JReviews — Geology of Colonmy and Oronsay. 37 



portions might have been inserted, in. the space of about nine square 

 inches, on the broad margin, of the map. An uncoloured geological 

 map, on the scale of 2 miles to 1 inch, of the two islands is, 

 however, given in the frontispiece to the memoir. The geology of 

 the islands, described in Part I, is by Messrs. Wright and Bailey. 

 The Lewisian Gneiss is exposed over a few acres in the north of 

 Colonsay, but it is mainly covered by Blown Sand. The two islands 

 of Colonsay and Oronsay are formed chiefly of Torridonian, a single 

 conformable series, probably 5,000 feet thick, composed of grits or 

 arkoses, epidotic beds, sandstones, flags, mudstones, piiyllites, and 

 limestones. These are shown on the map in eight divisions, which 

 embrace in all seventeen minor lithological divisions. The mapping 

 clearly shows the general structure of the strata, which dip to the 

 east and north-east, but it is remarked that there is much complexity 

 of minor folding in the series. In a footnote the authors state : 

 " Without in the least suggesting a correlation, it is interesting to 

 note that in all save metamorphism the Colonsay flags resemble the 

 banded semi-siliceous granulites known as ' Moines ' in the Central 

 Highlands." Considerable attention has been given to the cleavages 

 in the Torridonian and associated rocks. 



The igneous rocks include the syenite of Kiloran Bay, the diorite 

 of Scalasaig, and the kentallenite of Balnahard ; also minor intrusions 

 of lamprophyre, basalt, and felsite, and certain north-west dykes of 

 - dolerite and monchiquite. 



Evidences of severe glaciation are seen in the small area of Lewisian 

 gneiss and elsewhere — glaciation that was effected hj ice from the 

 mainland from east to west. Support is given to the view of the 

 glacial origin of the sea-floor basin between Oronsay and Colonsay, as 

 advocated many years ago by Professor J. Geikie. The few sections 

 of Boulder-clay show it in an oxidized form as a red and often 

 stiff and tenacious clay, which forms wet rushy ground. There 

 are also many scattered boulders. 



Post-Glacial raised beaches occur at the 100, 50, and 25 foot levels, 

 the last named having been formed during early jSTeolithic times, in 

 a period of marked cave formation. A brief account is given of the 

 submerged Forest of Loch Fada, Colonsay, where stumps and large 

 trunks of trees (including Goat Willow) occur. It is observed that 

 no trees of the size exist in the valley at the present day, with the 

 exception of the artificially protected plantations and woods at 

 Kiloran. The submergence seems to have been caused by a later 

 transgression of the lake due to the raising of its outlet by the growth 

 of peat. 



The islands nowhere attain an elevation of 500 feet, and are 

 probably a remnant of an old plain, 400 to 500 feet above sea-level, to 

 the age of which, however, there is no clue. There is evidence of 

 pre-Glacial marine erosion in a plain, with ancient sea-cliffs, at an 

 elevation of 135 feet. Among other interesting observations are those 

 made on the development of the present topography. There is a good 

 deal of fertile land, largely due to the distribution of shell-sand by 

 winds. The debris from crags of phyllite affords good soil for pasture- 

 land, and the raised beaches a lighter soil suitable for corn. The 



