608 



MR J. D. FALCONER 



fused, and the rounded quartz grains are the corroded remnants of the original clastic 

 fragments. * 



(c) The Tertiary Dykes. 



The sinuous dyke at the Bath House is remarkable in having the friable coarse-grained 

 interior sharply marked off from broad marginal bands which, being more compact, have 

 offered greater resistance to decomposition. The interior is a coarse-grained olivine 

 dolerite, while the margins are somewhat more acid with porphyritic groups of felspar 

 laths set in a basaltic groundmass. At the Inches, the Harbourback, and the Horse 

 Island numerous dykes rise along the line of the great wrench fault. On the accompany- 

 ing map these are shown as one ; but in reality there are always two, and there may be 

 as many as four, small dykes ramifying through the broken rock (fig. 2). Petrographi- 



Fig. 2. — Plan of Dykes in the shatter-belt at the Harbourback. Scale tsW 



cally, these are basalts of two types : a fine-grained micro-ophitic olivine basalt, and a 

 somewhat more acid type with phenocrysts of felspar in a groundmass of felspar 

 microlites and augite and magnetite grains. Dykes of similar material running parallel 

 with the fault line are found piercing the Castle Craigs picrite and sandstones of the 

 Horse Island. A small dolerite dyke in the east islet, running at right angles to the 

 shatter-belt, is noteworthy in possessing fine-grained sheaths to the coarse-grained 

 irregular polygonal masses into which it is broken up, the sheaths being usually provided 

 with a median suture, t Various dykes and thin intrusive sheets are found in the 

 vicinity of the cornstone at Burnfoot Bridge ; these vary from olivine dolerites to 

 felspar basalts, and differ somewhat in texture and in the relative proportions of augite 

 and felspar. 



* See The Tertiary Igneous Rocks of Skye, p. 246. 



t Cf. "The Geology of Cowal," Mem. Geol. Sur., p. 144. 



