134 THE INCLUSIONS OF THE VOLCANIC ROCKS 



connected genetically with the rocks in which they occur. The origin of olivine nochiles 

 has been the subject of a voluminous discussion into which there is no need to enter 

 here. The fact that the inclusions themselves form a differentiation series comparable 

 with that which would have been expected had the various basic volcanic rocks enclosing 

 them consolidated under plutonic conditions, suggests that they are actually frag- 

 ments from such a series which has arisen from the differentiation and consolidation 

 in depth of a portion of the primary basic magma. 



II. SANIDINITES AND MICROTIN1TES IN THE TRACHYTES AND KENYTES 



Sanidinite is the name given by Lacroix* to the coarse-grained miarolitic rocks con- 

 sisting predominatingly of sanidine or anorthoclase which are so often found as inclusions 

 of trachytes and certain kinds of phonolites. Similar rocks of more compact grain 

 and finer texture he has designated as " micro- sanidinites." Both types are represented 

 in the present collection. The analogous term " microtinite " f is used for similar in- 

 clusions consisting mainly of plagioclase felspar. J 



Prior § has described a coarse-grained inclusion in the trachyte of Cape Crozier 

 which consists of an aggregate of stout felspar- prisms with some pleochroic (yellow 

 to grass- green) segirine-augite and a little cossyrite-like hornblende ; the felspars are 

 partly oligoclase with symmetrical extinctions of about 8° and partly anorthoclase. This 

 rock appears to be a sanidinite, but differs considerably from those now to be described. 



Sanidinites and Microsanidinites in the Trachyte of Mount Cis 



By far the larger number of the felspathic inclusions have been found at Mount 

 Cis, a small parasitic cone 700 feet above sea- level on the foot of Erebus, about half-way 

 between Cape Royds and Cape Barne. The cone is composed of a dark grey highly 

 jointed rock determined by Dr. Jensen as a phonolitic trachyte. 



The sanidinites vary in dimensions from fractions of an inch to several inches, and are 

 more or less rounded in outline. Their immediate contact with the host is, however, 

 not smooth because of the injection of the ground-mass into the druses of the inclusion. 

 The rocks are seen in hand- specimens to be made up mainly of large platy felspar prisms, 

 more or less interlaced ; the spaces between the felspars are partly filled up with a brown 

 glass, partly with the dark grey vesicular ground-mass of the trachyte, but are seldom 

 completely filled, so that the. whole mass has a most distinctly miarolitic aspect. 

 Numerous black iridescent crystals are seen in the druses and to a less extent within 

 the felspars themselves, and these prove on crushing under oil to consist of a very yellow 

 olivine. Less frequently small dark prisms (sometimes twisted) of a?girine-augite 

 are found with the same mode of occurrence, and occasionally apatite and magnetite 

 are also noted. The length of the tabular felspars varies between a few millimetres 

 and 3 - 5 cm., their thickness in general being about one- fifth of their length. 



Although on a general inspection the felspars appear to be of the same nature, micro- 

 scopical examination shows that in most specimens they consist of sanidine, but in a 

 few cases of anorthoclase. Cleavage fragments of the former show no twinning, while 

 a fine albite lamellation may be observed in the latter. This is in accord with their 

 properties in thin sections, the sanidine showing only Carlsbad twinning, the anortho- 



* Les Enclaves, etc., p. 352. 



| From microtine, a term proposed by Tschermak for the glassy plagioclases. 

 J Lacroix, A., " Conclusions a tirer de Fetude des enclaves homocogenes pour la connaissance 

 d'une province petrographique — Santorin," Compt. Rend. Acad. Sc, cxl, 1905, p. 973. 

 § Loc. cit., p. 114. 



