478 TransacfAons. — Geology. 



in the rock as ic passed through underlying beds. They are 

 usually small, but I observed one a centimetre in length. 



The specific gravity of the rock is 2-65. This is rather low 

 for a syenite, but the small amount of basic mineral present 

 and the large predominance of orthoclase feldspar accounts 

 for it. 



A partial chemical analysis gave the silica-percentage 

 as 58. This is also low for a syenite. This may be explained 

 by the small amount of free silica and the large amount of 

 altered feldspar present in spite of the comparatively small 

 amount of ferro-magnesian mineral that occurs. 



A microscopic examination shows that the rock is prin- 

 cipally composed of idiomorphic feldspars in a crystalline 

 groundmass. The feldspars are much clouded by alteration, 

 and occasionally occur as binary twins ; others show undulose 

 extinction and zonal structure. They are mainly orthoclase, 

 but plagioelase is also present. In sections from the middle 

 of the dyke quartz is rare, but in sections from the edge it 

 was observed in small individual crystals, and also replacing 

 feldspar as a mosaic of small grains. This is perhaps due to 

 pressure as well as to the alteration of the feldspar. 



The ferro-magnesian mineral which, no doubt, occurred 

 originally in the rock has been replaced by chloritic material 

 which shows at times a doubtful pleochroism. The small per- 

 centage of ferro-magnesian mineral is characteristic of feldspar 

 porphyries. However, on digesting the powdered rock with 

 hydrochloric acid for twenty-four hours the solution gave, 

 with ammonia, a decided pi-ecipitate of ferric hydrate. This 

 must have come partly from the alteration products and from 

 small crystals of iron-ore forming an original constituent of 

 the rock. The groundmass is at times hard to determine, 

 owing to alteration, but it appears to consist of small feld- 

 spars, which are so completely clouded as to be opaque 

 between crossed nicols. In the finer portions of the rock the 

 groundmass is clearly holocrystalline. A larger quantity of 

 free silica is also present here. The porphyritic crystals, too, 

 do not exhibit a distinct crystalline outline, but have the form 

 of allotriomorphic grains. 



The inckisions in the rock, which I took at first for altered 

 feldspars, are most probably fragments of a trachytic or 

 andesitic lava. They consist of a semi-crystalline groundmass, 

 and in one I noticed small porphyritic crystals of plagioelase 

 which had undergone a certain amount of alteration, but not 

 sufficient to render tlieir determination at all doubtful. No 

 ferro-magnesian mincjrals occur in them, but I should say 

 they are most probably fragments of andesitic lava. It is 

 possible that they have been produced from feldspars by 

 inetasomatic processes, but I think it hardly likely. .\s vol- 



