Vol. 62.] STRUCTURE OF THE SGURR OF EIGG. 41 



the kind of evidence that is relied on, and as having also a direct 

 bearing- on the main subject of this communication, a summary 

 discussion will not be out of place. Part of the evidence in the case 

 of the Isle of Skye has already been set forth elsewhere, 1 and the 

 conclusion there arrived at has been even more firmly established 

 in the survey of the ' Small Isles ' to the south. 



The basalt-plateaux are made up of a monotonous succession of 

 sheets of basic rocks, individually from 2 or 3 to 100 feet thick, 

 and reaching an aggregate thickness of sometimes 2000 or even 

 3000 feet. The rocks composing these sheets are, broadly, of two 

 kinds, which may be distinguished as basalts and dolerites. 



The basalts are usually amygdaloidal, and more or less affected 

 by secondary changes. They are comparatively soft and crumbling 

 under exposure to atmospheric weathering, and are very largely 

 concealed under superficial accumulations of Drift and peat. They 

 are of fine texture, and have typically the microstructure which has 

 been styled granulitic. 



The dolerites are much fresher rocks, and much more durable, 

 forming all the strong features in the plateau-type of landscape. 

 Their escarpments show marked vertical jointing, sometimes as- 

 suming a very regular columnar habit. The rocks, excepting only 

 the thinnest sheets, are of decidedly coarser texture than the 

 basalts, and their microstructure is typically ophitic. These 

 dolerites are, in aggregate thickness, at least equal to the basalts 

 with which they alternate ; and their salient outcrop gives them an 

 appearance of very decided preponderance. 



Such is the constitution of all the north-western half of Skye, of 

 Eigg in general (excluding the upper part of the Sgurr), of Canna, 

 Sanday, and Muck, and of the greater part of Mull. . My observa- 

 tions go to prove that, while the basalts represent super- 

 ficial outpourings of lava at an early stage of igneous 

 activity, the intercalated sheets of dolerite are intru- 

 sive sills belonging to a later date. 



The dolerite-sheets run with remarkable regularity for very long 

 distances, but occasionally they may be observed to shift their 

 horizon among the basalts, to come together, or to separate. When 

 they encounter a mass of coarse volcanic agglomerate, they often 

 behave much more irregularly, being sharply diverted or abruptly 

 terminated. They sometimes enclose abundant fragments of the 

 amygdaloidal basalts, and in a few instances it has been verified 

 that a sheet of dolerite cuts through a dyke of earlier date. These 

 are more or less conclusive proofs of intrusion for the individual 

 sheets which afford such direct evidence ; but the cases in which 

 some of these tests can be applied are, naturally, not very frequent. 

 Much more convincing evidence is furnished by a comparative 

 survey of the areas in which the rocks are developed. 



In the first place, it is to be remarked that sheets of v dolerite in 

 all respects identical with those in question occur, not only among 



1 ' The Tertiary Igneous Rocks of Skye ' Mem. Geol. Suit. 1904, chap. xiv. 



