22 ROYAL SOCIETY OF CANADA 



The name " Hastingsite '"' was suggested by Dr. Harrington as a 

 varietal name for this hornblende, thus connecting it with the region 

 where it occurs. 



Muscovite. — This mineral seems to occur in two definite and dis- 

 tinct forms. First, in comparatively small individuals, somewhat similar 

 in dimensions and habit to the biotite, which is the usual and more alnm- 

 dant mica. In this mode of development it is often intergrown with 

 the biotite. 



In the second place, the nmscovite occurs in much larger plates and 

 aggregates, in more or less intimate association with corundum, in those 

 types of the syenite which have consolidated from a magma supersatu- 

 rated with alumina. It is, therefore, present in greater abundance and 

 is more characteristic of these somewhat unusual types of the nepheline 

 syenite, which mainly by the almost complete failure of the ferromag- 

 nesian minerals, favoured the separation of the excess of alumina in the 

 form of corundum. The mineral, under these conditions of association, 

 has always been described and regarded as secondary, resulting from the 

 alteration of the corundimi. The supporters of such a view, argued 

 that every gradation in the process of this alteration may be seen, from 

 those occurrences, in which a comparatively pure crystal has been replaced 

 by muscovite. On the other hand, the peculiar conditions which at- 

 tended and contributed to the replacement have never been satisfactorily 

 explained. Both minerals are developed side by side in perfectly fresh 

 and unaltered rocks, the surrounding constituent minerals having under- 

 gone little or no perceptible change. Moreover, it is well known that 

 corundum is one of the most unalterable of substances when subjected 

 to ordinary processes of atmospheric decay, this fact receiving the 

 strongest support from the Ontario occurrences. The critical and ex- 

 tended study of these Ontario deposits of corundum, both in the field 

 and in thin sections under the microscope, shows that this apparent al- 

 teration is closely connected with some phases of pneumatolytic or vein 

 action, which immediately preceded the complete solidification of the 

 rock. The extreme phases of this alteration are best seen in the peg- 

 matitic or coarser varieties of the syenite, although examples are not 

 lacking in the more normal grained portions of these rocks. Indeed it 

 seems to belong to the same class of phenomena as the " corona " or 

 " reaction rims " which so frequently surround some of the earlier 

 i'onned minerals in many plutonic rocks. (See Plate 10.) 



The alteration in the case of the Ontario corundum is always to 

 muscovite and this mineral may be considered cliemically as made up of 

 orthoclase, corundum and water. Morozewicz has shown experimentally 

 that a magma such as that which on cooling gives risa to a soda syenite 



