THE HIGHLAND BORDER ROCKS OE THE ABERFOYLE DISTRICT. 
205 
completely digested, the line separating the one from the other must be sought for 
further to the north-west. 
The Serpentine- Gabbro-Dolomite Complex. — The serpentine-gabbro-dolomite com- 
plex, whose petrographical characters and affinities have already been discussed 
(pp. 195—197), can be traced almost continuously along the line of the main Highland 
Fault, which forms its southern boundary, from Gualann to the old limestone quarry 
north-east of Aberfoyle. Its delimitation northwards is also determined by a fault 
or faults. Its petrographical characters, as we have seen, leave little room for 
doubting that it represents an intrusive complex derived from the magma which 
gave rise to the spilites of the Black Shale and Chert Series. It may, therefore, be 
regarded as an integral part of the Highland Border group of rocks. As in the case 
of the albite diabase and the hornblende schists, its period of intrusion has not been 
established definitely. There can be no doubt that it is of pre-Lower Old Red 
Sandstone age, since its petrographical affinities are with the spilites and albite 
diabase rather than with the basic and ultra-basic facies of the newer granites of the 
Highlands. Again, since it is strongly foliated in places, it must be older than the 
post-Margie crustal movements. Further, as Du Toit * has noted, in the region 
between Cualann and Balmaha well-rounded boulders, clearly derived from this 
complex, build up a serpentine conglomerate which may be the basement bed of 
the local Margie Series. It is possible, then, that the intrusion of the serpentines 
and gabbros took place in pre-Margie times. The dolomitisation and silicification of 
those rocks are, in part at least,' of much later date, since dolomitic and siliceous 
ramifications from the main dolomite exposures penetrate the adjoining Lower Old 
Red Sandstone beds. Dolomite occurs as fault-rock on the line of the Highland Fault 
and the Gualann Fault, and the occurrence of jasper, which is seen from its micro- 
scopic characters to have resulted from the silicification of a ferruginous dolomite, 
is a feature characteristic of the course of the Jasper Fault. 
Relation of the Highland Border Rocks to the Old Red Sandstone,. — Along the 
whole area under consideration the Highland Border Rocks are separated from the 
main tract of the Lower Old Red Sandstone by the Highland Boundary Fault. 
From a point near the Bonin ty Burn opposite Clashbeg Wood westwards, however, 
a group of lower beds of the latter formation intervenes between the serpentine- 
gabbro complex and the zone of metamorphic rocks. Its southern boundary is 
determined by the Gualann Fault, its extension northwards by the Jasper Fault. 
The outlier of rocks of Upper Old Red Sandstone age to the north-west of 
Gualann is faulted against the serpentine-gabbro mass along the line of the Gualann 
Fault. In the neighbourhood of that fault it is unconform able to the Lower Old 
Red Sandstone ; to the north and west it overlaps successively the metamorphic 
belt, the black shales, cherts, and spilites of the Lower Series, and the beds of the 
Upper or Margie Series ; finally is found resting directly on the Leny Grits. 
k Trans. Edin. Geol. Soc., vol. viii, p. 324, 1905. 
