﻿256 



DR. A. HOLMES ON THE TERTIARY 



[vol. lxxii, 



Water and other volatile fluxes. 



/ 



/ 



Residual Lime-zeolites and chlorite, 1 

 /'Liquors. Soda-zeolites and chlorite, V\ 

 / Silica and chlorite. J 



Sokoto Amygdaloidal (Containing 



Basalt. Basalt, any or all of 



\ f the amygdale 



\v / minerals.) 



Vesicular (Relatively enriched in 1 / 

 Basalt. soda.) J 



XVI. Hornblenhe-Andesite. (PI. XXI, fig. 5.) 



The specimen (No. 126) was collected from a small dyke 

 penetrating a flow of amygdaloid on the south side of the 

 Monapo, about 2\ miles north-west of Mochelia. In the hand- 

 specimen white phenoerysts of felspar (averaging 4 mm. x 2 mm.) 

 are seen distributed in a dark-grey ground-mass. Smaller crystals 

 of hornblende and flakes of biotite can also be distinguished. 

 The rock is compact, but a few very small amygdales containing 

 well-crystallized quartz are present. 



Under the microscope the chief minerals seen and their approxi- 

 mate proportions by weight, determined by the Rosiwal method, 

 are as follows : — 



Estimated Percentage 

 Minerals. by tveiglit. 



Andesine and inclusions 26] 



tt t.1 j n Jrhenocrvsts 



Hornblende 9 —40 



Biotite 5 J 



Plagioclase 



Orthoclase .. not determined ! Ground- mass 



Biotite:::::::::::::::::::::::: separately =eo. 



Hornblende J 



Total 100 



Part of the rock was crushed, and hj means of bromoform, 

 subsequently diluted with benzine, three groups of minerals were 

 separated : — 



(a) Hornblende and biotite sank in bromoform : in Thoulet solution 



diluted with water, the average specific gravity of the crop was 

 found to be 2-97. 



(b) Plagioclase sank in bromoform diluted with benzine at 2'67. 



(c) The remaining material — mainly ground- mass — was collected for 



analysis. Its average specific gravity was found to be 2:61. 



Mineral Group. Percentage by iveight. Sjwcijic Gravity. 



Andesine and inclusions 26 2" 6 7 



Hornblende and biotite 14 2'97 



Ground-mass 60 2 - 61 



4 Average specific gravity ... 2'67 



