1871.] GEIKIE — TERTIAKT VOLCANIC ROCKS. 307 



which the pitchstone rests. Again, at the east end of the Scur the 

 pitchstone wall is placed not fairly on the crest of the dolerite- 

 plateau, but on the south side of it. This cannot fail to arrest the 

 notice of every observer, even from a distance (see PI. XIV. fig. 1). 

 It shows us not only that the rocks of the Scur were erupted along 

 a hollow or valley, but that only the north or north-eastern side of 

 that valley is now preserved. 



Allusion has been already made to two minor tongues of pitch- 

 stone which project to the north-east from the main ridge of the 

 Sciir, and form small hills. Even in these offshoots the same 

 evidence of want of sequence between the rock of which they are 

 composed and the underlying basaltic sheets is clearly exposed. In 

 Beinn Tighe, for instance, the northern projection, a section taken 

 across the isle from east to west shows the basalts at a much 

 higher level on the one side than on the other. These offshoots 

 appear to have been originally either recesses of the main valley, or 

 tributary valleys descending into it, and to have been buried and 

 preserved under portions of the coulees of the pitchstone lava which 

 overflowed fi'om the main mass. 



Underneath the eastern end of the precipice of the Sciir, on its 

 southern or lower side, a bed of fragmentary materials is found to 

 intervene between the pitchstone and the dolerites. The base of 

 the pitchstone dips into the hill, forming the roof of a small cave. 

 The under surface of the pitchstone is tolerably smooth, but undu- 

 lating, and shows the ends of the columns as a polygonal reticu- 

 lation over the roof. The breccia is a pale-yellow or grey felspathic 

 rock, like the more decomposing parts of the grey porphyry of the 

 same cliff. Through its mass are dispersed great numbers of angular 

 and subangular pieces of pitchstone, some of which have a striped 

 texture. Fragments of basalt, red sandstone, and other rocks are 

 rare ; and the bed suggests the idea that it is a kind of brecciated 

 base or flow of the main pitchstone mass. A similar rock is found 

 along the bottom of the pitchstone on both sides of the ridge (c, in 

 fig. 9). At some points where this breccia is only a yard or two 

 in thickness, and consists of subangular fragments of the various 

 dolerites and basalts of the neighbourhood, along with pieces of red 

 sandstone, quartz-rock, clay-slate, &c. The matrix is in some 

 places a mass of hard basalt debris ; in others it becomes more cal- 

 careous, passing into a sandstone or grit in which chips and angular 

 or irregular-shaped pieces of coniferous wood are abundant*. A 

 little further east, beyond the base of the Sciir, a patch of simi- 

 lar breccia is seen, but with the stones much more rounded and 

 smoothed. This outlier rests against the denuded ends of the ba- 



* The microscopic structure of this wood was briefly described by Witham 

 (Foss. Vegetables, p. 37), and two magnified representations were given to show 

 its coniferous character. Lindley and Hutton further described it in their 

 ' Fossil Flora,' naming it Pinifes eiggensis, and regarding it as belonging to the 

 Oolitic series of the Hebrides — an inference founded perhaps on the erroneous 

 statement of Witham to that effect. William Nicol corrected that statement by 

 showing that the wood-fragments occurred, not among the "lias rocks," but 

 "among the debris of the pitchstone" (Edin. New Phil. Journal, xviii. p. 154); 



