Geological Society of London. 43 



the agglomei'ate in this case being connected with a thick and wide- 

 spread sheet of tuff intercalated among the basalts. Another example 

 is cited from the east end of the island of Canna, where the ejected 

 volcanic blocks are associated with records of contemporaneous 

 river-action. 



(3) The paper describes in some detail the evidence for the flow 

 of a large river across the lava-fields during the time when volcanic 

 activity was still vigorous. Thick sheets of well-rolled gravel are 

 intercalated among the basalts of the islands of Canna and Sanday. 

 These masses of detritus consist mainly of volcanic material, but 

 they include also abundant pieces of Torridou Sandstone and rocks 

 from the Western Highlands. The current of water which trans- 

 ported Iheni certainly came from the east. That it flowed while the 

 volcanic vents continued in eruption is shown by the bands of tuff 

 and the large blocks of slag contained in the conglomerates, as well 

 as by sheets of vesicular basalt iuterstratified in the same deposits. 

 From the terrestrial vegetation whereof the macerated remains are 

 enclosed in the tuffs and shales, and from the entire absence of 

 marine organisms, it may be confidently inferred that the water was 

 that of a river. The large size of many of the rounded blocks that 

 were swept along proves that this river must have been of con- 

 siderable volume and rapidity. The stream probably drained some 

 part of the Inverness-shire Islands, and wandered over the volcanic 

 plain, following the inequalities of the lava-fields, sweeping away the 

 loose detritus of volcanic cones, and being continually liable to have 

 its course altered by fresh volcanic eruptions. An interesting section 

 is cited from the island of Sanday, where what appears to be a portion 

 of the ravine of one of the tributaries of this river, trenched in the 

 basalts, filled with coarse shingle and buried under later basalts, 

 remains in a picturesque sea-stack. 



(4) Many additional details are given to illustrate the structure 

 and behaviour of the basic sills which are so abundantly developed, 

 especially at the base of the plateaux. Some of these sheets are of 

 colossal proportions, as in the Shiant Isles, where a single, columnar 

 bed is 500 feet thick. Others descend to extremely minute pro- 

 portions, such as the slender layers and threads which have been 

 injected into the coal and shale between the lower basalts in the 

 Portree district. A remarkable instance of a sill traversing the 

 centre of another is cited from the south-east of Skye, the younger 

 sheet having a strongly developed skin of black glass on its upper 

 and under surfaces. One of the most striking instances of a sill 

 rising obliquely across a thick mass of the plateau-basalts is described 

 from Stromo in the Faroe Islands. 



(5) The author adds some additional particulars, more especially 

 from Skye and St. Kilda, to his published account of the dykes 

 which have taken so important a place in the origin and structure 

 of the plateaux. 



(6) Further observations are narrated regarding the great bosses 

 of gabbro in the Inner Hebrides. In particular, the peculiar banded 

 structure, already described from a part of the Cuillin Hills, is shown 



