OF THE ROSS ARCHIPELAGO 145 



All these inclusions, from the abundance of quartz they contain where not exten- 

 sively melted, may be safely referred to fragments of impure sandstone. The presence of 

 augite, wollastonite, and calcite shows that there must have been some lime present, 

 possibly as a calcareous cement. 



Quartz-bearing Inclusions in the Kenytes 



Two specimens only of this nature have been collected. The one was included 

 in the kenyte of Cape Royds, the other in that of Sentinel Hill. The former is a fine- 

 grained brownish grey banded rock, with large rounded cavities. In section it is seen 

 to consist of quartz in small angular grains, with lesser amounts of felspar, augite, and 

 magnetite. The brownish colour is due partly to a glassy base, and partly to limonite 

 staining. The felspar occurs in short rectangular forms giving symmetrical extinction 

 on albite lamella* of 33°, and is therefore labradorite, although more acid species may also 

 be present. The augite, on the other hand, is found in highly irregular embayed forms 

 and freely includes quartz. 



The other inclusion (E 20) is a slightly reddish streaky rock with a sintery appear- 

 ance which microscopic examination shows to be due to an abundance of glass. The 

 clear streaks are composed mainly of small rounded quartz grains lying each isolated 

 in the glass. Occasionally, however, they are accompanied by thin needles of a colour- 

 less mineral, which from its straight extinction, positive elongation, and moderate 

 birefringence appears to be sillimanite. Where there is the richest development of 

 these needles there are also some small clear hexagonal overlapping plates of tridymite. 

 There are a few larger prisms and rounded plates of another clear mineral, with refrin- 

 gence and birefringence much superior to those of quartz. It is uniaxial or almost so, 

 and optically positive. 



The more turbid streaks are almost opaque in section, and appear to consist 

 largely of glass with smaller amounts of the above-named minerals, together with 

 much fine opaque dust. In a few patches they are excessively rich in minute needles, 

 probably of sillimanite. Magnetite and haematite are abundant in both parts of 

 the rock. 



This rock, like the last, is clearly an altered sandstone. 



VII. CLASSIFICATION OF THE INCLUSIONS 



From the above descriptions and discussions it will be obvious that the inclusions 

 can be sharply separated into two categories, the one including all those rocks which 

 are genetically connected with the volcanic hosts, the other comprising those that have 

 no such connection and have been broken off from the sedimentary or older igneous 

 rocks through which the volcanic lavas ascended. This underlying distinction has been 

 made the basis of all the classifications that have been proposed. As early as 1884 

 Sauer proposed the terms "endogene" and "exogene" for the two classes, and 

 his terms are still applied by some German writers.* Lacroix in 1893 rejected them 

 for the terms "homoeogene" (from «Va\Xo?, analogous, and ytwao,, I beget) and 

 "enallogene" (from 5 m <ho? ( different, and ycwam), since the genetic relationship is 

 not necessarily one of direct parentage, such as is implied in the term endogene.f 

 Sollas in 1894 proposed the terms "xenocryst" and "xenolith" for foreign 



* Sauer, Erlaut. Preuss. Geol. Landsanst, Sect. Wiesenthal, 1884, p. 70. 

 f Les Enclaves, etc., p. 7. 



