476 Reviews—Geological Survey of Islay. 
II.—Geroxocicar Survey oF Scornanp. 
Tae Gxorocy or Istay. By S. B. Wirxrinson!; with notes by 
J.J. H. Teatz, F.R.S., and B. N. Peacn, F.R.S. Memoirs of the 
Geological Survey. pp. vii, 82, with 6 figures and 8 plates. 
1907. Price 2s. 6d. lagna 
/ las island. of Islay is of the first importance in the interpretation 
of the geology of a large part of the Highlands of Scotland. 
The Lewisian and Yorridonian rocks of the west coast there come into 
juxtaposition with the Central Highland schists. The one series 
extends northwards to Cape Wrath, the other stretches through 
Argyllshire and Perthshire to the shores of the Moray Firth. In 
Islay they meet, as they do in no other part of Scotland. The 
geology is complicated by the presence of another group of rocks 
which may be Cambrian. Only the Moine Schists and the grits and slates 
of the southern border of the Highlands (with the Loch Tay limestone) 
are wanting to make Islay a complete epitome of Highland geology. 
For these reasons we turn to this memoir with much curiosity to 
know the results of the most recent work in this quarter. 
The structure of the island is sufficiently simple in its broad 
outlines. On the west there are Lewisian and ‘orridonian rocks, 
both in their typical development. The Lewisian is penetrated by 
many sills and dykes of epidiorite. Both sets of rocks have been 
locally much crushed. On the east side of Islay we have the Central 
Highland schists (Blair Atholl or Islay limestone, black schist, and 
Ardrishaig phyllites) with a group of quartzites, dolomitic shales, etc. 
Between those two formations there is a conglomerate, the Portaskaig 
conglomerate. There seems to be no doubt that the quartzite group 
rests upon the limestone, and that there is a slight unconformability 
at the junction. 
Nearly through the middle of the island there runs a great north 
and south thrust-plane, a continuation of the series of post-Cambrian 
thrusts of the north-west of Scotland. To the west of it lie the 
Lewisian and Torridonian, to the east the Central Highland schists and 
quartzite. The break is absolute. The two sides of the island are 
entirely unlike; only the epidiorite sills are found on both sides of the 
thrust (if we except the Tertiary dykes and later rocks). This is 
a great disappointment, for it means that Islay gives no direct clue to 
the time-relationships of the two sets of rocks which form its opposite 
sides. The interest centres, accordingly, in the north-east corner of 
the island, where the quartzite and Central Highland schists are 
folded together in intricate fashion. 
The quartzite group of Islay, resting unconformably on the Central 
Highland schists, possesses many features which give it a strong 
resemblance to the Cambrian rocks of Sutherland. The lithological 
characters and apparent succession are the same; there are sandy 
dolomitic shales, with worm-castings, which are described as ‘ fucoid 
beds.” In the quartzite there are several kinds of worm-burrows 
which are strictly comparable with those found in the Assynt 
‘ [We have every reason to believe that S. B. Wilkinson, above-named, is the 
same as B. 8. N. Wilkinson, part author of memoir on the Geology of West Gower, 
noticed in the August Number of the Gror. Mac. Bibliographers may, however, 
be puzzled.—Eprr. ] 
