Vol. 65.J THE CAULDRON-SUBSIDENCE OF GLEN COE. 623 



adjacent Schists by thin breccias, and therefore allied in their be- 

 haviour to the lavas of the district rather than to the intrusions. 

 The breccias here consist of small angular fragments of schists ; in 

 the Coupall River, along the same line, pieces of rhyolite are also 

 common. 



Although occasionally resting directly upon the Schists, the lavas 

 of Glen Coe are more commonly separated from the older rocks 

 by lenticular masses of sediment, examples of which have already 

 been mentioned. In addition to red and green shales and grits, 

 these sediments include several varieties of coarse breccia and con- 

 glomerate. In some cases the breccia is merely a thin layer of 

 shattered fragments of the subjacent rock into which it passes 

 imperceptibly, and from which it has been derived by simple 

 weathering without transportation. In other cases, peaks and 

 ridges of quartzite and quartzose schist have showered down their 

 debris in long trains of scree, to be met and covered by the 

 advancing lavas. Occasionally, too, mingling with the angular 

 local blocks are found rounded boulders from a distant source, 

 as if ephemeral torrents had assisted in the transportation. Of 

 such a mixed character is the basement conglomerate in Gleann 

 Ehaolain, on the southern flank of Bidean nam Bian. Lenticular 

 masses of conglomerate also are occasionally met with, as on 

 Aonach Eagach. They consist of well-rolled, rudely spherical 

 boulders of a great variety of rocks, the majority of which have 

 been brought from a distance. The boulders are packed into a 

 gritty matrix, and the whole deposit resembles nothing so much as 

 the boulder-gravel in the bed of a torrent. 



In addition to the basement beds, recurrent sheets of shale and 

 grit were spread out between successive flows, while screes trailed 

 down from neighbouring crags. The finer sediments, like the 

 coarser deposits, are marked by rapid variations in thickness, and 

 only in a few cases are they traceable for any distance. They may 

 have been deposited in lakes, temporarily occupying the floor of 

 the cauldron : or perhaps, they were laid down merely in pools 

 and back-waters by streams which washed over the surface of the 

 fissured lava flows, and filled every crack and crevice with soft and 

 penetrating silt. With these considerations in mind, there can be 

 little hesitation in believing that the eruptions in Glen Coe took 

 place under subaerial conditions. 



(d) Summary of the Volcanic History. 



(1) The Glen Coe volcanic rocks are of Lower Old Bed Sandstone 

 age, as is indicated by the plants, Psiloplujton and Pachytheca, found 

 by Mr. Tait in the basement beds on Stob Dearg and Aonach 

 Eagach. 



(2) Three main types of lava are represented, namely, augite- 

 andesite, hornbiende-andesite, and rhyolite. There is no evidence 



Q. J. G. S. No. 260. 2 u 



