376 SIR A. GEIXIE ON THE TERTIARY [May 1 896,. 



grey basalt, vary from less than 1 to 3 or 4 inches in thickness. 

 They are separated by thin partings of coaly shale, and as they 

 tend to break up into detached nodule-like portions, especially 

 towards the right hand of the section represented in fig. 21, they 

 might, on casual inspection, be easily mistaken for nodules in the 

 dark shales. Somewhat later in the time of intrusion are veins 

 of basalt which, as at c, break across the nodular sills, and sometimes 

 expand into thicker beds (c'). 



I have never seen such a congeries of minute sills among the 

 Tertiary basalt-plateaux as that here exhibited. In a space of 

 about 3 feet of vertical height there must be more than a dozen 

 roughly parallel cakes of intrusive rock. Veins (e) run up from 

 the chief band of eruptive material into the overlying finely 

 vesicular basalt (/). The dyke (g) is probably the youngest rock 

 in the section. 



The amount of contact-metamorphism effected even by such 

 thick sills as those of Trotternish and Shiant is much less than 

 might be expected. It seldom goes beyond a mere induration of 

 the strata for a few yards, often only for a few inches from the 

 surface of junction. In the Shiant Isles the shales on which the 

 sills rest have undergone a remarkable alteration. They have 

 been greatly indurated, and have acquired a globular or botryoidal 

 structure. The spheroidal aggregates vary from not more than a 

 line to more than half an inch in diameter, and appear on the 

 surface as dark, irregularly grouped, pea-like aggregates. This 

 structure is perhaps best developed immediately under the thick 

 sill that forms Eilean Muirhi. 1 



On the western side of Skye, owing to greater local subsidence of 

 the basaltic plateau, the base of the volcanic series is seldom seen y 

 and hence the platform of sills is for the most part concealed under 

 the sea. But where at one or two points the Jurassic strata are 

 brought up to the light of day, they have carried with them their 

 intrusive sheets of basic rock. Thus, at the mouth of Dun vegan 

 Loch, the islets of Mingay and Clett form parts of a sill which rests 

 on shell-limestones full of oysters (Ostrea hebridica), referable to the 

 Loch Staflin group of the Great Oolite Series. This rock, when 

 observed from a little distance, presents the usual regularly pris- 

 matic or columnar structure so well developed among the Trotter- 

 nish sills, but on a closer view shows this structure much less 

 distinctly. It is an olivine-dolerite of medium and fine texture, 

 which in thin slices displays under the microscope a distinctly 

 ophitic structure, the abundant light-brown augite enclosing the 

 striated felspars. Its lowest portion, from 3 to 7 or 8 inches 

 upward from the bottom, is much closer-textured than the rest of 

 the rock and is finely amygdaloidal. Its vesicles are in many 

 cases drawn out to a length of 3 or 4 inches, and the zeolites which 

 now fill them look like parallel annelid tubes or stems of Lithostrolion. 

 It is noteworthy also that the elongation of the vesicles has 



1 Macculloch, 'Description of the Western Islands,' vol. i. (1819) p. 441. 



