210 MR. A. HARKER ON THE OVERTHRUSX [May 1903, 



circumstances which in many other localities have been obscured 

 by a more noteworthy amount of flowing-movement. 



It may be remarked here, parenthetically, that the inclusion and 

 partial or complete destruction of basic rock-debris by an acid 

 magma, which will be seen to be so important a factor in the 

 production of the gneisses of Eum, is entirely in accord with our 

 reference of these rocks to the same great suite as the larger bodies 

 which form the principal hills of the island. Nothing is more 

 characteristic of the Tertiary intrusions of Britain as a whole than 

 the frequent intimate association of widely diverse rock-types, often 

 giving rise by admixture to rocks of very peculiar kinds. Sometimes 

 angular fragments of a rock have become involved in a newer 

 magma and entered into reactions with it ; sometimes the earlier 

 rock has been invaded before it was cooled, or even before it was 

 completely solid, by the later and different magma ; sometimes, 

 again, the imperfect admixture has been effected in some intra- 

 telluric reservoir prior to intrusion. The intensity of the mutual 

 reactions has been controlled by the temperature and other physical 

 conditions implied in these different circumstances ; but it has also 

 depended upon the degree of difference between the two rocks 

 involved, the maximum effects being found where a basic rock has 

 been attacked by an acid magma. There has thus resulted, in 

 different cases, a rock with evident xenoliths, more or less altered, 

 a hybrid product with scattered xenocrvsts, usually much disguised, 

 or, in the extreme case, a rock which shows in a given specimen no 

 direct indication of any foreign element. Even this last, however, 

 will often betray its origin by something unusual in its mineralogical 

 constitution. 1 Good examples of these various phenomena may be 

 studied without going beyond the Isle of Eum. I need cite only 

 one instance, that of the eastern border of the principal granite- 

 tract, where it is conterminous with part of the large area of ultra- 

 basic rocks. There is a zone of evident admixture, which in places 

 attains a width of about 50 yards. Where this well-marked zone, 

 with the appearance of a breccia, is wanting or much reduced, it is 

 because mutual reactions of a more advanced kind have resulted in 

 complete dissolution of the peridotite-xenoliths in the acid magma. 

 The evidence of this is seen in little clots or patches rich in ferro- 

 magnesian silicates (often including hypersthene) scattered through 

 the granite or granophyre to some distance from the line of contact. 

 The final breaking-up of these has doubtless been facilitated by a 

 certain amount of differential movement in the magma. 



Effects of the same general kind as those just noticed, but with 

 great variety in detail, have been studied by the present writer in 

 many of the Tertiary intrusions of Skye ; and a comparison of the 

 phenomena with those of the Eum gneisses leaves no doubt that a 



1 On this point, see ' Igneous Rock-Series & Mixed Igneous Eocks ' Journ. 

 of Geol. [Chicago] vol. viii (1900) pp. 389-99. I have described numerous 

 occurrences of xenolithic and hybrid rocks, with some discussion of the general 

 questions involved, in a forthcoming memoir of the Geological Survey on ' The 

 Tertiary Igneous Rocks of Skye.' 



