Yol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 357 



Immediately above the massive greenish pebbly sandstone (d) 

 -which caps the stratified series, lies a group of basalts (e), composed 

 of several distinct beds, having a united thickness of from 80 to 

 100 feet. The lowest of these has a regular columnar structure, 

 while those overlying it exhibit the confused starch-like grouping of 

 curved and rather indistinctly -formed prisms, which is so cha- 

 racteristic a structure in the plateaux. 



The next band in upward succession is one of conglomerate (/), 

 which runs as a continuous and conspicuous feature along the upper 

 part of the cliff. This rock presents in many respects a strong 

 contrast to the conglomerates underneath. It is dull green to 

 yellow in colour, and is well stratified, being marked by the inter- 

 stratification of finer layers, and passing down into a band of 

 pebbly sandstone, which rests immediately on the basalt (e). Its 

 component stones are thoroughly waterworn, ranging up to 6 

 inches or even more in length. But its most distinctive character 

 lies in the nature of its pebbles. Instead of consisting mainly of 

 volcanic materials, these stones have almost all been transported for 

 some distance. They include abundant fragments of Torridon 

 Sandstone, gneiss, schists, grits, and other rocks like those in Rum 

 and Western Inverness-shire. iSo such rocks exist in situ in Canna. 

 The nearest tract of Torridon Sandstone is in Rum, about 4 miles to 

 the eastward. But the pieces of schist and epidotic grit, like the rocks 

 of the Western Highlands, must have travelled at least 30 miles. 



It is important to observe that all these transported stones 

 indicate a derivation from some source lying to the eastward of 

 Canna. The evidence in this respect agrees with that furnished by 

 the ancient river-gravel under the pitchstone of the Scuir of Eigg. 

 It is clear that the waters which found their way across the lava- 

 fields of this part of the Inner Hebrides took their rise among the 

 mountains of Inverness-shire. 



The conglomerate now described is from 40 to 50 feet thick. It 

 can be followed along the face of the cliffs for more than a mile on 

 the northern side of Canna. Less persistent on the southern side, its 

 outcrop strikes from the edge of the precipice inland, keeping to 

 the south of the top of Compass Hill. It is well seen in the ravine 

 above the Coroghon, but cannot be followed farther westward 

 among the basalt-terraces. Yet, though this stratified intercalation 

 is not traceable very far as a band of conglomerate, the same strati- 

 graphical horizon is probably indicated elsewhere by other kinds 

 of sedimentary deposits, to which further reference will be made in 

 the sequel. 



The section now described establishes the existence of at least 

 two successive platforms of conglomerate in the volcanic series. 

 Following these platforms along their outcrop, we obtain additional 

 light on their origin, and on the topographical conditions under 

 which they were deposited, and we learn further that other pro- 

 longed intervals, which were likewise marked by intercalations of 

 sedimentary material, occurred in the outpouring of the basalts. 



Taking first the lower conglomerate of Compass Hill and tracing 



2b2 



