B. K. N. Wyllie & A. Scott — Plutonics of Garahal Hill. 505 



been only partly decomposed, but that partial decomposition has set 

 free a certain amount of lime, with the result that plagioclase is more 

 basic than is commonly the case in aplites. It should be mentioned 

 also that the diallage shows the usual alteration to brown hornblende, 

 though that is not nearly so advanced as in the davainite. 



When the case is considered from the purely chemical point of view, 

 this 'selective solution' is easily understood. Olivine, being an 

 orthosilicate, is unsaturated with respect to silica and hence would 

 be easily attacked, the resultant product being the metasilicate, 

 hypersthene (Mg Fe Sig Og). Enstatite, a metasilicate, is saturated 

 with silica and hence remains undissolved, unless the temperature is 

 very high. Diallage, while chiefly composed of metasilicate molecules, 

 also contains compounds of the type K" AI3 Si Og, and these can take 

 up more silica to form types such as R" Alg Sig Og. Thus the 

 formation of anorthite, and consequent basification of the plagioclase, 

 may be expressed by the equation 



Ca AI3 Si Og + Si O3 — > Ca K\ Sig Og. 



ToNALITE AND DlORITE. 



Garabal Hill itself is composed of a series much more acid than 

 those described above. There are clearly two separate rock masses, 

 the earlier one comprising diorites with a considerable variety of 

 structure and raineralogical composition, while the later one is 

 a more or less uniform tonalite. The latter is obviously intrusive 

 into the former, as it penetrates both the diorites and davainites 

 in numerous veins of varying thickness, and generally with 

 sharp junctions and no commingling on the margins. The tonalite 

 consists mainly of deep-brown strongly pleochroic biotite and 

 zoned oligoclase in a matrix of quartz, orthoclase, and plagioclase. 

 The quartz and felspar contain numerous inclusions of apatite, zircon, 

 and sphene, while in the ground-mass sporadic crystals of sphene 

 and muscovite are found. In the region of Garabal Hill the rock is 

 even-grained and not particularly coarse, although in one or two 

 places mica ' clots ' are found, consisting of aggregates of biotite, 

 with strong absorption, and subordinate interstitial quartz and 

 felspar. These probably represent foreign material which has been 

 absorbed and completely recrystallized. !Near Meall Breac, to the 

 west of the fault and in the area mapped by the Geological Survey 

 as tonalite, a much coarser rock is found. It differs from the 

 tonalite described above, not only in texture but also by the presence 

 of green hornblende, equalling in amount the biotite, and of fairly 

 large crystals of sphene. It closely resembles some of the more acid 

 diorites and should probably be classed with them. 



The diorites differ considerably in the different localities, and 

 grade from fairly acid, quartz-bearing rocks to pyroxene diorites and 

 hornblendites. The chief constituent minerals are quartz, orthoclase, 

 plagioclase, mica, hornblende, pyroxene and sphene. The plagioclase 

 in the more basic varieties is labradorite-andesine, but oligoclase is 

 found in the ' acid ' rocks. The quartz and felspar are rich in 

 inclusions of apatite, zircon, and occasionally rutile. The mica is 

 generally biotite with strong pleochroism and absorption, and often 



