PRESIDENT S ADDRESS SECTIOIV C. 



207 



and of lavas formed by mixing these in different proportions then 

 took place. Consistent with this view is the corrosion of the anortho- 

 clase in the more basic types. 



The Western District Area.— The alkali rocks of Coleraine, 

 Carapook, &c., in the western district have been partially des- 

 cribed by Dennant (172, 175), and by Hogg (174). 



Trachytic and basaltic rocks occm'. Their relative ages in 

 most places remains uncertain from lack of field evidence. The hill 

 called Adam, North of Coleraine, however, is composed of a basic 

 reck described by Hogg as an olivine-basalt, and penetrating it is a 

 dyke of the trachytic type suggesting the possibility that here the 

 order of eruption is from basic to acid. This cannot yet be regarded 

 as proved to be the general order for the district. A section of the 

 dense black rock from Adam shows that its genei-al characters are 

 those of an olivine-basalt. 



Olivine and augite phenocrysts are set in a ground mass 

 consisting of lath-shaped plagioclase, granular augite, and magnetite. 

 Its relation to the alkali series is suggested by the presence of a few 

 large corroded crystals of anortiioclase. Otlier basic rocks occur at 

 Mount Koroite, &c. 



The trachytic rocks are more widely spread and show consider- 

 f.ble uniformity of petrographic characters. The rock from the 

 Quany at Coleraine may be taken as a tj-jie. It is a greenish, dense 

 rock, weathering to a veiy light colour. It consists mainly of anortho- 

 clase in oi-thopliyric prisms with few phenocrysts. The rock is not 

 quite fresh, and scattered crystals of segirine have been partially 

 or wholly changed to chlorite. Some magnetite is also present. It 

 irt best described as an anorthoclase-trachyte. In some varieties of 

 the trachytes from this disti-ict little or no ferro-magnesian minerals 

 are present, and the rock consists' then almost entirely of felspar. 

 The two analyses quoted below are by Dennant, and show that these 

 are rather more acid than the anorthoclase-trachytes of Macedon. 



1. Trachyte, Little Rock North of Coleraine. 

 i. Trachyte, Carapook. 



