ANNIVERSARY ADDRESS OF THE PRESIDENT. 95 



and north-west of the Mainland and North Mavine ; others of a less 

 active and persistent type were blown out some 25 miles to the east, 

 where the islands of Bressay and jSToss now stand. In the western 

 district streams of slaggy porphyrite and diabase with showers of 

 fine tuff and coarse agglomerate were thrown out, until the total 

 accumulation reached a thickness of not less than 500 feet. These 

 volcanic ejections took place contemporaneously with the deposition 

 of the red sandstones, for they are intercalated in these strata. 



No trace of any of the vents of eruption has been found in the 

 western and chief volcanic district, but in Noss Sound a group of 

 small necks occurs, filled with a coarse agglomerate of pieces of 

 sandstone, flagstone, and shale. Messrs. Peach and Home infer 

 that these little orifices never discharged any streams of lava. 

 More probably they were opened by explosions which only gave 

 forth vapours and fragmentary discharges, such as the band of tuff 

 intercalated among the flagstones in their neighbourhood. 



But one of the most striking features of the volcanic phenomena 

 of this remote region is the relative size and number of the sills and 

 dykes which here as elsewhere mark the latest phases of subter- 

 ranean activity. Messrs. Peach and Home have shown that three 

 great sheets of acid rocks (granites and spherulitic felsites) have 

 been injected among the sandstones and basic lavas, that abundant 

 veins of granite, quartz-felsite, and rhyolite radiate from these 

 acid sills, and that the latest phase of igneous action in this region 

 was the intrusion of a series of bosses and dykes of basic rocks 

 (diabases) which traverse the sills. 



The basin of Lome has not yet been described, though various 

 writers have referred to different parts of it. My own observations 

 have been too few to enable me to give an adequate account of it^ 

 The volcanic sheets of this area form a notable feature in the scenery 

 of the West Highlands, for they are the materials out of which the 

 remarkable terraced hills have been carved, which stretch from Loch 

 Melfort to Loch Creran, and which reappear in picturesque outliers 

 among the mountains traversed by Glen Coe, Between the ancient 

 schists that form the foundation-rocks of this district and the 

 base of the volcanic series, lies a group of sedimentary strata which 

 in some places must be 500 feet thick. This group consists of 

 exceedingly coarse breccias at the bottom, above which come 

 massive conglomerates, volcanic grits or tuffs, fine sandstones, and 

 courses of shale. While the basement-breccias are composed mainly 

 of detritus of the underlying schists, they pass up into coarse conglo r 



