648 SIR A. GEIKIE AND ME. J. J. H. TEALL ON THE [NOV. 1 894, 



2. The Banded Gabbros. — It is in these rocks that the chief 

 interest of the series centres. Even from some distance their 

 remarkable structure is recognizable, owing to the striking contrasts 

 of colour which their weathered surfaces present. Parallel strips of 

 pale grey alternate with bars of dark brown, and, as the crags have 

 been intensely glaciated, the structure is revealed as distinctly as if 

 the rocks had been artificially cut and polished. The geologist who 

 is familiar with the aspect of the ice-worn hummocks of Lewisian 

 gneiss in the west of Sutherland, and who for the first time comes 

 upon these bare rocky knolls of Druim an Eidhne, may well be 

 pardoned if he for a moment should be inclined to insist that he 

 has before him another example of the same unmistakable outer 

 features and the same internal structures which characterize the most 

 venerable formation of the north-western Highlands. Nor is this 

 first impression immediately effaced by a closer examination. Not 

 until the whole mass of rock is considered and its relation to the 

 rest of the igneous masses is understood will the idea of an Archaean 

 age be definitely abandoned. 



The banded gabbros occur in successive sheets or sills which vary 

 from a few feet to many yards in thickness. Indeed, their upper 

 and lower limits are not easily fixed, except when they are marked 

 by the intercalation of the dark, fine-grained sheets, or where they 

 are truncated by the massive gabbros. Each of these banded sheets 

 consists of many parallel layers of lighter and darker material, 

 which correspond in direction with the trend of the sheet itself, and 

 are usually inclined towards the east or south-east at angles ranging 

 from 20° to 30°. The component layers vary in thickness from 

 mere pasteboard-like laminae to beds a yard or more in thickness. 

 On a single exposed face of rock they may seem to be as parallel, 

 regular, and continuous as sedimentary deposits. But, as we trace 

 them along the strike, we observe that they are apt to vary in thick- 

 ness and even to die out. In this general parallelism and discon- 

 tinuity they present a strong resemblance to the arrangement of the 

 darker and lighter bands among the old banded gneisses. 



In yet another particular the analogy with these ancient rocks is 

 sustained by the Tertiary gabbros. The parallelism of the bands 

 sometimes gives way to undulations or puckerings, and even to rapid 

 plications. A remarkable example of this structure is represented 

 in PI. XXVL, where a group of bands some 10 feet thick has been 

 doubly folded between parallel bands above and below. 



Even before a minute examination of the banded structure is 

 made, the observer recognizes it to be due to an aggregation of the 

 several constituent minerals in distinct layers. The paler bands 

 are seen to be those wherein the felspar more especially pre- 

 dominates. The dark brown bands are particularly rich in the 

 ferro-magnesian minerals and magnetite, which project from the 

 weathered surfaces as garnets do from the face of a crag of mica- 

 schist. The thin ribs of glistening black mark where the iron ore 

 is well developed. 



A closer inspection reveals the fact that an intimate union exists 



