Vol. 62.] STRUCTURE OF THE SGURR OF EIGG. 43 



II. General Structure of the Pitchstone-Complex. 



The general geological structure of Eigg is too well knowu to 

 need any detailed account in this place. The Jurassic strata dis- 

 played in the northern part of the island have sunk below sea-level 

 in the southern part ; and here the crumbling basalts, and the more 

 durable dolerite- sills which alternate with them, rise inland in a 

 terraced slope, culminating in the Sgurr. Resting on this pile, as 

 if poised upon a pedestal, is the great mass of the pitchstone. Its 

 crowning position and its abrupt outline make it a conspicuous 

 object, even in a distant view. The bold ridge which it makes has a 

 curved course with a general direction from north-west to south-east, 

 terminating abruptly at the highest point (1289 feet). Excepting 

 this most elevated portion to the south-east, the crest of the ridge runs 

 usually at an altitude of 1000 to 1100 feet. The pitchstone presents 

 generally precipitous walls on both sides : its base on the north- 

 eastern side being in most places between 900 and 1000 feet above 

 sea-level, while on the opposite side it sometimes sinks to 700 feet, 

 showing a general inclination in this direction (south-westward or 

 southward). The base also declines along the direction of the ridge, 

 being at about 1060 feet at the easterly termination, and sinking 

 to about 550 feet at the opposite end, where the ridge is truncated 

 by sea-cliffs. There are also minor variations of level, and it is 

 clear that the lower surface of the pitchstone has in places an 

 undulating form. The breadth of the ridge varies up to about 

 500 yards, exclusive of some arm-like extensions on the north- 

 eastern side, which make the hills named Beinn Tighe and Corn- 

 bheiun. The maximum thickness of the pitchstone is about 

 400 feet. In its south-eastern portion the base, in so far as it can 

 be regarded as an inclined plane, has a general dip of about 15° to 

 the south. At the actual termination eastward the inclination is 

 much steeper, the observed dip being about 40°. 



The fine natural section at this last-mentioned place shows clearly 

 that the pitchstone truncates obliquely the alternations of basalt 

 and dolerite upon which it rests (see PI. III). The same is true in 

 every part of the boundary, as is well brought out by the mapping 

 of the hill. The relation of the pitchstone to the underlying rocks 

 is, then, of a transgressive kind. Further, the partly undulating 

 form of the surface of contact is not due to any subsequent folding ; 

 for the basalt-group below is undisturbed, with a gentle south- 

 westerly dip. (See map, fig. 1, and section, fig. 2, pp. 44 & 45.) 



The rocks which build the Sgurr are not a single body, but 

 constitute a number of sheets, which in most places are sharply 

 divided frCm one another. The dominant rock is the well-known 

 porphyritic pitchstone, containing clear crystals of felspar in a 

 velvety-black resinous-looking groundmass. Intercalated in this 

 are sheets, usually of no great thickness, composed of a dull-grey 

 porphyritic felsite, which, as seen in the escarpment, weathers 

 more rapidly than the pitchstone, and so forms grooves or recesses 



