264 Proceedings of the Royal Physical Society. 



Parallel arrangement of the felspar laths gives the rock in 

 parts a well-marked fluidal structure. 



The specimens sliced were stream boulders. A rock of 

 similar appearance, forming a flow or series of flows which 

 overlie the brown trachytes, is probably the source of these 

 boulders. The agglomerate mentioned above is composed 

 largely of angular fragments of a pale grey rock, which is 

 identical in microscopic character with that just described 

 (G. 8). 



II. Basalts (including Trachy-dolerites). 



Specimens were obtained from four lava flows and from 

 two dykes. 



(G. 2.) Microscopically this is a rock of ashy grey colour, 

 with scattered phenocrysts of felspar, augite, and olivine. 

 In thin section the phenocrysts are seen to be plagioclase 

 felspar, green augite, and olivine. From the extinction 

 angles in sections cut at right angles to the albite lamella 

 the felspar is seen to be basic labradorite. The olivine is 

 very fresh. None of the phenocrysts are abundant. The 

 bulk of the rock consists of a fine grained ground -mass, 

 which is largely felspathic. Laths of oligoclase, pale green 

 augite (prismatic in habit, and often occurring in cross twins), 

 minute needles of apatite, along with particles of iron ore, 

 are embedded in poecilitic fashion in a felspathic base. The 

 felspathic base has a refractive index well under that of 

 Canada balsam, and is probably orthoclase. 



(G. 6.) In this specimen the porphyritic constituents are 

 much more abundant and the ground-mass is much lighter 

 in colour. The phenocrysts of plagioclase frequently occur 

 in clusters, and have a glassy appearance in hand specimens. 

 Examined microscopically they exhibit albite, pericline, and 

 Carlsbad twinning, and well-marked zonary banding. Since 

 this banding disappears simultaneously with the albite 

 lamellse, it is probably due to ultra-microscopic twinning. 

 The inclusions consist of large plates of magnetite, small 

 rounded olivines, and needles of apatite. Many of the 

 crystals show also a marginal zone of inclusions of magnetite, 

 augite, etc., which have been enclosed by a later growth of 



