658 SIR A. GEIKIE AND ME. J. J. H. TEALL ON THE [Nov. 1 894, 



extremely diversified. In some places no definite order of arrange- 

 ment can be made out ; in others we find veins of more acid material 

 penetrating masses of more basic material ; and in others a kind of 

 breccia has been formed, in which lumps and fragments of basic rocks 

 lie in a matrix of acid rocks. Then, again, every possible kind of 

 parallel structure may be observed. In some cases this is only 

 faintly indicated in the arrangement of the minerals, or in the 

 forms and mutual relations of the more or less differentiated masses, 

 and from this condition every intermediate phase may be followed 

 to the most perfect parallel banding. Many causes have doubtless 

 operated in the production of the results which we now see, and 

 much work will have to be done before all these causes are clearly 

 recognized, and the effects of each accurately defined. One great 

 difficulty, with which the geologist has to contend in his attempt to 

 unravel the complicated story of the Lewisian gneiss, is that of 

 separating the effects due to causes operating before or during the 

 consolidation of igneous magmas from those due to dynamic action 

 operating upon the rocks after consolidation. 



In the middle area — that is, in the district between Scourie and 

 Loch Inver, where innumerable basic and a few ultrabasic dykes 

 clearly cut the fundamental gneiss — the existence of narrow belts of 

 country, ' shear-zones,' as they have been termed, along which the 

 rocks have been affected by secondary dynamic action, can be clearly 

 demonstrated. Along these zones the granitic gneiss often becomes 

 granulitic, hornblende takes the place of augite, and quartz-veins 

 often make their appearance. The dolerite-dykes, where they abut 

 against the shear-zone, which is often also a line of fault, tail off 

 into hornblende-schist, and the ultrabasic dykes (in some cases at 

 least) into talc-gedrite-siderite-schist. Between these zones we find 

 areas which have certainly not suffered deformation since the dykes 

 were formed, and it is precisely in such areas that structures most 

 nearly allied to those of the banded gabbros are to be found. 



The ultrabasic portions of Lewisian gneiss about Scourie and 

 Drumbeg may be especially referred to in this connexion, although 

 probably much of the banding in other localities and affecting other 

 kinds of rocks is of the same nature and origin. The separation of 

 the component minerals of these varieties of gneiss into definite 

 parallel bands presents so remarkable a resemblance to the structure 

 which we have described from the Tertiary basic rocks of Skye, that 

 it is difficult to believe that they cannot have arisen from the 

 same conditions. 



Geologists now generally agree in regarding the older gneisses as 

 mainly rocks of igneous origin. And this view, as it seems to us, 

 is strengthened by the detection of so close an analogy between the 

 banding of these rocks and that of basic eruptive bosses. There 

 seems to be good reason to believe that this structure in the undis- 

 turbed igneous rocks is not a mere local accident, but that it occurs, 

 as at least an occasional phenomenon, among basic eruptions of 

 many different ages from Archaean to Tertiary time. 



We suspect that much of the banding among the old gneisses, as 



