432 MR. E. M. ANDEBSOK" OX THE GEOLOGY OF [vol. lxxix, 



also present in the Main Boulder Bed. These constituents may 

 have been derived from the main bed by fluviatile or marine 

 erosion. This theory assumes that the Quartzite is the later 

 deposit, and that there is a certain amount of overlap at its 

 base. It is unnecessary to suppose a readvance of the hypothetical 

 ice-sheet. 



Relations of the Boulder Bed. — It is probable, in any 

 case, that the base of the Quartzite overlaps the Main Boulder Bed. 

 A ground-moraine deposited on an irregular land-surface might 

 well be absent in places, and this may explain the fact that the 

 Quartzite is sometimes in very close proximhy to the White Lime- 

 stone. This is seen, for instance, in a tributary of the Errochty, 

 about a mile west of the old bridge which crosses the parent stream. 

 (It imiy, however, be due in part to lack of exposures that the 

 Main Boulder Bed cannot be more continuously traced.) The 

 White Limestone, as before remarked, is, for one reason or another, 

 very inconstant. The Banded Series is relatively persistent, but 

 at the northern end of the projecting area of Dalradian rocks most 

 of this series appears to be absent, as well as the two members 

 above it, in such wise that the Quartzite approaches the underlying 

 Grey Limestone. How far these facts are due to erosion, and how 

 far to subsequent movement, is difficult to determine. It seems, 

 however, reasonable to assume that there was a period of elevation 

 either before or after the formation of the Boulder Bed. which 

 accounts for its irregularity. Even if the conglomerate was formed 

 on a flat surface, its partly calcareous matrix and the limestone- 

 fragments may be explained by glacial erosion. But, perhaps more 

 probably, the surface over which the ice advanced was already an 

 uneven one. 



The hornblende-schists. — In this account of the strati- 

 graphy of the district little mention has been made of the 

 hornblende-schists. These are exceedingly numerous, and for the 

 greater part they have not been shown in the accompanying map 

 (PI. XXY). They occur from top to bottom of the sequence, and 

 do not appear to be constant at any particular horizon. They are, 

 therefore, probably not original lava-flows, but altered dykes, or 

 more likely, from their relation to the bedding, at least in large 

 part altered sills. Isolated fragments of a similar amphibolite are 

 seen in the Boulder Bed of the Errochty Water section. These 

 must be the result of the regional metamorphism acting on pebbles 

 of a rock like that which was concurrently transformed into 

 hornblende-schist. It seems possible, therefore, that there were 

 two periods of intrusion : one which preceded, and one which 

 followed the formation of the Boulder Bed. 



