430 MR. E. IT. ANDERSON 01ST THE GEOLOGY OF [vol. lxxix, 



horizons of Boulder Bed, the main bed already mentioned in the 

 text, and a thinner bed which is intercalated in the Schichallion 

 Quartzite. 



As recognized in the Geological Survey memoir, the main bed 

 consists of two subdivisions. One of these has a micaceous, 

 merging in places into a partly siliceous, matrix, while the other 

 is highly calcareous. On the hill-slope east of Druimchastle the 

 former division borders the Quartzite, and the latter the White 

 Limestone. The same arrangement holds in the Errochty Water, 

 except Avhere the limestone is margined by a fault. There can be 

 little doubt that this is the general relation, though one or both 

 of the subdivisions may be missing, and the sequence is often 

 confused by minor folds. 



The non-calcareous part of the Boulder Bed is extraordinarily 

 unbedded. The uniformity of the matrix and the haphazard 

 arrangement of the boulders strongly suggest that it is a tillite, 1 

 or altered boulder-clay. The character of the boulders is dis- 

 cussed in the Memoir, and it need only be mentioned that they 

 consist for the greater part of quartz and quartzite, and ' granite ' 

 or nordmarkite. The calcareous division has often markedly 

 carious weathering, and has been named by Grant Wilson the 

 ' honeycomb rock.' It contains abundant inclusions of a sub- 

 stance which may be the White Limestone. Their derivative 

 nature might possibly be questioned, as they resemble the segre- 

 gations of calcite which occur in some parts of the Ben Lawers 

 Schist. There is, however, a small but definite admixture of 

 quartzose fragments, which prove this division to be an integral 

 part of the Boulder Bed. 



It is worthy of note that what are apparently limestone frag- 

 ments are not confined to the calcareous division of the Boulder 

 Bed, but occur more sparingly in the non-calcareous part. Within 

 my experience these are best seen in the strip of Boulder Bed which 

 borders the Schichallion quartzite-belt, beside a small stream not 

 shown in the 1-inch map, at a point 1500 yards north-east of the 

 summit of Schichallion. This observation has an important 

 bearing, as it is hardly possible to question that, in this case, we 

 are dealing with fragments of a pre-existing calcareous rock. The 

 material is creamy weathering, and non-dolomitic, and may well be 

 derived from the White Limestone. This would imply that the 

 Boulder Bed was the later formation. If this be not the chrono- 

 logical order, there is no member of the succession nearer than the 

 Ben Lawers Schist that can be regarded as a source. 



1 A glacial origin is assigned to the Portaskaig Conglomerate by James 

 Thomson, who, however, did not class it as boulder-clay (' On the Geology of 

 the Island of Islay' Trans. Geol. Soc. Glasg. vol. v, 1877, p. 211). The point 

 has been discussed by Mr. Bailey (' The Islay Anticline ' Q. J. G. S. vol. lxxii, 

 1916-17, p. 142). The identity of this conglomerate with the Schichallion 

 Boulder Bed has been regarded as probable since the time of Macculloch ; 

 but, as regards the latter bed, the suggestion that it is a tillite does not yet 

 seem to have appeared in print. 



