Vol. 50.] BANDED STRUCTTTEE OF SOME TERTIARY GABBBOS. 659 



distinguished from mere foliation, which has been ascribed to late 

 mechanical deformation, may be an original structure due to the 

 conditions in which the igneous magma was erupted and consoli- 

 dated. In view, however, of the undoubted evidence of seconda^ 

 dynamic action in many regions, and in the absence at present of 

 any well-established criteria by which we can in all cases discrimi- 

 nate between original and secondary structures, we are not yet in 

 a position to define the exact limits within which the hypothesis of 

 the intrusion of heterogeneous magmas is applicable to the explana- 

 tion of the structures of the Lewisian gneiss. 



EXPLANATION OF PLATES XXVI.-XXVIII. 



Plate XXVI. 

 Folded and Banded Grabbros at Druim an Eidhne, 10 feet broad. 



Plate XXVII. 



Granulitic and foliated gabbro, traversed by later veins of felspathic gabbro, 

 at Druim an Eidbne. Tbe irregular wbite patcbes are the relics of veins 

 of felspathic gabbro. 



Plate XXVIII. 



Fig. 1 (5373 in Surv. Collect.). From one of tbe light-coloured felspathic 

 bands. The minerals represented are plagioclase, uralitic hornblende, 

 and magnetite. 



Fig. 2 (5377). From one of the dark bands rich in augite and magnetite. 

 The minerals present are plagioclase, augite, and magnetite. Some of 

 the augite-grains occur as inclusions in the interstitial magnetite. 



Fig. 3 (5375). From a band of intermediate character. The minerals represented 

 are plagioclase, uralitic hornblende (scarce), and iron ore. The portion 

 of the slide selected for representation is exceptionally rich in magnetite, 

 which is seen to fill up the spaces between the felspars. 



Fig. 4 (5376). From one of the black ultrabasic ' schlieren.' The only two 

 minerals present in the portion represented in this figure are augite 

 and magnetite. 



Fig. 5 (5374). From one of the black ultrabasic ' schlieren.' Composed of 

 olivine, felspar, and magnetite. The magnetite occurs in veins tra- 

 versing the olivine, and also as grains independent of that mineral. 

 The plagioclase, which occupies the centre of the figure and fills up 

 the irregular space between the other minerals, belongs to one crys- 

 talline individual. 



Fig. 6 (5374). Another portion of the same slide. The minerals here repre- 

 sented are olivine, augite, and magnetite. 



Discussion. 



Dr. Johnston-La vis wished to know if there was any special 

 orientation of the crystals in the different bands, and also whether 

 the thinner bands were the more basic, as these facts would help to 

 elucidate their origin. He quite agreed with the Authors that the 

 differentiation of the magma was anterior to its taking up its 

 present position. He remarked that frequently effusive rocks 

 showed a banding of this nature due to differential shearing between 

 portions of the magma of different viscosity, and such might be the 



Q.G.J. IS. No. 200. 2z 



