308 Alfred Harker — The Sgurr of Eigg — Comments. 



rocks ; and even there any noteworthy development of tachylyte is 

 found only in places remote from the plutonic centimes. In the 

 dolerite sills themselves there is an unmistakable relation between 

 tlie degree of chilling and the distance from the special centres. 

 Indeed, the thing is a feature of the landscape ; for any tourist in 

 the Inner Hebrides must observe how the columnar jointing becomes 

 more strongly pronounced as we recede from the gabbro and 

 granite hills. 



Another important factor is the varying thermal conductivity of 

 the country-rocks. A basalt dyke, which exhibits strongly chilled 

 edges where it intersects a sound crystalline dolerite, may show only 

 a very slight modification against a decaying amygdaloidal basalt, 

 and absolutely no sign of chilling where it cuts one of the bands 

 of clay or bole. Professor Kendall long ago drew attention to these 

 differences dependent on specific thermal conductivity, and every 

 student of Hebridean geology must have had many opportunities 

 of observing such phenomena. Again, it is true, as Mr. Bailey 

 remarks, that the disputed dolerite sheets are often intimately 

 associated with the bands of red earth or bole ; and this is not 

 surprising, for a sill -intrusion naturally follows the plane of least 

 I'esistance. But when a sill rests on such a blanket, and is covered 

 by another blanket, viz. the scoriaceous base of the overlying flow, 

 it is too much to demand that it shall show evidence of rapid chilling. 

 The appearance of blending of the dolerite with the bole may be 

 seen equally at the edge of a dyke. I am not attempting to set 

 forth all the elements of the problem, but rather to point out that 

 it is not so simple as is sometimes assumed. In such a case, I submit, 

 we are more likely to reach the truth by trying to understand the 

 conditions than \>j applying a universal formula of the ' rule of 

 thumb ' order. 



Finally, Mr. Bailey considers the inclination of the lower surface 

 of the pitchstone sheet at certain localities. Since we unfortunately 

 possess no large-scale contoured map of Eigg, this question can 

 scarcely be discussed with profit except on the spot. But Mr. Bailey 

 passes over without notice a point which seems to me the most 

 decisive of all. The sides of every driftless valley in the basaltic 

 region are conspicuously terraced, the strong sheets of dolerite standing 

 out beyond the crumbling amygdaloidal basalts. Sir Archibald 

 Greikie's hypothetical valley has nothing of this character. The base 

 of the pitchstone is an irregularly curved surface, for acid intrusions 

 seldom attain to the perfectly conformable habit seen in many basic 

 sills; but the surface is everywhere a clean-cut one, passing across 

 harder and softer sheets indifferently. Let any geologist look at 

 the eastward termination of the Sgurr, or at any good photograph 

 of it, or even at the small outline sketch reproduced in Mr. Bailey's 

 paper : let him compare the pitchstone base with the terraced slope 

 of the hillside below, and pronounce whether the former represents 

 an erosion-surface or, as I maintain, an intrusive junction. 



