436 ME. E. M. ANDERSON ON THE GEOLOGY OF [vol. lxxix, 



course of the flagstone margin, but no line has been drawn that can 

 be regarded with confidence. The reason is that, although the 

 Struan Flag Group of this district contains no rock of a thoroughly 

 pelitic nature which matches the Grey Schist, it does contain 

 horizons which resemble the less thoroughly micaceous Killie- 

 kran'kie Schist, and probably also quartzites of Schichallion type. 

 It is, however, certain that neither the Grey Schist, Grey Lime- 

 stone, Banded Series, White Limestone, nor Boulder Bed can be 

 seen south of the Errochty Water on the western margin of the 

 syncline. On the other hand, one passes from areas of Killie- 

 krankie Schist, through rocks of the same type which may belong- 

 to either series, into unquestionable flags. In a stream, which rises 

 about a mile west of Ben a' Chuallaich, and joins the Tummel at 

 Kinloch Rannoch, rocks resembling the flagstone series are seen in 

 contact at one point with what may be the pebbly quartzite, and 

 at another with what is almost certainly Ben Eagach Schist. 



We are, in any case, justified in regarding the junction of the 

 two series as, in part at least, a strong line of discordance. If 

 it is an unconformity, the amount of erosion, or the amount of 

 overlap, must extend from Grey Schist certainly to Killiekrankie 

 Schist, and probably to Ben Eagach Schist. 



The following considerations appear to tell against the explanation 

 of the facts by means of an unconformity. For the sake of argu- 

 ment, let it be supposed that the unconformity exists. Then 

 there are two alternatives : either the Dalradian Series has been 

 deposited over the flagstones, or the flagstone series over the 

 Dalradian. If the Dalradian Series be the later, the discordance 

 already noted must be due to the overlap one over the other of 

 different members of this group. 1 The Grey Schist, being for so 

 long a distance marginal to the flagstones, must be taken as the 

 oldest of the formations which have succeeded them, and it was 

 followed by the Grey Limestone. In this case it is difficult to 

 explain the observed adherence to type of these two Dalradian 

 members, where they abut against their shoreline. The former 

 is an altered carbonaceous muclstone, and the latter a clear- 

 water deposit. Some degree of lateral variation at least was to be 

 expected, and we have also to account for the absence of con- 

 glomerate. 



It may be supposed, however, that, while an unconformity exists, 

 the order in time is the reverse of that already considered, and 

 the Struan Flag Series originally overlay the Dalradian. It then 



1 This relation has been figured by Prof. J. W. Gregory (' Handbuch der 

 Pegionalen Geologie' vol. iii, pt. 1, 1917, p. 38). Prof. Gregory, however, 

 shows the Blair Atholl Limestone and Graphitic Schists as resting upon the 

 Quartzite. As the former presumably correspond to the Grey Limestone and 

 Grey Schist of this paper, it is difficult to explain the consistent intervention 

 of a belt of Grey Schist, Grey Limestone, etc., between the flags and the 

 Quartzite, along the eastern margin of the ' syncline ', on the basis of Prof. 

 Gregory's diagram. 



