Vol. 50.] BANDED STR.T7CTUKE OF SOME TERTIAKY GABBROS. 657 



mass during intrusion that the handed structures were produced. 

 This is our hypothesis. We do not offer it as an original one, hut 

 merely as that to which we have been led from a consideration of 

 the facts above described. 



(ii) "We pass on now to consider the extent to which the phe- 

 nomena exhibited by these Tertiary gabbros may serve to elucidate 

 the structure and origin of some of our oldest gneisses. The extra- 

 ordinary resemblance of these gabbros to the JSTorian (anorthosite) 

 rocks of the North American Continent was clearly recognized 

 by earlier observers. More detailed observation has served to 

 extend the points of resemblance. The deposits of titaniferous iron 

 ore and the gneissose structures of the Norian formation have their 

 analogues in the black masses, extremely rich in titano-magnetite, 

 and in the banded structures to which attention has been directed 

 in this paper. It seems impossible, therefore, to avoid the conclusion 

 that the cause, whatever it may have been, which produced the 

 phenomena in the one case operated in the other, and that the 

 pre-Cambrian anorthosite rocks of Canada originated under physical 

 conditions closely allied to, if not identical with, those which give 

 rise to the Tertiary gabbros of Skye. 



It will be interesting, however, to compare these gabbros with 

 Archaean rocks nearer home. The Lewisian gneiss of the North- 

 west of Scotland is a petrographical complex, largely but not 

 entirely composed of gneisses having marked affinities with plutonic 

 igneous rocks. The area of this ancient formation already mapped 

 in detail by the Geological Survey forms a narrow border along the 

 coast, extending from Cape Wrath to Loch Torridon. In no portion 

 of this area do rocks of uniform character cover any large tract of 

 country, but extreme petrographical diversity may be said to 

 characterize the formation. Between Cape Wrath and Loch Laxford 

 hornblendic and micaceous gneisses predominate, and these are 

 often traversed by intrusive veins and sills of gneissose granite and 

 pegmatite. The area south of this district, extending from Scourie 

 to some distance beyond Loch Inver, is very largely composed of 

 augitic gneisses, with which banded rocks of ultrabasic composition 

 — peridotites, pyroxenites, and pyroxene-granulites — are associated. 

 Hornblendic and micaceous gneisses also occur. Another feature of 

 this area is the extraordinary abundance of basic dykes. South- 

 wards, in the neighbourhood of Gairloch and Loch Torridon, horn- 

 blendic and micaceous gneisses again predominate, and bands of 

 hornblende-schist, which appear to represent the dykes of what we 

 may refer to as the middle zone, are extremely numerous. In the 

 neighbourhood of Loch Maree limestone, garnetiferous mica-schists, 

 and graphite-schists occur. As these are probably metamorphosed 

 sediments, we need not further refer to them in the present com- 

 munication. 



The petrographical changes are sometimes gradual and sometimes 

 abrupt. The mode of association of the different varieties, or the 

 architecture of the rock-masses, as Prof. Brogger would say, is 



