part 4] THE SCHISTS OF THE SCHICHALLION DISTRICT. 429 



IV. Chronological Order of Succession. 



General considerations. — The evidence for the relative 

 order of these four divisions, and their relation to the Boulder Bed, 

 lies wholly in their surface-distribution, as shown by the mapping. 

 The Banded Series is an almost unmistakable type, and there are 

 numerous sections in which one passes from it across Grey Lime- 

 stone into Grey Schist ; while, again, one constantly passes from 

 Banded Series into Boulder Bed, with or without an intervening 

 White Limestone. The White Limestone, it is true, is sometimes 

 absent. This may be due to an unconformity, or to the fact that, 

 being the thinnest member of the series, it is the most liable to be 

 cut out by unrecognized lines of movement. 



Alternative methods of reading the succession have been tried, 

 but without success. For instance, the Banded Series contains 

 what is here regarded as an intercalated quartzite. Could this 

 quartzite, however, be that of Schichallion ? The answer is 

 that banded material is not observed along the margin of the 

 Schichallion belt, and it is unlikely that, if this division had rapids- 

 thinned out, its shore-line should everywhere be concealed by a 

 comparatively narrow strip of Boulder Bed. 



Errochty section. — A study of the district south of the 

 River Tummel is enough in itself to decide the sequence, but 

 ample confirmation can be found in the area farther north. East 

 of Druimchastle bands of Killiekrankie Schist, Schichallion 

 Quartzite, Boulder Bed, White Limestone, and Banded Series 

 descend the hill-slopes in that order. A more complete section 

 is seen in the Errochty Water, near Trinafour. West of the 

 junction of the Dalradian System with the Struan Flags, some 

 distance above the new, but below the old bridge, the Grey Schist 

 is first met with. The old bridge rests upon a foundation of Grey 

 Limestone, and above it the Banded Series is crossed. This ends 

 with a narrow band of White Limestone, the other side of which 

 is a small fault, bringing on the Boulder Bed. The limestone is, 

 however, repeated a little farther up stream, and seen in unfaulted 

 contact with a calcareous rock, which merges into the con- 

 glomerate. Higher up stream the latter gives way to Quartzite, 

 and finally to a broad belt of Killiekrankie Schist. 



The evidence for the lower part of the succession shown in the 

 table (p. 427) has now been presented. The upper part extends 

 through a large area of Perthshire, and has not been a subject of 

 dispute. If the lower part is accepted, and proves capable of 

 extension to other districts, it is suggested that the whole sequence 

 should be known as the Perthshire Dalradian Succession. 

 So far, I have not been dealing with the question whether the 

 time-sequence of the table is up or down. 



Boulder Bed. — There are, as shown in the table, possibly two 



