CHAP. XXXVII 
THE PLATEAU OF SMALL ISLES 
217 
in a range of mural precipices rising to a height of about 600 feet above 
the sea. From the top of that escarpment the ground falls by successive 
rocky terraces and grassy slopes to the southern shore-line. Sunday, con- 
nected with the large island by a shoal and foot-bridge, is two miles long and 
220 to about 1200 yards broad. Its highest cliffs range along its southern 
shore to a height of 193 feet, whence they slope gently northward into 
the hollow between the two islands. This peculiar topography accounts for 
the manner in which the geological sections of most interest are distributed. 
The first, and still the best, account of the geology of these islands is 
that of Macculloch. He showed that the rocks all belong to the series of the 
plateau-basalts, and he described the presence among them of a “ trap-con- 
glomerate.” He noticed the occurrence also of trap-tuff and the occasional 
appearance of carbonized wood in these deposits. Reasoning upon these 
observations in his characteristically vague and verbose manner, “ bewildered 
in the regions of conjecture,” he concludes that the basalts instead of 
belonging to “ one general formation ” have been successively deposited on 
the same spot, “ since lapse of time is evidently implied in the formation of 
a conglomerate.” He inclines to believe that they have been discharged by 
ancient volcanoes from which in the course ot time all traces ot their 
original outline have been more or less completely removed, the existing 
basalts being merely fragments of once more extensive masses. 1 
Macculloch regarded the intercalated-conglomerates as having been 
arranged under water and as marking pauses in the deposition of the sheets 
of “ trap.” He gave two diagrams in illustration of the relations of these 
detrital deposits, but he expressed no definite opinion as to their .origin, 
though from one passage it would seem that he inclined towards the belief 
that they were formed in the sea. 2 Since his time, so far as I am aware, no 
fresh light has been thrown upon the subject. 
During a yachting cruise in the summer of 1894 I visited Canna for 
the first time and found so much that was new to me in regard to the 
history of Tertiary volcanic action, and which demanded a careful survey, 
that I returned to the locality the following summer and remained in the 
island until 1 had mapped it and its dependencies upon the Ordnance Survey 
sheets on the scale of six inches to a mile. The following narrative is the 
result of the observations then made. 
As far back as the year 1865 1 published an account of an ancient 
river-channel which, during the volcanic period, had been eroded on the 
surface of the basalt-plateau, and of which a small portion bad been pre- 
served under a stream of pitchstone-lava that had flowed into and buried it. 3 
This water-course, now marked by the picturesque ridge of the Scuir ol Eigg, 
was shown to have been excavated by a stream which came from the north- 
east or east, and to be younger, not only than the plateau-basalts of the 
district, but than even the dykes which cut these basalts. Yet that it 
1 Western Isles, vol. i. pp. 448-459, and pi. six. Figs. 2, 3 and 4. See also Jameson’s 
Mineralogy of the Scottish Isles. 2 Op. dt. pp. 449, 457, pi. xix. Figs. 2 and 3. 
3 Scenery of Scotland (1865) ; Quart. Journ. Geo. Soc. vol. xxvii. (1871), p. 303. 
