ORIGINAL ARTICLES. 
The Maégnesian Group of Igneous Rocks’ 
By F. Dixey, M.Sc., F.G.S., Government Geologist of Sierra Leone. 
1. Jntroduction. 
2. Description of the Magnesian Group. 
3. The Magnesian Provinces. 
4. Age of the Magnesian Group. 
5. Origin of the Magnesian Group. 
6. Relation to well-known Suites and Series. 
I. INTRODUCTION. 
[ROCESS rich in magnesium are abundantly developed in a certain 
few areas of the world, whereas they are rare or absent in the 
remaining areas. With reference to the abundance of magnesian 
rocks in Southern India, for example, Holland has stated? :— 
The most remarkable feature which has heen revealed by the recent 
microscopic study of the crystalline rocks in Southern India is the very 
great predominance of the pyroxenes (and especially of the rhombic 
forms of that group) amongst the ferro-magnesian silicates. In addition 
to the series described in this paper [Charnockite Series], which is 
probably the most abundant of all the rock groups in the southern portion 
of Peninsular India, and in every variety of which hypersthene is a constant 
and characteristic constituent, the central and eastern parts of the Madras 
Presidency are cut through by an enormous number of basic dykes, in 
which rhombic pyroxene is also a leading mineral. Judging thus by the 
mineralogical composition of the rocks, which is confirmed by the few 
chemical ‘analyses that have so far been made, the bases magnesia and ferrous 
oxide must take an unusually prominent place amongst the chemical 
constituents of the southern portion of the Peninsula. Besides occurring 
as a constituent of the enstatites, which are so widely distributed through 
the Madras rocks, magnesia appears in even larger proportions in the 
peridotites, which, either as dunites, saxonites, picrites, or their decomposed 
forms magnesite, serpentine, and steatite, arenow known to be far more 
abundant than was suspected at the time of the first recognition of these 
highly magnesian ultra-basic rocks near Salem in 1892. 
Again, Lacroix has given a description of two series of analogous 
igneous rocks in West Africa; the first of these, the gabbro- 
peridotite series of French Guinea,® to which probably belongs the 
enormous noritic intrusion of the Sierra Leone Peninsula,’ is 
characterized throughout by abundance of magnesian minerals, while 
the second,’ a series comparable with the Charnockite Series of 
1 Paper read before Section C (Geology) of the British Association for the 
Advancement of Science, Edinburgh Meeting, September, 1921. 
2 “ Charnockite Series of Igneous Rocks’’: Mem. Geol. Surv. India, 
vol. xxviii, 1900, p. 192. 
° M. A. Lacroix, “ Les Syénites Nephelinique de PArchipel de Los”: Nouv. 
Arch. du Mus. @ Hist. Nat., Paris, 5éme Serie, Tome 3, 1911, p. 108. 
4 ¥F. Dixey, ‘‘ The Narive of Sierra Leone’’?: Abs. Proc. Geet. Soc., 1921, 
p- 103. 
> M. A. Lacroix, ‘‘ Sur Existence & la Céte d’Ivoire d’une Série Petro- 
graphique comparable & celle de la Charnockite’’?: Comptes Rendus Hebdom. 
des Séances de lV Acad. des Sci., 150, 1910, p. 18. 
