﻿486 MR. J. A. THOilSOX OX THE HOEXBLEXDIC [XoT. I908, 



absorption of the amphibolite-series by an acid magma. The 

 presence of acid veins in the adjoining schists was difficult to 

 verify owing to the number of granite-erratics, and to the dense 

 covering of fern, but Prof. SoUas has described their occurrence. 



In section, the minerals identified are pyrite, ilmenite, garnet, 

 apatite, sphene, clinozoisite, epidote, allauite, hornblende, mica, 

 labradorite, and quartz. Of these the brown hornblende, clinozoisite, 

 epidote, and in part the sphene, are xenocrysts : the other minerals 

 being due to the absorption of basic material in the acid magma. 



The key to the sequence of events is given by the apatite. This 

 mineral occurs rarely in the peridotite and its variants, but is 

 extraordinarily developed in some specimens of the rock under 

 consideration. It can safely be assumed to belong to the intrusive 

 magma, and proves that the intruding veins were not mere quartz- 

 veins, but at least pneumatolytic apophyses of a plutonic magma, if 

 not dykes of granitic material itself The apatite occurs in long, 

 often very slender prisms, occasionally in stouter and unusually 

 large individuals. The cross-fracture is well- developed, and by 

 movements along these planes the crystals are often bent and 

 broken. It is enclosed most generally in the felspars, or in the 

 quartz-felspar mosaic, but is also found within the garnet, the mica, 

 and the radiating tufts of actinolite. It thus appears that the re- 

 crystallization of the hornblende into actinolite, and the formation of 

 mica and garnet are subsequent to the consolidation of the apatite. 



Ilmenite is an occasional constituent in skeletal growths, 

 generally within the actinolite. Sphene is much more abundant, 

 with varying habit. Small granules are moulded on ilmenite and 

 occasionally on clinozoisite. Larger independent crystals are 

 often acicular, and occasionally have a habit resembling apatite. 

 The sphene occurs in greatest abundance within actinolite and mica, 

 or in their neighbourhood. It is occasionally developed in parallel 

 needles, lying in the directions of cleavage of the mica-plates, but 

 extending beyond their boundaries (fig. 8, p. 489). 



The minerals of the epidote-group are colourless for the most 

 part, and consist then of clinozoisite, with occasional yellow 

 interference-colours denoting the passage to iron-poor epidote. 

 They form large well-cleaved prisms, often penetrated by actinolite, 

 and moulded by quartz. Such an association of well-crystallized 

 zoisite and water-clear basic felspars is quite abnormal in an igneous 

 rock (fig. 6, p. 487). Besides the bigger crystals, these minerals 

 occur in a kind of graphic structure with felspar. Groups of 

 small angular crystals are enclosed poecilitically in a large plate of 

 labradorite. Each group in itself has a prismatic or somewhat 

 rounded outline, and the individuals of the group are all in optical 

 continuity. It is difficult to imagine a graphic intergrowth of 

 zoisite or epidote and felspar. It seems rather probable that the 

 larger prisms of these minerals have been attacked by the magma, 

 but not totally dissolved, and the remaining skeletons included in 

 the growing felspar-crystal. A similar inclusion of a partly- 

 destroyed xenocryst is observed within the actinolite. 



