

 this phenomenon resembles certain " reaction rims, " and it is thus ex- 

 plained by Holland;^ but what seems a more reasonable explanation of 

 the Ontario occurrences is that as the plagioclase succeeded and to a 

 certain extent overlapped the crystallization of the hornblende, it would 

 have been attracted to such centres of crystallization as had already been 

 formed by the solidification of the earlier and more basic mineral. This 

 curious occurrence is well illustrated in certain of the hornl)lendic 

 varieties of the syenite exposed at the dam at Bancroft and at Egan 

 Chute on the York river. The larger crystals of corundum occurring in 

 the nepheline rich variety of the syenite at Craigiuont are also frequently 

 surrounded by a zone of plagioclase, separating tlie former mineral from 

 the nepheline. 



"■ Can. Rec. Sc. Vol. VII. No. 4, 1896-97. 



2 Mem. Geol. Surv. Ind. Vol. XXX, Part 3, 1901, pp. 190-101. 



