77 



ever this compound at a low red heat he produced typical anor- 

 thite. This novel synthesis opens up a fascinating field for 

 speculation as to what would happen if water and oxygen gained 

 access to a highly heated zone of silicides which may form a 

 shell below what we ord ; narily speak of as the crust of the 

 earth. 



' ' With this slight digression into a field which lies beyond 

 the domain of geology I will -conclude by reminding you that 

 my main object has been to emphasize the importance of frac- 

 tional crystallisation as a factor in the evolution of igneous 

 rocks. " 



The lecture was illustrated by lantern slides. 



Explanation of Plate V. 



Fi^. 1. Hypersthene-angite-andesite. Cheviot Hills. Ordin- 

 ary light. Mag. 30. 



The larger constituents are labradorite (colourless), hyper- 

 sthene (grey with dark patches due to alteration ; section 

 of crystal on lower margin), and augite (grey with parallel 

 lines representing cleavage planes ; fragment of crystal 

 near centre of figure). The groundmass or matrix con- 

 tains small felspars, grains of augite, etc., and a con- 

 siderable quantity of glass which can only be distin- 

 guished under a higher power. 



This rock is of Devonian age, but it is a common 

 type in Tertiary and recent volcanoes, especially those 

 surrounding the Pacific Ocean. 



Fig. 2. Olivine-basalt. Ardtornish, Isle of Mull. Ord. light 

 Mag. 15. 



Group of olivine-crystals partially altered to serpen- 

 tine along margins and cracks (lower part of figure). 

 Augite in irregular patches (pale grey), labradorite 

 (colourless) and iron ores (black). Similar rocks are com- 

 mon in the central portions of the Pacific. 



Fig. 3. Basalt, Carnmonev Hill, near Belfast. , Ord lipht 

 Mag. 15. 



Felspar (colourless), augite (grey), and interstitial 

 matter (dark). 



Fig. 4. Portion of same slide more highly magnified lo show 

 the character of the interstitial matter. Ord. light Mag. 45. 



The interstitial matter occurs partly in the condition 

 of brown glass (centre of figure) containing the iron-oxide 

 in solid solution, and partly in the form of an aggregate 



