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VOLCANIC EOCKS OP' MOZAMBIQUE. 



229 



the neighbouring gullies were empty. This fact, together with the 

 comparatively high temperature (about 60° C), suggests that the 

 springs may be of juvenile origin, representing the last flicker 

 of expiring activity on the border of a volcanic field already much 

 reduced by erosion. Nearly a gallon of the water was evaporated 

 to. dryness in a clean prospecting-pan, in order that the residue 

 might be tested by blowpipe reactions. Carbon dioxide was present 

 in just sufficient abundance to be recognized. The residue was 

 very small, and only soda and silica could be detected. 



The rocks described in the present paper are confined to a 

 district rather more than 30 miles in length, extending from the 

 Mitikiti River to beyond the Sanhuti River. Amygdaloidal and 

 vesicular basalts are not the only types of lava represented. In 



Fig. 4. — Specimen of amygdaloidal basalt from the Sanhtiti- 

 River district. (Photograph by Mr. D. A. Wray.) 



the neighbourhood of the Monapo River — 2 miles north-west "of 

 Mochelia — a narrow dyke of hornblende-andesite penetrates a sheet 

 of amygdaloid (H.A. in fig. 1, p. 226), and Mr. Wayland found 

 pebbles of pyroxene-andesite on the debris-strewn flats of the same 

 district. Here also fragments of tephritic pumice are found, 

 though again not in situ. This rock clearly indicates another line 

 of variation which is more strongly represented in the north, for 

 the late Mr. R. L. Reid discovered an interesting occurrence of 

 phonolite associated with basalt and picrite-basalt in the neigh- 

 bourhood of the Sanhuti River. Other rocks not found in situ 

 are solvsbergite and segirine-trachyte. Thus, there occur together, 

 within the limits of what is a comparatively small volcanic field, 

 lavas of both alkali and calc-alkali types. 



VI. Period op Eruption. 



The period of eruption of the amygdaloids cannot be definitely 

 ascertained, but three lines of evidence point to their Tertiary, 

 and almost certainly post-Oligocene, age. 



Q. J. Gr. S. No. 287. s 



