258 STRATIGRAPHY 



of this trachydolerite or kenyte group, the trachytes, basalts, and limburgites 

 being quite subordinate in volume. 



As seen in the field, the rocks of the kenyte group occur in dome-shaped 

 elevations or rounded ridges ; both are jointed, and, where they have for some time 

 been exposed to weathering, very much frost splintered, so much so that the 

 traveller has to beware of trusting to any support from what appear to be solid 

 masses of lava. When a slight weight or pull was put upon the mass we frequently 

 found that it crumbled to fragments immediately. This of course was the result 

 of frost weathering. The large phenocrysts of anorthoclase usually resist weathering 

 better than the ground mass, and so come to stand out in relief, giving the rock 

 a rough bristling appearance. In the most recently erupted kenyte, as already 

 described under the head of Vulcanism, we found that the anorthoclase crystals 

 had been dropped clean out of the glassy matrix, and had fallen in showers at some 

 distance from the central active crater, and they have accumulated in such numbers 

 to the north-west of crater No. 2 that they have contributed in no uncommon 

 measure towai'ds filling that crater. In places the ground mass of the lavas of the 

 kenyte group becomes pitchy and glassy. This is notably the case with the recently 

 erupted kenyte bombs of Mount Erebus. 



Apart from the section afforded by the precipitous inward slope of the active crater 

 of Mount Erebus, as well as by the great inner Avail of the second crater, it is difficult 

 to make out any traces of stratification in the lavas and tuff's of Mount Erebus ; in 

 fact it is often difiicult to decide whether the rock material belongs to a massive 

 lava flow or to a tuff bed, inasmuch as frost weathering for the most part covers the 

 rock surfaces with a mantle of fine rubbly and earthy material in which fragments 

 of fresh felspar are very conspicuous. At first sight we took these zones of finely- 

 comminuted material produced by weathering for tuft' beds, but subsequent examination 

 showed that they were merely the weathered residues of what were probably super- 

 imposed lava sheets, the jimction line between which was hidden by the debris. 

 Flagstaft" Point was an exception to the above rule, in that there the lava showed 

 distinct evidence of bedding, having beneath it a somewhat glassy pumiceous layer. 



A description has already been given under the heading of Vulcanism of the 

 various parasitic cones in the neighbourhood of the Skuary * and Cape Barne. At 

 the Skuary there is also a trace of stratification. The clifts there are from 80 to 

 100 feet in height, and seemed to consist of lava flows one above the other, with 

 scarcely any dip. Examined under the microscope, one of the specimens secured 

 showed large phenocrysts of anorthoclase and a few porphyritic crystals of olivine 

 embedded in a hollow crystalline ground-mass of interlacing felspar laths, with 

 small grains of augite and some magnetite. 



Of the numerous small cones in the neighbourhood of the Skuary at least one 

 was undoubtedly, in the opinion of one of us (R. E. Priestley), the plug of an eroded 



* Cape Evans. 



