June 19, 1921 bowen: monticelute alnoite 279 



attention the fact that some new occurrences had been found in the 

 course of a road-materials survey of the area which involved, in a 

 petrographic way, merely the identification of the rock as alnoite.'* 

 An examination of one of these occurrences, that at Isle Cadieux, 

 proved it to be of particular interest, and I am greatly indebted to Dr. 

 Harvie and to the Director of the Geological Survey of Canada for 

 the opportunity of making a more detailed study. 



The rock is on the whole particularly fresh for a rock of this kind 

 and the paragenesis of its minerals unusually clear. A fuller descrip- 

 tion will be given in a paper now in course of preparation which em- 

 bodies also experimental studies designed to throw some light on the 

 mineral relations. In the meantime it was thought desirable to pub- 

 lish a note on the petrography of the rock mass. 



The greater part of the rock is a fine-grained dark gray mass in 

 which the individual minerals are not distinguishable in the hand 

 specimen except in the case of large poikilitic biotites that are its 

 principle constituent. 



Under the microscope the rock is seen to consist of biotite, olivine, 

 augite, melilite, perovskite, apatite, titaniferous magnetite, and al- 

 teration products, largely carbonates. It is therefore a typical alnoite 

 but differs from all alnoites as hitherto described in that it contains 

 two olivines, namely, ordinary chrysolite and also monticellite, the 

 latter usually in amounts considerably in excess of the amount of 

 chrysolite. 



IVIonticellite, biotite, and melilite constitute groundmass minerals 

 and often poikilitically include augite and chrysolite. Plainly a great 

 deal of resorption of augite and chrysolite has occurred, their places 

 being taken by biotite, monticellite and melilite. There is therefore 

 a marked difference between the chrysolite and monticellite in the 

 manner of their occurrence. Not infrequently monticellite forms a 

 reaction rim about chrysolite with the two olivines in optical contin- 

 uity. In such cases the great difference in birefringence is particu- 

 larly clear on account of their uniform orientation. In addition to 

 the different mode of occurrence and the greatly inferior birefringence 

 of the monticellite it is further contrasted with the chrysolite in being 

 plainly optically negative, whereas the chrysolite is just upon the 

 border between the positive and negative members of the forsterite- 

 fayalite series and its sign is therefore doubtful. 



* Geol. Survey Canada, Summary Rept. 1916: 198-206. 



