118 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Another example of the thicker type of sills may be selected from the promontory 

 of Ardnamurchan.* The general form and position of this mass will be understood from 

 the small map in Plate I. Forming the rugged shore for three quarters of a mile, it slopes 

 thence inland in a series of rocky knolls, which in rather less than a mile culminate in the 

 summit of Ben Hiant, 1729 feet above sea-level. The rock which covers this large space 

 is disposed in numerous rude beds, which have a seaward dip of perhaps 15° to 20°. They 

 are distinctly prismatic, and the prisms are not infrequently grouped in fan-shape. They 

 are evidently due to different eruptions, but I observed no trace of any other rock 

 intercalated between them. They are never, so far as I could discover, amygdaloidal 

 nor do they present the ordinary external characters of the beds of the plateaux. They 

 distinctly overlie the bedded basalts on their eastern and southern margins ; but west- 

 wards they appear to lie transgressively across the edges of these rocks, and to the north- 

 west they rest on quartzites and schists. An outlier from the main mass forms the pro- 

 minent hill of Srbn Mhbr, and can be seen distinctly overlying the bedded basalts as well 

 as the neck of agglomerate already described (fig. 30). The rock of Ben Hiant is for the 

 most part a well crystallised, ophitic olivine-dolerite. A specimen taken from the shore 

 on the west side of the mass was found by Dr Hatch to present under the microscope 

 its augite in large plates, which enclose narrow laths and needles of plagioclase felspar as 

 well as grains of olivine. All the felspars are in lath-shapes, sometimes extremely long and 

 narrow. The iron-ore likewise assumes an ophitic character, enclosing rectangular portions 

 of felspar. Another specimen, taken from the south-east side of the hill, showed under 

 the microscope " a curious intermixture of two different structures. Scattered portions 

 which show the usual ophitic structure, their felspar and augite occurring in large 

 crystals, are, so to speak, imbedded in a ground mass which presents rather a basaltic type, 

 its felspar, augite, and magnetite, in long thin needles, microlites, and other skeleton 

 forms, being enclosed in a dark devitrified base." A third specimen, selected from one 

 of the columnar sheets near the top of Ben Hiant, is " a fine grained dolerite (or gabbro) 

 showing little ophitic structure, the augite occurring in roundish grains, and only slightly 

 intergrown with the felspars, which are more or less lath-shaped. The rock contains a 

 considerable quantity of black iron-ore in irregular grains and some dirty-green viridite." 

 Still another variety of structure occurs in a specimen which I broke from one of the 

 shore crags on the S.W. side of the hill. Under the microscope, it presents a beautiful 

 aggregate of " skeleton crystals and microlites of plagioclase, with here and there a 

 rectangular crystal, long slender microlites of augite, and short serrated microlites of 

 magnetite, the whole being confusedly imbedded in a dark glassy base powdered over 

 with a fine magnetite dust." 



In rambling over this Ardnamurchan rock 1 was often reminded of the great intrusive 

 mass of Fair Head. One of the features in which the rocks of the two localities resemble 

 each other is their tendency to assume a coarsely crystalline texture. In some parts of 



* This locality has been described by Professor Jddd, who believed the dolerites to be streams proceeding from a 

 volcanic vent (Quart. Jour. Geol. Soc, xxx. (1874) p. 2G1). 



