[ADAMS & BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 9 



plutonic activity. They belong to one petrographical province; never- 

 theless for purposes of convenience of description, they may be con- 

 sidered as divisible into four groups, although it must be understood 

 that no arbitrary lines exist in nature between these respective sub- 

 divisions. 



1. The ISTepheline Syenite. 



2. Eocks of the Urtite and allied groups. 



3. The White Alkali Syenite. 



4. The Eed Alkali Syenite. 



1. The Nepheline Syenite. — This is made up essentially of an acid 

 plagioclase, usually albite, with nepheline and biotite (lepidomelane), 

 hornblende or pyroxene. Orthoclase, microchne and microperthite are 

 occasionally found, but when present rank merely as accessory con- 

 stituents. 



2. RocJcs of the Urtite and Allied groups.— The nepheline syenite 

 exhibits extreme variations in the relative proportion of its constituent 

 minerals, passing by a decrease in the amount of plagioclase present 

 into rocks composed exclusively of nepheline and ferro-magnesian 

 minerals, and of these into varieties composed on the one hand, almost 

 exclusively of nepheline (Monmouthite) or, on the other hand, into very 

 basic varieties composed almost exclusively of iron magnesian con- 

 stituents and approaching Jacupirangite in composition. 



The rocks of groups 1 and 2, in addition to their essential consti- 

 tuents almost invariably contain a certain amount of calcite. Scapolite 

 is also a frequent constituent. Some of the less common accessory con- 

 stituents are, garnet, sodalite, cancrinite, fluorite, muscovite, corundum, 

 magnetite, pyrite, sphene, zircon, apatite, spinel (automolite), graphite 

 and eudialite. 



3. White All-all Syenite. — This differs from the nepheline syenite 

 in that the nepheline occurs merely as an accessory constituent, or may 

 be entirely lacking. The rock is thus composed of an acid plagioclase 

 and the ferro-magnesian constituents, the latter, however, being present 

 usually in very subordinate amount. 



4. Red AlJcali Syenite. — This rock is distinguished at once from 

 the preceding syenite by its pinkish or reddish colour. Like the white 

 syenite it contains plagioclase, usually albite, as the predominant feld- 

 spar, but orthoclase and microcline are usually present and are relatively 

 more abundant. Occasionally a little nepheline occurs, but when found 

 is generally decomposed to a reddish or pinkish gieseckite. Magnetite 

 in small irregular crystals and grains is usually present. Biotite is 

 the iron magnesian constituent and is, as a general rule, present in very 

 subordinate amount. When specimens are examined by the unaided eye, 



