Alfred Harker — The Sgiirr of Eigg — CoTnments. 307 



of tlie pitchstone surface here, M'ith its steep northern boundary, 

 I ascribe to the intrusion ploughing into the agglomerate and occupying 

 part of its funnel. It is for Mr. Eailey, who distrusts coincidences, 

 to explain why, on his alternative reading of the relations, the 

 thick dolerite sheet should select this critical spot for dying out 

 abruptly. 



My generalization concerning the intrusive nature of most of the 

 strong dolerite sheets in the region was not based especially upon 

 the mapping of Eigg, though I find it borne out in that island. 

 When I wrote the passage (juoted by Mr. Bailey, I had in mind 

 more particularly the fine cliff-sections on the eastern coast of Skye, 

 rendered classic by Macculloch. The close resemblance in behaviour 

 there between the strong dolerite sheets in the basalt series and 

 those in the Jurassic strata below seems to me most striking. It 

 must surely have been noticed by Sir Archibald Geikie when he 

 set down both as lavas. He has changed his opinion as regards 

 the lower sheets, but apparently retains it as regards the higher 

 ones. If any reader is interested in this question, so fundamental 

 in British Tertiary geology, let him turn to Sir Archibald's paper 

 of 1896 (Q.J.G.S., vol. lii, pp. 331-405) and examine the numerous 

 sections by which it is illustrated. My contention is broadly (though 

 I have no personal knowledge of the Faroe Isles) that the sheets 

 there marked by vertical lines are in general intrusive sills. Further, 

 I maintain that this is in some cases proclaimed by the drawings 

 themselves: in figs. 7, 12, 13, 17, for instance, the transgressive 

 behaviour is faithfully rendered. Similar sections may be seen in 

 Eigg; I remember pointing out to Dr. Peach, on the slopes of the 

 Sgurr, a dolerite sheet cutting obliquely across well-marked flow- 

 lines in a basalt. 



It would appear from Mr. Bailey's remarks that he regards chilled 

 edges as one of the criteria of an intrusive sill. For some of my 

 friends on the Geological Survey this matter of chilled edges seems 

 to have become, in these latter years, a kind of cheap and infallible 

 touchstone. Chilled edges, an intrusion ; no chilled edges, a lava- 

 flow : it is at least a rule easily remembered. Now, plainly, an 

 igneous rock will have chilled edges if its magma has been rapidly 

 cooled at its boundary. Rapid cooling of the surface is assured in 

 the case of an ordinary lava-flow (I say nothing of freaks like 

 Matavanu), while for a sill or dyke the question is one of conditions. 

 Nor is it difiicult to see what some of the conditions are. There is 

 in the first place the temperature of the country-rocks. The 

 principal sill epoch came immediately after a prolonged succession 

 of plutonic intrusions, when the rocks of the whole region, and 

 especially of the neighbourhood of the plutonic centres, had been 

 raised to a considerably high temperature. Cooling must have been 

 a very gradual process ; and it is a matter of observation that, in the 

 long succession of minor intrusions (basic dykes and sheets) which 

 followed, sharpness of boundary and all the indications of rapid 

 chilling become marked, and increasingly marked, as we pass from 

 earlier to later groups of intrusions. Tachylytic selvages, for instance, 

 are scarcely seen except on sheets and dykes which cut all other 



