[ADAMS A barlow] ALKALI SYENITES OF EASTERN ONTARIO IS 



SiO^ 



AI2O3 



Fe.Oa 



Cab 



MgO . . . . 



K2O 



FaaO . . 



Loss on ignition • • 



100.34 100.40 



The appearance of the nepheline on the weathered outcrops of the 

 nepheline syenite is remarkable. When surfaces of the rock, which have 

 been exposed to the action of the atmosphere are examined, each grain 

 or individual of nepheline will be found to be represented by a depres- 

 sion. At the bottom of this the nepheline grain can be seen with a 

 smooth rounded surface, as if it had been partially dissolved away, the 

 feldspar and iron magnesia constituents standing up above it on all 

 sides. The surface of the nepheline is coated with a mere film of de- 

 composition products and is of a faint bluish grey colour, the feldspar 

 weathering chalk white, and on breaking the rock open the nepheline 

 appears to be perfectly fresh. Evidently the nepheline is destroyed 

 much more readily by the weather than the other constituents of the 

 rock, and the alteration products are of such a character that they are 

 at once removed, leaving the surface of the mineral fresh and hard. This 

 peculiar method of weathering makes it possible to determine the pre- 

 sence or absence of nepiieline in any s]iecinien of the syenite from a care- 

 ful inspection of the weathered surface of the rock alone. In fact its 

 presence can be quite as certainly determined in this way as by means of 

 chemical tests or a microscopical examination. This simple method has 

 furthermore the advantage that it can be applied to large areas of rock 

 surface. (See Plates 3 and 4.) 



Sodalite. — This mineral was observed at a large number of widely 

 separated localities along tlie great belt of these syenite rocks in the 

 township? of Glamorgan, Fai-aday, Dungannon. Monteagle, Raglan, 

 Bnidenell, and as far as Clear Lake near the north-east end of the belt. 



It usually occurs in ill-defined irregular masses and patches, of 

 comparatively small size, in tlie nepheline syenite, especially in those 

 portions which are unusually rich in nepheline. It is also developed 

 along and in the immediate vicinity of certain cracks and fissures in the 

 nepheline, with no sharp line of division between the two minerals, the 



