616 MESSRS. CLOT/GH, MATJTE, AND BAILEY ON [Nov. 1909, 



(2) Rhyolites. — Tbe augite-andesites are succeeded towards 

 the head of the corrie by three rhyolite flows. Each of these lavas 

 is about 150 feet thick. It is usually found in Glen Coe that the 

 more acid, and presumably more viscous, rocks give rise to the 

 thicker lava flows of the series. They also furnish bolder and more 

 massive crags than their associates, and the three now under con- 

 sideration can on this account be readily recognized, even from the 

 road. Flow-banding is generally conspicuous in these lavas, and 

 the uppermost shows flow-brecciation also. 



(3) Agglomerates. — The next group in the sequence consists 

 of a mass of agglomerate underlain by about 20 feet of greenish 

 sandy shale. The agglomerate is made up of blocks of augite- 

 andesite and rhyolite, of all sizes up to a foot in length. 



(4) Hornblende-andesites. — The hornblende-andesites of 

 Bidean nam Bian, which seem to consist of a series of flows, are 

 readily distinguished from the augite-andesites of group (1). They 

 carry numerous small, but conspicuous, phenocrysts of plagioclase 

 in addition to prisms of hornblende. They are more acid in cha- 

 racter than the augite-andesites, and frequently show flow-banding,, 

 which is conspicuous on the weathered crusts. They occasionally 

 exhibit a hexagonal columnar jointing due to cooling. 



(5) Rhyolite.— The rhyolite which succeeds the hornblende- 

 andesites on Beinn Fhada is not divisible, and may, indeed, be a. 

 single flow. On a fresh fracture the rock is black and vitreous, rich 

 in phenocrysts of felspar, but poor in respect of quartz. Its main 

 characteristic is the prevalence of the brecciated structure already 

 noticed in the case of the uppermost lava of group (2). Numerous- 

 fragments of rhyolite and hornblende-andesite, together with occa- 

 sional pieces of quartzose schist, are included as xenoliths in a 

 matrix which shows clear signs of flow-banding. 



(6) Shales and grits, well stratified and greenish grey in 

 colour, now interrupt the succession of volcanic rocks. Lying upon 

 the irregular surface of the rhyolite, they are naturally variable in 

 thickness, and point to the deposition of volcanic detritus in a local 

 body of water. Their varying angles of dip form an important 

 index to the deformation which the rocks have undergone since 

 their deposition (PI. XXXIII, Section III). 



(7) Andesites and rhyolites. — The group of lavas suc- 

 ceeding the sediments consists of rhyolites, hornblende-andesites, 

 and one basic andesite, and serves as a striking example of 

 irregular accumulation. The older members are found only in the 

 south-east, on the hill above Dalness, while the later members 

 overlap the older in such a way that the youngest lava preserved 

 to us rests on the sediments of group (6), and caps the southern 

 summit of Beinn Fhada. It is a hornblende-andesite characterized 

 by an abundance of relatively large plagioclase phenocrysts, a 

 feature which gives it a superficial resemblance to certain dykes of 

 hornblende-porphyrite. 



