122 MR. J. MORRISON ON [vol. lxxiv r 



quartz, which it frequently encloses. Perthite, or microperthite, 

 is a common phenocryst, sometimes indeed the most important. 

 It builds good crystals with Carlsbad twinning. An acid oligo- 

 clase giving low angles of extinction is common ; but a more 

 basic variety is met with, the angles occasionally approaching those 

 of andesine. The crystallization periods of oligoclase, orthoclase, 

 and quartz overlap to a considerable extent. Glomeroporphyritic 

 groups consisting of numerous felspar individuals, both micro- 

 perthite and oligoclase, are found in both the earlier and the later 

 sets of acid rocks. 



Quartz plays an important part in all the rocks, and in the 

 extreme types is the only prominent phenocryst. It is frequently 

 moulded and enclosed by later orthoclase (a familiar feature in 

 the granite), more rarely by oligoclase. It is sometimes surrounded 

 by a granular aggregate of quartz and orthoclase which may even 

 be of coarser texture than the remainder of the ground-mass, the 

 boundaries following the outlines of the original crystal rather 

 than of the existing corroded form. The excess of quartz in these 

 bands is probably attributable to the fact that, at the time of 

 intrusion, the relief of pressure enabled the magma to react on 

 the crystal until chemical equilibrium Avas restored, by which 

 time the viscosity consequent on the fall of temperature was 

 sufficient to prevent complete diffusion of the dissolved silica, the 

 portion remaining in the vicinity being subsequently added in 

 optic continuity. Small patches are occasionally found, in which 

 the quartz exists as a nucleus only. With orthoclase it forms the 

 great bulk of the ground-mass, oligoclase being present as a rule 

 in small quantity only. 



Biotite occurs as hexagonal plates and scattered shreds in varying 

 amount. The common change is to chlorite, sometimes giving a 

 dark fibrous appearance under crossed nicols, sometimes the dee]) 

 colour characteristic of penninite : epidote is usually associated 

 with it, and less frequently calcite, rutile, or muscovite. The last- 

 mentioned mineral also appears in some rocks in the form of plates 

 recognizable macroscopically, but it is not common. Sphene is 

 abundant in some dykes near the granite, both as wedges and as 

 grains, and in others is apparently wanting. It is rare on the whole 

 in the earlier acid set, and, with one or two notable exceptions, 

 almost entirely absent from the later. Apatite is of constant 

 occurrence, especially in association with the biotite, and is sometimes 

 enclosed bj sphene. 



Iron-ores, though not abundant, are of interest. Secondary 

 pyrite is frequently met with, and rarely ilmenite ; but interest 

 centres chiefly in pyrrhotite, which was first recognized macro- 

 scopically in a fragment freshly chipped from a dyke near the 

 granite. Further search showed it to be fairly common, and 

 microscopic investigation revealed its presence in small amount in 

 every section. It is common in some dykes, rare in others, and 

 apparently absent from the remainder. In one case it occurs 

 as a constituent of a small basic segregation. Zircon, rutile, 



