CHAP. XLII 
THE BASIC SILLS OF ARDNAMURCHAN 
3i9 
mile culminates in a summit, 1729 feet above sea-level. 1 The rocks which 
cover this large space are disposed in numerous rude beds, which have a 
seaward dip of perhaps 15° to 20°, and are sometimes distinctly prismatic, 
the prisms being not infrequently grouped in fan-shape. They are evidently 
due to successive intrusions. Although generally coarsely crystalline in 
texture, they include also intermediate and fine-grained sheets. They 
are never, so far as I have been able to discover, amygdaloidal, 2 nor do 
they present the ordinary external characters of the beds of the plateaux, 
though here and there they appear to have caught up portions of the 
plateau-series. They distinctly overlie the bedded basalts 011 their eastern 
and southern margins ; but westwards they appear to lie transgressively 
across the edges of these rocks, while to the north-west and north they rest 
on quartzites and schists and on Jurassic limestones. An outlier from the 
main mass forms the prominent hill of Sron Mhor, and can be seen distinctly 
overlying the bedded basalts as well as the neck of agglomerate already 
described (Fig. 302). 
The prevalent rocks of Ben Hiant are well crystallized, ophitic olivine- 
dolerites and gabbros. 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. Dr. Hatch 
observed in another specimen, taken from the south-east side of the hill, “ 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 groundmass 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 south-west side 
ol the hill. Under the microscope. Dr. Hatch found in it 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.” 3 A sill 
1 This locality has been described by Professor Judd (Quart. Jour. Gcol. Soc. xxx. (1874), p. 
261 ; and xlvi. (1890), p. 373). 
2 As amygdaloidal structure is occasionally to be found among both dykes and sills its 
presence in the Ben Hiant rocks would not be inconsistent with their intrusive orgin. 
3 Professor Judd has called the rocks of Beinn Hiant augite-andesites, and has given descrip- 
tions and figures of their structure, and analyses of tlieir chemical composition (op. cit. ). 
