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.[248] Since his time, so far as I am aware, no fresh light has been thrown upon the subject. [Footnote 248: _Op. cit._ pp. 449, 457, pl. xix. Figs. 2 and 3.] 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 I 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 I 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 had been preserved under a stream of pitchstone-lava that had flowed into and buried it.[249] This water-course, now marked by the picturesque ridge of the Scuir of 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 belonged to the volcanic period was proved by the manner in which it had been sealed up and preserved under the black glassy lava of the Scuir. Its history and the data from which this history is compiled will be narrated in a later part of this chapter. [Footnote 249: _Scenery of Scotland_ (1865); _Quart. Journ. Geo. Soc._ vol. xxvii. (1871), p. 303.] My examination of the islands of Canna and Sanday, however, brought to light other and more abundant evidence of river-action in the same region of the Inner Hebrides, but belonging to an earlier part of the volcanic period. This evidence reveals that a powerful river, flowing westwards from the Highland mountains, swept over the volcanic plain, while the sheets of basalt were still being poured forth, and while volcanic eruptions were taking place from cones of slag.