B. K. N. Wyllie & A. Scott— Phitonics of Garabal Hill. 507 



pale-green and somewhat larger in size than the crystals found in 

 similar veins in Cornwall. Minute crystals of blue magnesia-bearing 

 spinel are found bordering the epidote. In the centre rock the 

 vesuvianite occurs interstitially, and is obviously the last mineral to 

 crystallize, but towards the margins aggregates of large hoinblende 

 and idocrase crystals occur, both being subhedral. The vesuvianite 

 crystals are brownish to colourless, Avith occasional lamellar cleavage 

 and the characteristic low double refraction. The amphibole 

 resembles the green hornblende of the diorite, but is obviously 

 changed in places to a mineral resembling glaucophane, while else- 

 where epidote and vesuvianite have developed at its expense, the 

 crystals being very much corroded round the margin. The vein is 

 bordered by a narrow band of typical hornblende schist, consisting of 

 blue-green amphibole, quartz, orthoclase, and albite, with occasional 

 patches of the original dioritic hornblende in a state of partial 

 alteration. 



Epidosites have been described by the Geological Survey from 

 both sedimentary and igneous schists and gneisses in the Lizard 

 district ^ and also from the Kilmartin district, vphere they occur in 

 epidiorites.^ Those occurring in the sedimentary rocks are generally 

 quartzose, and are probably produced by the shearing of rocks which 

 were weathered before being metamorphosed. The ' igneous ' 

 epidosites, however, are assumed to be due to chemical segregation 

 during movement, and it is this explanation which seems to be most 

 applicable in the case of the Garabal Hill rock. The epidote rocks 

 of the latter area differ from those of Cornwall by the almost 

 complete absence of minerals other than the lime-bearing ones — the 

 amount of spinel is exceedingly small. They seem to have formed 

 along shearing planes in the diorite, where the country rock has 

 undergone strain of such intensity that it has been rendered com- 

 pletely fluid. ^ The neighbouring rock is a hornblende-pyroxene- 

 diorite with a basic plagioclase as the only felspathic mineral. The 

 shearing has resulted in chemical interaction between the felspar 

 and the ferromagnesian minerals, with the formation of epidote. 

 Several cases of the production of epidote as a contact mineral 

 between felspar and hornblende^ have been recorded, and it seems 

 probable that the reaction between these two minerals in a fluid 

 state in presence of water is comparatively simple. The crystals 

 on the margins would be easily attacked, the large hornblendes 

 which are partly corroded being those which have been incompletely 

 dissolved. This solution effect would also explain the partial 

 aggregation of the vesuvianite at the margins, as the latter is richer 

 in lime and poorer in alumina than epidote, while hornblende is 



^ Flett in Geology of Lizard and Meneage (Mem. Geol. Surv. England), 1912, 

 pp. 36, 46. 



- Flett in Geology of Seaboard, Mid-Argyll (Mem. Geol. Surv. Scotland), 

 1909, pp. 47-50. 



^ At several other places on Garabal Hill evidence of faulting is found ; thus 

 at loc. iv (see Map, Fig. 1) a line of brecciated diorite with numerous quartz-veins 

 can be traced for about 20 yards. 



* G. H. Williams, Bull. U.S. Geol. Surv., 1886, No. 28, p. 31 ; F. D. Chester, 

 BuU., 1890, No. 58, p. 35. 



