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TOLCAKIC ROCKS OE MOZAMBIQUE. 



249 



rocks of the same type as those found in the north of the 

 Mozambicpie area. Microscopically, the east-and-west dykes of 

 Northumberland — Croolidene, Collywell, and Tjmemouth 1 — are 

 very similar to those described here. 





A. 



B. 



C. 



D. 



E. 



sib.j 



51-21 



52-42 



49-47 



51-31 



51-20 



Al,6 3 



17-99 



17-40 



18-70 



14-55 



18-89 



Fe 2 6 3 



4-63 



4-21 



15-05 





7-57 



Feb 



5-81 



5-16 





9-02 





IVlg \J 



(in 



0 ou 





0 oO 



o * o 



CaO 



9-36 



9-08 



8-90 



11-61 



10-52 



Na,0 



2-92 



2-65 



2-87 



1-79 



1-71 



K 6 



1-03 



1-77 



0-79 



0-60 



0-51 



H^O 



1-04 



1-26 



0-81 



1-41 



1-70 



Tib, 



0-42 



0-67 





1-00 



1-14 



PoOt 



tr. 











MnO 



0-27 



0-24 





0-47 





CO., 









1-47 





Totals 



100-79 



100-16 



101-93 



100-08 



99-99 











A. Dyke, Mochelia (an. Holmes). 



B. Sill, Sokoto Hill (an. Harris, Kirby, & Spencer). 



C. Basalt, Kallele Biver, 56 miles from coast, Somaliland. 



D. Crookdene Dyke, Northumberland (an. Smythe). 



E. Tynemouth Dyke, Northumberland (an. Stead). 



Contact-Metamorphism. 



The metamorphism of the sediments through which the dykes 

 and sills intrude is everywhere of a simple character. Usually, 

 the changes are restricted to bands extending only a few inches on 

 each side of the contact. Limestones are recrystallized to granular 

 calcite, and in places crystals of quartz are developed, indicating the 

 percolation of silica-bearing solutions from the lavas. Mr. Way- 

 land found that the upper part of a dyke, intrusive into calcareous 

 sandstone near Mochelia, had been removed by weathering, thus 

 exposing the sandstone walls which had enclosed it. Upon these 

 well-marked hexagonal jointing had been superinduced. 



Near the contact of this dyke steam-holes have been developed 

 in the sandstone, and these are now filled with opal and chalcedony, 

 the latter being pale brown owing to the incorporation of limonite 

 which was present in the original cement. Examination of the 

 unaltered sandstone in thin section shows that it consists of slightly 

 rounded grains of quartz, orthoclase, and microcline, cemented 

 together by calcite and limonite. Small nummulites are found 

 embedded in the cement. Near the dyke, calcite is replaced by 

 opal, the metasomatism having equally affected the nummulites. 

 In the amygdales of the sandstone, opal was first deposited, 

 chalcedony followed, and the infilling was completed by a clear 

 mosaic of quartz. (See PI. XXI, fig. 2.) 



1 M. K. Heslop & J. A. Smythe, Q. J. G. S. vol. lsvi (1910) p. 3. 



