345 



they are so large as to show crystallographic outlines. The largest 

 are, however, extremely small as compared with the porphyritic 

 constituents occurring in the same rocks. Gradations in size may 

 frequently be seen in one and the same slide (pitchstones of Corriegills 

 shore and near Tormore on the west side of the Island) and when 

 this is the case the larger microlites are aggregated together so as 

 to form most exquisite arborescent groups. Some of these groups 

 remind one of fir trees ; others resemble, as Mr. ALLPORT has pointed 

 out, the delicate sprays and tufts of Batrachospermum. These 

 arborescent groups are always surrounded by a zone of clear glass ; 

 the interspaces between the zones being occupied by glass crowded 

 with acicular microlites which can only be distinctly seen with a 

 high magnifying power. 



Various opinions have been held as to the nature of these 

 microlites. Professor ZIRKEL referred to them as hornblende without 

 however, giving any decided evidence as to their character. Mr. 

 ALLPORT, having determined the existence of crystals of pyroxene 

 in the rock, assumed in his earliest papers that the microlites were 

 of the same nature. The pyroxenic nature of the microlites was 

 believed in by VOGELSANG and is accepted by ROSENBUSCH in the 

 last edition of his work on the massive rocks (p. 406). In 1881 } 

 however, Mr. ALLPORT showed conclusively that he had been mistaken 

 in his original identification, and that Professor ZIRKEL had been 

 right in regarding them as hornblende. Cross-sections of the larger 

 prisms, which often form the central stems in the arborescent 

 aggregates, give approximately the angle of hornblende (124 30) and show 

 only the form, 110 (see Fig. 4, Plate XXXIV.). The maximum extinction 

 in the prismatic zone is 15. There is another point about the large 

 microlites, which may easily be observed, but which does not appear 

 to have been recorded. They have in most, if not in all cases, a 

 central core of glass and the form of the core is that of the 

 hornblende prism the core is in short a negative crystal. Very 

 frequently the hornblende prism is imperfect in its periphery, a 

 small portion being required to completely enclose the central core 

 of glass. The ends of the minute prisms which show this structure 

 are generally very ragged. 



In a ground-mass of the above character we find porphyritic 

 crystals and crystalline fragments of quartz, sanidine, plagioclase, 

 pyroxene and magnetite. These porphyritic constituents sometimes 

 occur in abundance, at other times they are entirely absent. The 

 quartz occurs in corroded grains and bipyramidal crystals, The felspars 

 are often completely honey-combed with inclusions. The inclusions, 

 both in the quartz and in the felspar, often take the form of negative 

 crystals. Thus, in quartz they are often seen to be hexagonal or 

 rhomboidal, in felspar rectangular. The pyroxene occurs in grains 

 and well-formed crystals. Jn some rocks the forms of the augites are 



