TRANSACTIONS OF SECTION C. 653 



materials of its gravel deposits, had flowed from east to west. The channel of 

 the main river, as well as those of some of its tributaries, had been sealed up 

 under a flow of pitchstone, which, after ages of waste, now forms, owing to its 

 greater durability, the prominent ridge of the Scuir, the original higher ground of 

 bedded basalt having been worn down into lower slopes. The river-course thus 

 entombed must be assigned to the volcanic period of older Tertiary time in this 

 country. Its western end is truncated by a precipitous sea-cliff, at the top of 

 which a section of it is displayed, with its underlying shingle and overlying pitch- 

 stone, at a height of some 500 feet above the sea. This summer the author had 

 enjoyed a favourable opportunity of visiting Hysgeir, a small low islet about 

 eighteen miles to the west of Eigg, which had recently been identified by Professor 

 Heddle as a continuation of the rock of the Scuir. He was able completely to 

 corroborate this identification. The pitchstone of Hysgeir in its external forms 

 and internal structure precisely resembles that of Eigg, presenting, indeed, so close 

 a resemblance that it looks like a detached piece of the high ridge of the Scuir. 

 Unfortunately, the columnar rock everywhere slips under the sea, and allows no 

 trace to be seen of what it rests upon. If it be approximately as thick as it is in 

 Eigg, its base may be 200 or 300 feet below sea-level. The gradual fell of the 

 river-bed from east to west had been noticed at the Scuir, and the position of the 

 pitchstone at Hysgeir showed a continued declivity in the same direction of 

 perhaps as much as 35 feet in the mile. No visible rock rises to the surface of 

 the sea between Hysgeir and Eigg. The region has been intensely glaciated, and 

 the low ridge and rocky slopes of Hysgeir are strewn with erratics, which show 

 that the ice moved westwards from the Inverness-shire highlands. 



A much older river, but one still belonging to the volcanic period, has left 

 some interesting records in the islands lying to the north of Hysgeir. A succession 

 of coarse river-gravels are there found intercalated on different horizons among 

 the bedded basalts. The materials of the lowest of these conglomerates are 

 remarkably coarse, blocks 6 feet in length being occasionally visible. They con- 

 sist in large measure of volcanic rocks, especially slaggy and amygdaloidal 

 varieties. These constitute the largest and least water-worn blocks. Pieces of 

 Torridon sandstone, epidotic grit, quartzite, and various granites and schists' are 

 generally well-rounded and smooth, and especially abound in the finer and more 

 stratified gravels. The rapid dying out of thick sheets of coarse conglomerate is a 

 conspicuous feature of the deposits, their place being sometimes taken by layers of 

 fine tuff" or volcanic mudstone, or by shales with remains of land-plants. Some 

 portions of the conglomerate pass into true volcanic agglomerate, and this latter 

 rock can in one place be seen to rise as a neck enclosing blocks of scorise and basalt 

 sometimes 15 feet in length. 



The sequence of events which these various deposits indicate appears to be aa 

 follows. During the outpouring of the lavas of the great basaltic plateaux of the 

 Inner Hebrides a river flowed across the volcanic plain from the Western High- 

 lands, whence it carried large quantities of shingle. By successive violent floods 

 these materials, togetlier with the detritus of the lava-fields, were strewn irregu- 

 larly far and wide beyond the immediate channel of the river. In the pools 

 left behind, fine volcanic silt gathered and entombed leaves and stems of the 

 surrounding terrestrial vegetation. But volcanic activity still continued, and, 

 though cones of slags and pumice were swept down, new eruptions took place by 

 which masses of rock, sometimes 9 feet in diameter, were thrown out to a distance 

 of a mile or more, and fresh streams of lava were poured out, completely burying 

 the previous accumulations. Renewed river-floods of gradually lessening severity 

 spread fine detritus over the cooled sheets of basalt, and again these later fluviatile 

 deposits were entombed beneath fresh outbursts of lava. Perhaps no more 

 striking evidence can be elsewhere obtained of the conditions of the land-surface 

 over which, from many scattered vents, the materials of the volcanic plateaux of 

 the Inner Hebrides were slowly piled up. 



