49 



These syenites show a marked variation in the relative 

 proportion of the principal mineral constituents. Indeed 

 extreme and rapid variation in composition is one of the 

 most noteworthy features of their development, so that 

 it is possible to obtain from the same exposure, and even 

 from contiguous bands, different and quite distinct types 

 of rock. 



Some varieties are abnormally rich in nepheline, 

 while others contain a much smaller proportion of this 

 mineral, passing by insensible gradation into alkaline- 

 syenites, composed almost wholly of feldspar. Certain 

 types are relatively rich in ferromagnesian constituents, 

 which may be either mica or hornblende, while other 

 phases of these rocks contain very little, if any, of these 

 coloured minerals. Some exposures are characterized by 

 the interesting alkali-hornblende to which the name 

 hastingsite has been given, while in some localities garnet 

 is present in considerable amount. The actual relation 

 of these syenitic occurrences to the granite batholith is 

 not observable along this line of section, since the actual 

 contact between the two cannot here be seen, but examina- 

 tion elsewhere throughout the region has led to the con- 

 clusion that the nepheline and associated alkaline syenites 

 represent a peripheral differentiation phase of the invading 

 soda-rich granite along its contact with the limestone. 

 The actual relations of the nepheline syenite to the Grenville 

 series, on the other hand, is well illustrated in the railway 

 cutting in the village of Bancroft, which exposes the line 

 of contact. The character of this junction is shown in the 

 accompanying section through this rock cut at Bancroft. 



The nepheline syenite, which is clearly the invading 

 rock, seems to permeate the limestone, the latter being in 

 process of replacement by the syenitic magma. The 

 masses of limestone caught up in the syenite become 

 gradually disintegrated in the magma until they survive 

 merely as separated, irregularly rounded grains of calcite 

 often enclosed in the individuals of perfectly fresh nepheline, 

 hornblende or other minerals of the nepheline syenite or 

 lying between these with the form of the latter impressed 

 upon them. The derivation of these isolated grains of 

 calcite from the neighboring limestone is quite evident 

 even under the microscope for they sometimes show the 

 deformation and strain shadows observable in the invaded 

 limestone, while the minerals of the nepheline syenite in 



35064—4 



