MOUNT CIS AND TUFF CONE 221 



debris of Cape Royds by any members of the scientific staff of the expedition. 

 Erratic specimens, described by Mr. J. Allan Thomson in his notes on the " Inclusions 

 in the Lavas of Ross Island," in the second part of this volume, seem undoubtedly 

 to have come from this source, or from a similar cone. 



The main mass of the rock composing this plug is a homogeneous grey trachytic 

 lava with long drawn-out vesicles with, however, numerous inclusions of sauidinites, 

 consisting of felspar (sanidine or anorthoclase) and a little olivine. The olivine is 

 of very secondary importance, and appears in hand specimens as a few small grains 

 of a very clear gem olivine of a yellowish colour, mostly grouped about the exterior 

 of the sanidiuites. In some cases ground mass is not excluded from these sanidinites, 

 but is only much more sparsely distributed, and it seems probable that these 

 particular specimens are formed by the segregations of porphyritic felspars. 



Besides the coarse-grained sanidinites, there are a number of fine-grained in- 

 clusions which may be referred to two main types : — 



1. Microsanidinites, very similar to the sanidinites in composition and structure, 

 but aphanitic in grain. 



2. Enallogenous inclusions of the rocks through which the lava has burst. 

 These are especially important, since they include fragments of altered sandstone 

 and dolerite, which may be referred without doubt to the Beacon Sandstone 

 Formation. 



This is exceptionally interesting, as confirming the theory that the Ross Sea is 

 a sunkungsfeld, due to a north and south fault along the Victoria Land Coast. 



TUFF CONE 



On October 20th, when collecting specimens from Mount Cis, we noticed another 

 conical-looking hill about 400 yards to the N.N.E. On examination this proved 

 to be also a portion of a parasitic cone, and the rock was a very beautiful agglomerate 

 of angular fragments of variously-coloured volcanic rocks. 



Several types of kenyte and kenyte tuffs were represented, some of the fragments 

 of kenyte being of a far more acid type than any hitherto found at Cape Royds. 

 Basalts also appear to be represented by occasional fragments, and yellow lapilli 

 were common, and the whole is cemented together very firmly by fine material of 

 a dark grey colour. This was by far the most compact kenyte tuff found. 



On the southern side of the cone the rock was strongly weathered, the weather- 

 ing having worked along the cement much faster than along the fragments, and 

 in many cases this weathering had gone so far as to completely chisel out large 

 fragments of lava, which could only be known to have belonged to the cone by the 

 adhesion of a little of the cement. The only place where erratics of this rock were 

 found was on some hills a little to the south of west from the cone. 



2 F 



