202 
PROFESSOR T. J. JEHU AND DR ROBERT CAMPBELL ON 
mass is somewhat obscured by alteration products ; it contains stout laths of labra- 
dorite, granular calcite and chlorite doubtless representing original augite, iron 
oxides, apatite, and decomposed intersertal mesostasis. The rock is an olivine- 
enstatite porphyrite, and its petrographical characters point to community of origin 
with the local lavas of Lower Old Bed Sandstone age. The other intrusion occurs 
further west in the Corrie Burn on the same line of strike. 
The quartz dolerite dykes are seldom fresh, and their original characters are 
often obscured by the development of secondary chlorite and carbonates. The 
freshest specimens are seen to be free from or poor in olivine, and to contain quartz 
which is possibly, in part at least, primary. A dyke in the old limestone quarry 
north-east of Aberfoyle is rich in brown mesostasis, through which are scattered 
stout laths and forked microlites of labradorite, granular augite, iron oxides, and 
quartz. The distribution of the dykes is shown in the accompanying map (Plate VI). 
Their general trend is between north-east and south-west and east-north-east and 
west-south-west. In their direction and in their petrographical characters they 
recall the widely distributed quartz dolerites of Central Scotland, which are usually 
regarded as of late Carboniferous age. 
XII. Tectonics. 
From the account given in the preceding pages it will be seen that the rocks 
within the narrow belt between the main Highland Fault and the Leny Fault com- 
prise a great diversity of types — igneous, sedimentary, and metamorphic. The struc- 
tural relations are no less complicated. Our interpretation of the tectonics is set 
forth in the acompanying map and in a series of horizontal sections (figs. 3-10, 
pp. 206, 207) taken at intervals along the area from west to east. Some of the more 
important tectonic features have already been incidentally referred to ; in the 
succeeding paragraphs it is proposed to discuss them in somewhat greater detail. 
The Unconformability between the Lower or Black Shale and Chert Series and 
the Upper or Margie Series. — The rocks of the Lower Series, as we have seen, were 
laid down at and beyond the limits of terrigenous sedimentation ; the Upper Series 
was deposited in close proximity to a land area. The basement breccias and grits of 
the latter contain fragments of the black shales and cherts and of the underlying 
spilitic lavas. It is clear, therefore, that the deposition of the Margie beds was 
preceded by a period of crustal movement and elevation. The old sea floor became 
a land area, the denudation of which supplied, in part at least, the materials for the 
sediments of the younger formation. That no inconsiderable amount of erosion was 
accomplished in that interval is shown by the presence in the basement beds and 
grits of the Margie Series of fragments of the spilites, which are the lowest visible 
part of the Black Shale and Chert succession. Materials derived from the Lower 
Series are scarcer in the grits than in the basement breccias. This points to a 
transgression of the Margie sea, the deposits of which were ultimately spread over 
