part 1] AXD ASSOCIATED ROCKS OF MOZAMBIQUE. 51 



A chemical analysis of the mineral gave the following results : — 



Composition. Constitution. 

 Constituents. Per- Molecular Molecular Per- 

 centages. Proportions. Proportions centaqes. 



SiO-2 5110 852 CaSi0 3 =378 43*85 



A1.,0 3 377 37 MgSi0 3 =274 27*40 



FhIO-5 2-05 13 FeSiOj =108 14*26 



Feb 778 108 NaFe(Si0 3 ) 2 = 26 6-ol 



MgO 10-96 274 



CaO 21*21 378 NaAl(Si0 3 ).> = 4 0*81 



Na.,0 0-91 15 



K.,0 trace — Al Oj = 35 3*60 



H">0 0-87 — TiO.; = 3 021 



HoOatllG°C. ... 0*89 — Si0 2 =31 1"85 



TiOo 0-21 



97*99 



Total _99*75 Water 1*76 



Total 99-75 



Specific gravity = 3'27. — -*-~~ 



The augite is thus mainly composed of diopside (85 per cent.), 

 with a small excess of hypersthene and alumina that together 

 constitute the hypothetical Tschermak molecule, and of aegirine 

 (6 per cent.). 



Origin of the Crystalline Limestones. 



The occurrence of small and sparsely distributed bands of 

 crystalline limestone interfoliated with gneisses that are apparently 

 of igneous origin, at once raises a number of puzzling problems. 

 Like the limestones of Ceylon, 1 the Mozambique examples are 

 usually unassociated with schists of definitely sedimentary origin. 

 Near Memba the association is with a hornblende-schist probably 

 derived from a basic igneous rock. South of the Lurio the lime- 

 stones occur with haematite-schists. 



The probabilities are that the limestones are of exogenetic 

 origin, for the gneisses are certainly intrusive into them, and have 

 in places such a composition as to suggest that great but localized 

 masses of limestone have been more or less completely assimilated. 

 However, whether the limestones were vein- deposits or chemical 

 or organic sediments, they must have been deposited in or on some 

 other formation, and the main question is to determine where this 

 formerly coexisting material is now to be found. A number of 

 possibilities may be considered : — 



(a) The materials of the rocks formerly coexisting with the limestones 

 may have been above the present surface, and therefore now entirely 

 removed by denudation, or they may be below the present surface, 

 and therefore still unexposed. 



(6) The materials of the coexisting rocks may be wholly or in part those 

 actually exposed at the present surface ; in which case the gneisses 

 may be regarded either as the product of their refusion, or as 

 composite rocks due to their assimilation by a granitic magma that 

 advanced laterally from greater depths by a process of lit-par-lit 

 injection. 



1 A. K. Coomaraswamy, Q. J.G.S. vol. lviii (1902) p. 424. 



e2 



