part 4] the schists of the schichallion distkict. 431 



The siliceous fragments in the Boulder Bed may he derived in 

 part from intercalations in the Banded Series, although it is pos- 

 sible that many of them were originally vein-quartz. The ' granite '- 

 boulders are the only type for which there is no possible local 

 source. The latter have not been found in the calcareous division, 

 and are nowhere more abundant than quite close to the Quartzite 

 edge. If the order in time is Quartzite — Boulder Bed — Limestone, 

 this implies that the only constituents of the conglomerate which 

 are certainly far-travelled were among the very earliest to arrive 

 in situ. If this order holds, it may also be noted that the Boulder- 

 Bed, when deposited, must have had a calcareous top. The early 

 arrival of the ' granite ' seems to be an unlikely feature in a tillite. 

 The calcareous top — if it be a top — is overlain by limestone, and if 

 it be a boulder-clay top this can only be regarded as a curious 

 coincidence. Except, however, where, as noted below, it comes 

 against the Quartzite, the Limestone is always flanked by the 

 : honeycomb ' rock, and the two appear to have a fundamental 

 connexion. In fact the glacial hypothesis almost certainly implies 

 that the order was Limestone — Boulder Bed — Quartzite, and, even 

 apart from an ice-age, the facts are most readily explained if this 

 was the case. 



If this conclusion be correct, the order of superposition is that 

 given in the amended table (p. 427), and the Grey Schist is the 

 oldest Dalradian rock in the district. 



This conclusion is in agreement with the facts recorded by 

 Mr. Bailey in Islay. 1 The Portaskaig Conglomerate has, as is well 

 known, a remarkable resemblance to the Schichallion Boulder Bed, 

 and Mr. Bailey has found reason to believe that the former is 

 succeeded in point of time by the Islay Quartzite, which may well 

 be the equivalent of the Perthshire Quartzite Group. 



The conglomerate which is supposed to be inter- 

 calated in the Schichallion Quartzite has a matrix that is 

 more siliceous than any exposure which certainly belongs to the 

 Main Boulder Bed. It forms two narrow strips on the northern 

 slope of Schichallion, which are flanked on both sides by quartzite. 

 One of these is only about 100 yards from the upper margin of 

 the bounding hand of Main Boulder Bed, which has already been 

 mentioned. The narrow strip is here typically siliceous, while the 

 main bed has a micaceous matrix, right up to the edge of the 

 quartzite. Unless, therefore, one regards the former as an inter- 

 calation, one must suppose the margin of the main bed to undergo 

 very rapid lateral variation. Although the point is far from 

 certain, I regard the former as on the whole the more probable 

 hypothesis, and have followed the Survey map in separating the 

 two conglomerates. The siliceous type contains large boulders of 

 ' granite,' and pieces of a finer-grained acid igneous rock, Avhich is 



1 'The Islay Anticline' Q.J. G. S. vol. lxxii (1916-17) pp. 132-59; see, in 

 particular, p. 143. 



