Vol. 62.] 



STRUCTURE OF THE SGURR OF EIGG. 



53 



it in the latter way, as a mass of volcanic agglomerate filling a 

 small vent. Only a small part of it, on the south side, is accessible 

 to direct observation. 



Fig. 4. (Key to PI IV.)—Bidein Boidheach, showing the sea-cliff in 

 which the ridge of the Sgurr terminates north-westward. 





7 



V/ /T '0, 



The outcrops of the basaltic lavas and dolerite-sills rim in nearly horizontal 

 lines. PPP is the base of the pitchstone, and A is the volcanic agglomerate 

 which underlies it in the cliff. The dolerite-sill D, impinging upon the 

 agglomerate and deviated by it, is not distinctly seen in the photograph. 

 At B are basalt-dykes, cutting the basalt-lavas and dolerite-sills, and one 

 of these is seen to terminate abruptly at the agglomerate. 



The first point to note is the evidence that this fragmental 

 accumulation is older than the dolerite-sills. The large sill which 

 makes the top of the cliff on the south side is seen to abut upon the 

 agglomerate, but does not enter it. It is, however, not cut off, but 

 sharply deviated. As Dr. Peach observed, it turns upward, with 

 diminished thickness, along the boundary of the agglomerate, and 

 then, curving away, rapidly dies out. No corresponding sill is 

 seen on the opposite side, although the unbroken succession below 

 shows that the steep boundary on that side is not made by a fault. 

 Instead we find some thinner sills which, so far as can be seen, 

 appear to terminate abruptly at the agglomerate. The sharp 

 deviation or abrupt termination of sills where they come against a 

 mass of coarse agglomerate is a phenomenon seen in many places in 

 Skye, Canna, and Sanday. These and some other massive rocks 

 appear to have the property of arresting the progress of a minor 

 intrusion, either sill or dyke, which encounters them. 1 Some basalt- 

 dykes occur, as noted by Sir Archibald Geikie, in the cliff of Bidein 

 Boidheach, and one of them is seen to terminate abruptly at the 



1 See ' The Tertiary Igneous Rocks of Skye ' Mem. Geol. Surr. 1904, pp. 242, 

 293. 



