604 MR J. D. FALCONER 



of lava at the Inches. At the Harbourback and the Horse Island there occurs a thin 

 series of dark red ashy sandstones whose igneous constituents are probably to be referred 

 to the volcanoes of the plateau. The smallest neck in the Horse Island is partially 

 filled with this broken ashy sandstone ; the others are full of a dull green or reddish 

 agglomerate composed of blocks of ashy sandstone and shales, masses of earlier basic 

 lavas and ash, occasional fragments of plutonic igneous rocks, and, more rarely, portioDS 

 of charred wood impregnated with pyrites, the whole bound together by a matrix of 

 sandy ash. A central dip may in places be observed, and at the S.E. end of the east 

 islet the agglomerate may be seen lying conformably upon a hard white sandstone. 

 This agglomerate, and the Castlehill and Inches lava, probably occupy the position of the 

 red ashy sandstones elsewhere. 



Under the microscope the lavas (A) are basalts of a more felspathic type than the 

 Castlehill and Inches lava (B). Their ground-mass is composed of small lath-shaped 

 felspars, arranged as a rule in rude fluxion streams. They belong mostly to andesine 

 and labradorite, and are commonly clear and little decomposed. Augite granules have 

 been abundant, but are now as a rule replaced by calcite. (See PI. II., fig. 1.) Some 

 flows have originally possessed large porphyritic felspar crystals which are now replaced 

 by calcite and chalcedony. Large irregular phenocrysts of olivine are characteristic of 

 these lavas at the Inches ; they are never fresh, but always replaced by a mass of 

 serpentine and calcite. The ash associated with these rocks consists of small lapilli of 

 the same felspathic lavas and fragments of various clastic rocks, bound together by a 

 fine-grained, decomposed, and much calcified volcanic dust. 



The lava (B) is much finer-grained and less felspathic than the representatives of 

 the plateau basalts described above. Samples from the interior and margins vary 

 somewhat in texture, the former being slightly coarser and less markedly fluidal than 

 the latter. The marginal portions are frequently also minutely vesicular and much 

 brecciated. Small crystals of olivine in idiomorphic, pyramidal, or rectangular sections, 

 or in irregular corroded forms, are the only phenocrysts, and give the rock a characteristic 

 appearance under the microscope. Occasionally an intergrowth of olivine and augite 

 may form a large irregular porphyritic mass. In the groundmass the felspar microlites, 

 which vary from oligoclase to labradorite, are as a rule subordinate in quantity to the 

 augite grains, which are frequently idiomorphic. There is a little apatite and much 

 black magnetite dust, and in places a small quantity of brown glassy base. (See PI. II., 

 fig. 2.) The basalt in the neck in the Horse Island has similar microscopical characters. 



The most interesting feature about this rock, however, is the number and variety of 

 xenoliths and xenocrysts which it contains, and the effect of the caustic action of the 

 basic magma upon these. The brecciated margins of the flow contain abundant frag- 

 ments of earlier felspathic lavas very similar in character to the lavas (A), as well as 

 fragments of shale, limestone, and microcline-bearing sandstone. These have probably 

 been caught up during the flow, and have as a rule suffered little or no change beyond 

 induration or recrystallisation. The interior of the flow, however, contains numerous 



