RevieiDs — Geikie's Ancient Volcanoes of Great Britain. 373 



suffice to prove the strata to be the equivalents of the Lower Lime- 

 stone shale and part of the Carboniferous Limestone of England " 

 (pp. 463-4). 



One of the most interesting features of the massive relics, left by 

 denudation of ancient volcanic lavas, is to be seen in the Basalt- 

 plateau of the " Pai'ish of Small Isles," which includes the islands 

 of Eigg, Eum, Canna, Sanday, and Muck. 



" That the fragments of the basaltic plateau preserved in each 

 member of the group of the Small Isles were once connected as 

 a continuous volcanic plain, can hardly be doubted. Indeed, as 

 already stated, they were not improbably united with the plateau of 

 Skye on the north, and with that of Mull, Morven, and Ardnamurchau, 

 on the south. Taking the whole space of land and sea within which 

 the basalt of Small Isles is now confined, we may compute it at 

 not much less than 200 square miles. 



" The most varied and interesting of the fragments of the basaltic 

 plateau in the area of the Small Isles is that which forms the island 

 of Canna, with its appendage Sanday. Canna measures five miles 

 in length by from half to a mile in breadth, and consists entirely 

 of the rocks of the plateau and their accompaniments. The 

 basalts are exposed along the north coast in a range of mural 

 precipices rising to a height of about 600 feet above the sea. From 

 the top of the escarpment the ground falls by successive rocky 

 terraces and grassy slopes to the southern shore-line. Sanday, 

 connected with the large island by a shoal and foot-bridge, is two 

 miles long and 220 to about 1,200 yards broad. Its highest cliffs 

 range along its southern shore to a height of 193 feet, whence they 

 slope gently northward into the hollow between the two islands." 



The author had observed, as far back as 1865, an ancient river- 

 channel, which, during the volcanic period, had been eroded on the 

 surface of the basalt-plateau, and of which a small portion had been 

 preserved under a stream of pitch stone-lava that had flowed into 

 and buried it. 



This watercourse was shown to have been excavated by a stream 

 coming from the north-east, or east, and to be younger than the 

 plateau-basalts of the district or even the dykes which cut the 

 basalts. Yet that it belonged to the volcanic period was proved 

 by the manner in which it had been sealed up and preserved under 

 the black glassy lava of the Scuir. Other and more abundant 

 evidence of rivex'-action was brought to light by Sir A. Geikie, 

 during his later examination of the islands of Canna and Sanday, 

 belonging to an earlier part of the volcanic period. This evidence 

 reveals that a powerful river, flowing westwards from the Highland 

 mountains, swept over the volcanic plain while the sheets of basalt 

 were still being poured forth, and while volcanic eruptions were 

 taking place from cones of slag. In one spot, the former existence 

 of an old land-surface is attested by the presence of some shaly 

 layers, which have been deposited on a remarkable slaggy basalt ; 

 above these is a thin coaly parting enclosing a small tree stump still 

 standing erect upon the coaly parting and shale, which in its turn is 

 immediately overlain by and enveloped in a massive flow of basalt. 



