74 ROYAL SOCIETY OF CANADA 



the great majority of other occurrences in that instead of augite, which 

 usually occurs in these rocks, it contains either biotite (lepidomelane) 

 or hornblende, as the iron magnesia constituent; augite being seldom 

 present and when it occurs, being always in very subordinate amount. 

 In this respect it resembles the Indian occurrences as well as those at 

 Miass, in which biotite is the prevailing dark constituent. 



4. The nepheline syenites of this area show a fine series of differ- 

 entiation products. These range from granite at the acid end of the 

 series, through alkali syenites rich in soda, corundum syenites and allied 

 rocks and nepheline syenites, to nepheline rocks, and with increase in 

 the quantity of iron magnesia constituents to varieties having the com- 

 position of Essexites, and even to more basic forms. 



As a result of this ahundance of soda in the magma is the frequent 

 occurrence of varieties of the nepheline syenite which are extremely rich 

 in nepheline, and which, by the disappearance of the feldspar, pass over 

 into rocks which are composed almost exclusively of nepheline and iron 

 magnesia constituents. These rocks are closely allied to the Urtite* 

 described by Eamsay from Finland, but contain hastingsito instead of 

 segerine as the prevailing dark constituent and when the norm is cal- 

 culated, are found to be relatively richer in the salic components, so that 

 they fall into class I (Persalane) of , the Quantitative System, while the 

 Urtite belongs to class II (Dosalane). These rocks thus constitute a 

 new group, the type whose analysis is given on page 42, belonging to 

 a new rang which has been called IMonmouthose of the order Ontarare 

 It is proposed to call this rock Monmouthite. 



In the terminology of the Quantitative Classification, the various 

 differentiation products which have been analyzed are embraced by rangs 

 1, 2 and 3 of orders 4, 5, G, 7 and 8, of the Persalanes and Dosalanes, 

 Sub-class I, with the exception of one of the new types which is a Dosa- 

 lone. A somewhat similar, though less extreme, differentiation of an alkali 

 rich magma has been described by TealP in the district about Lake 

 Borolan, concerning which he writes : " The facts above described 

 clearly prove that we have in the plutonic complex of Lake Borolan, a 

 connected group of rocks formed by the consolidation of alkaline magmas 

 rich in soda the evidence available suggests that the quartz- 

 syenites shade into the quartzless syenites and these again into the 

 nepheline syenites." 



' Ramsay, W. — Das Nephelinsyenltgebiet auf der Halbinse] Kola-Fennla. 

 15. 2, Helsingfors, 1899, p. 22. 



' Teall, J. J. H. — Nepheline Syenite and its associates In the North West 

 of Scotland— Geol. Mag., Sept. 1900, p. 390. 



