508 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



been examined in detail by one of us (H.S.J.), though the 

 description is not yet published. It is composed of plagio- 

 clase (53%), namely labradorite zoned nearly to albite on 

 the periphery, of titaniferous augite (20%), olivine (15%), 

 magnetite, ilmenite, and a little biotite, and in fact differs 

 only from the Prospect rock in having more felspar and 

 olivine and much less augite, and in having no aplite of 

 alkaline character associated with it. It also exhibits to 

 perfection secondary analcite in enlarged cracks in the 

 labradorite crystals. 



On the other hand, though distinctly less alkaline than 

 the normal essexites, the Prospect rock is closely related 

 to that group, for in composition it is almost identical with 

 the Norwegian essexites described by Brogger — those of 

 Solvsberg, Brandberg, and Tof teholmen — which occur in an 

 alkaline comagmatic region, and are well recognised as 

 subalkaline members of the essexite family. The analysis 

 of the essexite of Tofteholmen (see column VI) may be 

 compared with the analyses of the main rock of Prospect 

 (Nos. IV and V). The constituents of the Solvsberg 

 essexite are a violet titaniferous augite (26%), labradorite 

 and a little orthoclase (together 46%), olivine (12J-%), biotite 

 (9j-%), iron ores (5%), and its chemical composition is shown 

 in column VII. 



The Prospect essexite resembles the foregoing rocks in 

 mineralogical as well as chemical composition ; but it com- 

 pares closely in the latter though not in the former respect 

 with the subalkaline (alkali-calcic) members of the alkaline 

 lamprophyres. As examples we may mention the camp- 

 tonites of Kjose-Aklungen, of the La Plata Mountains, 

 Colorado, and of Mount Ascutney, Vermont, U.S.A.; and 

 the monchiquite of the Rio do Ouro, Sierra de Tingua, 

 Brazil. Analyses of the two latter rocks are quoted here 

 (Nos. VIII and IX respectively in Tables III and IV), and 



