342 T. G. TAYLOR AND D. MAWSON. 
ing the rocks of the Mittagong district. To this end 
analyses have been made of the more typical igneous rocks, 
and magmatic names assigned. The blue hornblende men- 
tioned as occurring in the Gib syenite and in the trachyte 
of Mount Jellore has been carefully examined, and found 
to be allied most nearly to arfvedsonite. It is practically 
a silicate of iron and the alkalies, and it is noteworthy 
that it contains ‘75° of titanic oxide and °6* lithium oxide. 
The ‘‘norms’’ are set forth below, for comparison with the 
actual mineral composition noted under Section IV. 
I. Jellore Trachyte.—This specimen was taken from the 
very top of Mount Jellore close to the trigonometrical 
station. It hasa slight greenish appearance in the hand 
specimen due to decomposition of the hornblende. 
Composition of the “‘ norm ’’:— 
Quartz vas 2742 Ilmenite wes "46 
Orthoclase ... 23°91 Pyrite... a "10 
Albite ... Los SOL SS Apatite ae 67 
Anorthite | .j;.- "36 Water ... woo agelezal 
Diopside woo ORAS ea 
Hypersthene ... °26 100°42 
Magnetite ... 3°25 ——— 
The chief difference between the norm and the mode 
here is that instead of diopside the rock contains aegirine 
and arfvedsonite. 
Classification.—Class I., Order 4, Rang 2, Subrang 3. 
Magmatic name.—Toscanose. 
II. Melanocratic Gib Syenite.—This specimen probably 
represents the more central portion of the Gib rock syenite, 
and is distinguished from the light variety by having a 
higher silica percentage, and containing a considerable 
amount of magnetite which is absent in the latter. 
