68 THE GEOLOGICAL STRUCTURE [Feb. 1906, 



The speaker, on the contrary, believed that a large proportion of 

 these columnar sheets were as true lava-flows as the amygdaloidal 

 basalts with which they so regularly and abundantly alternated. 



The same tendency to see sills everywhere had apparently led 

 the Author to frame his theory of the Scuir. Sills were commonly 

 supposed to have forced their way along the planes of least resist- 

 ance, as for instance between the bedding ; but the pitchstone of 

 the Scuir, instead of inserting itself between the nearly-level sheets 

 of basalt, had filled up a hollow in these rocks, which, if not there 

 before the intrusion, must have been ploughed out by the intrusive 

 mass — an operation which would require a good deal of explanation. 

 But that this hollow did exist before the advent of the pitchstone 

 seemed to the speaker to be demonstrated by the section shown in 

 the vertical cliff at the western end of the ridge. The pitchstone was. 

 there distinctly seen resting in the hollow, the bottom of which was. 

 occupied by a mass of coarse shingle or conglomerate. The hollow 

 and the detrital material filling it had a close parallel in a remark- 

 able section, which the speaker had described from the neighbouring- 

 island of Canna, where a gully with vertical walls had, during the 

 volcanic period, been eroded out of the bedded basalts and had been 

 buried under succeeding outflows. The Author asserted that the 

 mass at the western end of the Scuir was a volcanic agglomerate, but 

 he adduced no evidence in support of that assertion. Any ordinary 

 volcanic neck would have descended vertically through the rocks 

 below, but the basalts underlie the Scuir conglomerate, and preserve 

 their unbroken regularity along the face of the cliff. In like 

 manner, in Canna the gully referred to shows its shingle lying upon 

 a flat unbroken pavement of basalt. It is manifestly the buried 

 gorge of a watercourse, and the speaker held that the same expla- 

 nation applied to the similar structure at the Scuir of Eigg. 



The supposition that the pitchstone is a sill raises some insuper- 

 able difficulties. The rock consists not of one continuous mass, but 

 of a series of layers or beds, superposed one upon the other. According 

 to the Author, the original glassy pitchstone has been invaded by a 

 number of sheets of pale felsite. He offered, however, no explanation 

 of how these sheets could be supposed to have split up the original 

 solid sill by insinuating- themselves, not between the columns, but 

 across them, in a direction along which it might have been supposed 

 that they would meet the greatest obstacle to their progress. These 

 felsites resemble the brecciated bottom of the main pitchstone, like 

 which they were regarded by the speaker as parts of a succession of 

 flows which filled up a long and wide hollow worn by erosion out 

 of the plateau-basalts. 



The Author of the paper seemed disposed to minimize the amount 

 of denudation during the volcanic period. The speaker, on the other 

 hand, believed that it must have been large, and he referred in 

 proof to the evidence of erosion and widespread accumulation of. 

 waterworn shingle to be seen in Canna and elsewhere. In con- 

 clusion, he submitted that, while he was ready to abandon his 

 published conclusions when they were shown to be erroneous, much 



