522 Reviews — Magnesite Deposits of Grenville District. 



titanium; the quantities are, however, very small, ranging from 

 •0005 to -0031 per cent, while iron does not exceed '006 per cent. 

 The authors believe that the colour of amethyst is mainly due to 

 manganese, which probably exists in the form of colloidal particles of 

 ultramicroscopic size, consisting of manganese oxide. Since the 

 colour of rose quartz when destroyed by heat cannot be restored by 

 daylight or by exposure to radium, it is considered not to be due to 

 any inorganic substance. 



The cause of the peculiar blue colour of quartz from Virginia was 

 also investigated. This i= fourd : ~ a ] 'ghly titaniferous petro- 



d to be in the main an 



;ring of light by minute 



>hic regularity. This is 



>ur of the quartz of the 



[eating does not destroy 



oured varieties are com- 



)0° C. for ten minutes. 



lggestive piece of work, 



.. Inch io wen wortny ot imitation on other minerals whose colour is 



due to minute quantities of some foreign substance. By way of 



criticism it may perhaps be suggested that the authors have scarcely 



paid sufficient attention to the possible influence of the iron oxides 



present in determining the colour of these varieties of quartz. 



R. H. Rastall. 



YI. — The Magnesite Deposits of Grenville District, Argenteuil 

 County, Quebec. By M. E. Wilson, Geological Survey of 

 Canada. Ottawa, 1917. 



IN recent times magnesite has become increasingly important as 

 a refractory mineral for metallurgical purposes, and during the 

 last three years a demand has sprung up in America for a supply 

 from home sources, most of the material used having been formerly 

 imported from Austria and Greece. This has led to the development 

 of a considerable industry in Quebec. The magnesite is found in the 

 rocks of the Grenville Series, the oldest subdivision of the Pre- 

 Cambrian of the district. These were originally sediments consisting 

 of sandstone, shale, and limestone, which have undergone an intense 

 degree of metamorphism ; the limestones have been converted into 

 crystalline marbles, partly dolomitic, while some varieties contain 

 a proportion of magnesium carbonate in excess of that required to 

 form dolomite ; some samples show as little as 7 per cent of lime. 

 From the descriptions given in this memoir it appears that the 

 enrichment in magnesia may be due in part at any rate to intrusion 

 of masses of pyroxenite and other basic rocks rich in magnesia con- 

 nected with the Buckingham Series. The process may have been 

 effected largely by the aid of pneumatolytic solutions arising from 

 basic or ultrabasic magmas. The magnesite is largely associated with 

 serpentine and diopside rocks, and the whole lias been crushed by 

 dynamic metamorphism into lenticular masses of varying size and 

 composition. Petrographically the rocks are of great interest : the 



