Geological Society of London. 283 
and hornblende-schists, the peridotites and palzeopicrites into talcose 
schists, the microcline-mica rocks into mica-schists, and the granites 
into granitoid gneiss; (6) the effects on the gneiss resulted in the 
formation of sharp folds trending generally N.W. and 8.E., the 
partial or complete reconstruction of the original gneiss along the 
old foliation-planes, and finally the development of newer schistosity 
more or less parallel with the prominent disruption-lines. 
There is an overwhelming amount of evidence to prove that all 
these various changes had been superinduced in the Archzan rocks 
in Pre-Cambrian time. 
After reviewing the facts bearing on the denudation of the Archean 
land-surface, the order of succession and thickness of the Cambrian 
strata were given, from which it is apparent that the deposits 
gradually increase in thickness as we pass southwards from Durness 
to Loch Broom. 
Prior to the deposition of the Silurian sediments the Cambrian 
strata were folded and extensively denuded. By these means various 
Cambrian outliers were formed far to the east of the present limits 
of the formation. 
The order of succession of the Silurian strata along the line of 
complicated structure from Eriboll to Ullapool was described, refer- 
ence being made to the further subdivision of the ‘“ Pipe-rock” and 
the Ghrudaidh Limestones (Group I. of Durness section). None 
of the richly fossiliferous zones of Durness is met with along this 
line, as they occupy higher horizons. An examination of the fossils 
recently obtained by the Geological Survey from the Durness Lime- 
stones confirms Salter’s conclusions that they are distinctly of an 
American type, the Sutherland quartzites and limestones being 
represented by the Potsdam Sandstones and Calciferous Sand Group 
of North America. 
After the deposition of the limestones, the Cambrian and Silurian 
strata were pierced by igneous rocks, mainly in the form of sheets, 
producing important alterations in the sedimentary deposits by con- 
tact-metamorphism, the quartzites becoming crystalline, and the 
limestones being converted into marble. 
When this outburst of volcanic activity had ceased, terrestrial dis- 
placements ensued on a stupendous scale. By means of powerful 
thrusts the Silurian strata were piled on each other, and huge slices 
of the old Archean platform, with the Cambrian and Silurian strata 
resting on it, were driven westwards for miles. With the view 
of illustrating the extraordinary complications produced by these 
movements, a series of horizontal sections was described drawn 
across the line between Hriboll and Ullapool. 
The evidence relating to regional metamorphism was next referred 
to, from which it is obvious that with each successive maximum 
thrust there is a progressive amount of alteration in the displaced 
masses, as the observer passes eastwards to the higher thrust-planes. 
Eventually the Archean gneiss is so deformed that the Pre-Cam- 
brian foliation disappears and is replaced by new divisional planes ; 
the Cambrian grits and shales are converted into schists ; the Silurian 
