MOUNT HOLMES BYSMALITH. 67 



but composed of the same minerals as the dacite-porphyry: alkali feldspar, 

 quartz, and biotite, with magnetite and zircon. The microscopical char- 

 acters of these minerals are like those of the minerals in the porphyry, 

 and it is possible that these coarse-grained inclosures may be nothing but 

 coarsely crystalline portions of the dacite magma. The gneissic structure 

 may be the result of irregular differentiation, as in the case of the banded 

 gabbro of Skye, 1 described by Geikie and Teall. 



The transition from the more coarsely crystalline central portion to 

 the denser and finer-grained aphanitic marginal portion is shown in the 

 changes in microstructure in specimens (83, 84, 86, 87, 88) collected from 

 Echo Peak and in the contact zone in the gulch between this and Three 

 River Peak. 



As the constituent minerals become smaller the poikilitic quartz in two 

 cases (84, 86) assumes more of an idiomorphic form, interrupted by small 

 feldspar crystals lying at various angles. The quartz sections appear in 

 nearly rhombic forms, the direction of extinction being diagonal to the 

 rhombs. The crystals are more or less perfect hexagonal bipyramids, formed 

 by ± P- They sometimes lie in a finer-grained mixture of feldspar and 

 quartz, which, however, does not amount to a groundmass, being in rela- 

 tively small quantity. In other cases the finer-grained modifications of the 

 rock are still micropoikilitic, and have essentially the same structure as the 

 coarser kinds. It would seem as though the idiomorphism of the quartzes 

 occurred in cases where the feldspar was a little more abundant. The rock 

 from which the second analysis (87) was made is minutely micropoikilitic. 

 This aphanitic variety is mottled with small dark spots that prove to be 

 chlorite, containing small scales and plates of muscovite, which also occurs 

 scattered through the rock in small amount. No biotite is present. Mag- 

 netite occurs in small crystals. The chlorite and muscovite are seen in 

 some cases to be alteration products of biotite, so that in all the specimens 

 examined it may be assumed to have had the same origin. 



This more lithoidal or aphanitic form of the rock occurs in broad 

 bands parallel to the plane of contact around the margin of the bysmalith. 

 The banding is recognizable at a distance, and is shown in the photograph 

 (PI. XII) of the north side of Echo Peak. The banding- stands at steep 

 angles, which are more nearly vertical in lower exposures, suggesting a 



'Geikie, A.., and Teall, J. J. H., On the banded structure of some Tertiary gabbros iu the Isle of 

 Skye: Quart. Jour. Geol. Soc., London, Vol. L, No. 200, 1894, pp. 645-G60. 



