GABBEOS ETC. IN^ SCOTLAND AND IKELAND. 77 



with short prismatic microlites of felspar. These microlites seem 

 to represent on a very minute scale the rounded crystals of the 

 granulitic rocks, and, like them, appear to indicate that the whole 

 mass was subjected to internal movements while crystallization was 

 taking place in it (see Plate VI. figs. 2, 4, 6, 8). 



Coming to the most highly vitreous forms, we find the same di- 

 stinction. In some cases we see the whole of the'glass filled vdth 

 delicate black rods (trichites) or beaded rods (margarites), with very 

 minute transparent crystallites (belonites &c.). But in other cases 

 the whole glass is filled with clouds of excessively minute, rounded 

 particles of magnetite (" magnetite dust "), which renders it nearly 

 opaque. In the case of the Beal dyke near Portree, in Skye, we 

 find the rod-like crystallites at a short distance from the edges ; but 

 at the actual edges, where some friction may be supposed to have 

 occurred, the rods resolve themselves into magnetite dust. 



With respect to the origin of the porphyritic crystals, it may 

 be pointed out that while we may in some cases regard them as 

 having separated from the magma, when it existed at a greater 

 depth from the surface than when the ground-mass crystallized, in 

 other cases there appears to be a difiiculty in applying this mode of 

 explanation. 



In some instances the mechanically injured condition of the crystals 

 and other appearances strongly suggest their actual transport from 

 below in the midst of the materials of the surrounding ground-mass. 

 But in others the porphyritic crystals exhibit zoned structures 

 and other characters not found, perhaps, in the deeper-seated rocks 

 of the class in the same area. May we not in these cases explain 

 the phenomena in the way suggested by M. Michel-Levy, by the 

 consolidation having taken place at two different periods ? It is not 

 difficult to imagine conditions which would bring about such a 

 result. If, for example, a mass of igneous materials were in a 

 liquid state at a great depth from the surface, the conditions 

 might be favourable to the separation of a felspar of a given com- 

 position from the magma. The continued abstraction of certain 

 elements from the base would alter the composition of the sur- 

 rounding magma, and this would modify slightly the conditions 

 causing the successively formed zones of the crystal to vary slightly 

 in composition. But if a fissure were formed above such a molten 

 mass, then the pressure upon it would be greatly and suddenly 

 relieved, even though no actual movement occurred in the deeper- 

 seated portion. Under the entirely new conditions thus originated, 

 the magma surrounding the zoned crystals already formed might be 

 induced to crystallize in a totally different, manner, the order 

 of the separation of the minerals and the forms and relations 

 of their several crystals being determined by these new conditions. 



It might at first sight appear that the unaltered or the corroded 

 appearance of the porphyritic crystals would afford a test as to 

 whether those crystals had consolidated in the midst of the rock in 

 which they are now formed or had been transported from a deeper- 



