NEPHELINE-BEARING ROCKS. 181 



This intergrowth of augite and nepheline, though not un- 

 known, is a very rare petrological feature. It is developed 

 in the Lobauer Berg rock above mentioned to a finer degree 

 than here figured. In the Stewart's Brook rock there are 

 small aggregates of chlorite, probably pseudomorphous 

 after olivine, but these are absent from the Barrington 

 rock. This type of rock seems best classed as a theralite. 

 The low percentage of silica suggested by the abundance of 

 nepheline is confirmed by the fact that a determination of 

 silica showed the presence of 42*54°/° only. 



No. 2 of the Barrington series, which Mr. Wilkinson 

 remarks appears to pass by transitions into the other types 

 (3 and 4), has a very different fabric, the tabular plagio- 

 clases having a more parallel arrangement (fig. 2, Plate 6). 

 There seems to be no definite nepheline, but there are 

 present, interstitially, cloudy areas of a very low refractive 

 index, which may represent altered orthoclase, though the 

 occasional presence in them of spherulites of natrolite 

 suggests the possibility of the derivation from nepheline. 

 The augite occurs in large irregular grains with a slight 

 development of the ophitic structure. The presence of 

 numerous small inclusions of olivine and felspar gives it a 

 curiously pitted appearance. Olivine also forms numerous 

 large idiomorphic crystals. Ilmenite is abundant in small 

 plates, but apatite is rare. 



In the majority of the doleritic pebbles of the Peel River 

 gravels, the structure is rather intermediate between those 

 of the rocks described above. The tabular felspars have 

 not much parallel arrangement, the augites are varied in 

 the extent to which they show ophitic structure or idio- 

 morphism, but are always purple. Olivine is abundant, 

 ilmenite frequent in small grains, and apatite is not very 

 common. In one instance orthoclase is present, forming 

 interstitial intergrowths with plagioclase, and occasionally 



