544 B. K. N. Wyllie & A. Scott — Plutonics of Garabal Hill 



iucomplete knowledge of magmatic phenomena that they must remain 

 as theories, and nothing more, for the present. 



Note on the Diorites of Glen Lednock, Comrie. 



Numerous isolated patches of diorite occur to the north-west of 

 Loch Lomond, but no intrusion of any size is met with, except in 

 Glen Lednock near Comrie. This occurrence is of interest because 

 of the fact that the plutonic mass gradually passes to a hj'pabyssal 

 variety of very similar composition. The plutonic rock is a fairly 

 coarse biotite-diorite of uniform character, and is made up of mica, 

 hornblende, felspar, quartz, and sphene, while magnetite, apatite, 

 and zircon occur as common accessories. The mica is a deep-brown 

 biotite with strong absorption, and is intergrown with a green horn- 

 blende, which is generally very much altered to chlorite. The felspar 

 is partly andesine, partly orthoclase, and has apparently undergone 

 crushing to some extent. Sometimes it is enclosed by the ferro- 

 magnesian minerals. Quartz is subordinate, while fairly large 

 crystals of sphene occur occasionally. On the whole, this rock 

 resembles the typical biotite-hornblende-diorite 'of Garabal Hill, 

 though it has suifered much greater alteration. The rock is very 

 uniform, the chief variation being in the relative amounts of biotite 

 and hornblende. Very occasionally, anorthoclase occurs in addition 

 to the other felspars. 



The hypabyssal rock may be described as a diorite-porphyry, and 

 has obviously formed as the result of quicker cooling of part of the 

 magma. The phenocrysts include felspar, hornblende, and sphene, 

 and show a tendency to occur in glomeroporphyritic aggregates. The 

 felspars seem to be orthoclase, though they are generally so much 

 altered that their determination is difficult. The hornblende has 

 usually been replaced by fibrous aggregates of a secondary blue-green 

 amphibole resembling actinolite ; while occasionally the alteration 

 has gone further and chlorite has developed. Sphene occurs as 

 granular aggregates, intimately mixed up with the hornblende, 

 though sometimes larger crystals are found. The groundmass is 

 a micro-crystalline aggregate of quartz, felspar, biotite, hornblende, 

 and granular sphene. The amount of quartz is small, but areas rich 

 in sphene occur locally. The ferromagnesian minerals are found as 

 small irregular crystals, and are accompanied by numerous cubes of 

 magnetite. These rocks bear a close I'esemblance to the so-called 

 porphyrites of Argyllshire,^ not only in structure but also in the 

 presence of two generations of hornblende. 



Numerous veins of aplite occur in association with these rocks, 

 differing, however, from those of Garabal Hill in some respects. 

 The aplites of the latter area have granitic affinities, and are made 

 up of quartz and alkali-felspar with subordinute muscovite. Those 

 of Glen Lednock, on the other hand, are related to the syenite family, 

 and contain orthoclase with abundant biotite, while quartz and 

 plagioclase are subordinate. 



^ Geology of Coioal (Mem. Geol. Surv. Scotland), 1897, p. 103; Hill in 

 Geology of Mid- Argyll (ibid.), 1905, pp. 103-15 ; Flett in Geology of Oban and 

 Dalmally (ibid.), 1908, pp. 96-102. 



