Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 373 



the Eigg rock, and if it is about as thick as that mass at the western 

 end of the Scuir, then its bottom lies 200 or 300 feet under the 

 waves. The river-channel occupied by the Eigg pitchstone un- 

 doubtedly sloped from east to west. The position of Hysgeir, 18 

 miles farther west, indicates a further fall in the same direction at 

 the rate of perhaps as much as 35 feet in the mile. 1 Unfortunately, 

 however, as no trace of the river-bed can now be seen on this 

 island, any statement in regard to its prolongation must rest on mere 

 conjecture. 



IV. The Basic Sills. 



One of the most characteristic structural features in the basalt- 

 plateaux of North-western Europe is the number, thickness, and 

 extent of the basic sills or intrusive sheets which accompany these 

 piles of volcanic material. As I have formerly shown, 2 the sills, 

 though they may be observed in any part of the basalt series, are 

 more particularly developed at its base, and are notably interpolated 

 among the Secondary formations which underlie it. In addition to 

 the examples which I have already described, the following localities 

 are here cited as affording excellent illustrations of the more 

 characteristic features of intrusive sheets. 



The eastern coast of Skye has been classic ground for this part of 

 volcanic geology since the publication of Macculloch's descriptions 

 and diagrams. Erom the mouth of Loch Sligachan to Rudha 

 Hunish, at the northern end of the island, a series of sills may be 

 traced, sometimes crowning the cliffs as a columnar mural escarp- 

 ment, sometimes burrowing in endless veins and threads through 

 the Jurassic rocks. The horizontal distance to which this band of 

 sills extends in Skye is not far short of 30 miles. But it stretches 

 beyond the limits of this island. It forms the group of islets 

 which prolong the geological structure and topographical features of 

 Trotternish for 4 miles farther to the north-west. It reappears 

 10 miles still farther on in the Shiant Isles. Thus its total visible 

 length is fully 40 miles. As a display of intrusive basic igneous 

 rocks it ranks next to the Great Whin Sill among the British 

 instances of this tectonic type. 



The larger sheets in this belt have certain characteristic fea- 

 tures. They are generally somewhat coarsely crystalline ophitic 

 dolerites or diabases, and exhibit the persistent uniformity of com- 

 position and structure so characteristic of intrusive sheets and dykes. 

 They display in many cases a regularly prismatic arrangement, the 

 columns being much thicker and longer than those of the basalts 

 of the plateaux or those of the dykes and veins. The regularity of 

 this structure is well shown in the great sill of which the Kilt Rock 

 is one of the most noted portions (fig. 20, p. 374). But the most 

 astonishing example is that which forms the Garbh Eilean of the 

 Shiant Isles, where the sill presents to the sea a vertical columnar 



1 Rep. Brit. Assoc. 1894 (Oxford meeting), p. 653. 



3 Quart. Journ. Geol. Soc. vol. xxvii. (1871) p. 296; Trans. Eoy. Soc. Edinb. 

 vol. xxxv. (1888) p. 111. 



2c2 



