144 Geological Society : — 



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 Highlands, and wandered over the volcanic 

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

 the loose detritus of volcanic cones, and 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. 



An account is given of the little islet of Hysgeir, about 

 18 miles to the west of the island of Eigg, which has been identified 

 by Dr. Heddle with the rock of the Scuir of Eigg, and which the 

 author has visited in two successive years. The 'pitchstone' is 

 precisely like that of the Scuir down to the most minute structure, 

 as is shown by an examination of the rock under the microscope 

 by Mr. A. Harker. There can be little doubt that this rock was 

 a superficial lava like that of Eigg, and there seems every prob- 

 ability that it is really a westward continuation of the Scuir. The 

 Hysgeir pitchstone everywhere dips under the sea, so that its 

 bottom cannot be seen. 



The author considers that the Canna river may have been the 

 same as that which at a later part of the volcanic period had its 

 channel filled up by the pitchstone of the Scuir of Eigg. 



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 

 to have a wide distribution in Skye, and to occur also in Rum. 

 The remarkable alteration of the plateau-basalts as they approach 

 the gabbros of Loch Scavaig is referred to, and the mioroscopic 

 structure of these metamorphosed rocks is described in notes supplied 

 to the author by Mr. Harker. An account is also given of the 



