BANDED STRUCTURE OF SOME TERTIARY GABBROS. [NOV. 1 894, 



case also in injected rocks : hence the reason of the questions that 

 he had put. 



Prof. Blake remarked that one of the Authors, after paying 

 a visit to Anglesey, had described the general aspect and structure 

 of certain gneiss-like rocks in almost identical terms with those now 

 used for the gabbros of Skye, which the Authors regarded as 

 Tertiary ; yet, on the ground of such aspect and structure, the 

 Anglesey rocks were considered to be of the age of the Hebridean 

 gneisses. He hoped that it would now be admitted that the age of 

 rocks could not by these particulars alone be determined. 



Dr. Hicks said that the facts stated by the Authors were very 

 interesting, as bearing on the possible cause of the banded structure 

 in some of the Lewisian gneisses. It was now generally admitted 

 that the more massive of the pre-Cambrian gneisses must have had 

 an igneous origin ; but the mode by which the banding had taken 

 place remained somewhat doubtful. 



The point raised by Prof. Blake was easily disposed of, as it was 

 seldom necessary to rely upon petrological characters only ; and in 

 the case of the Anglesey gneisses there was good stratigraphical 

 evidence to show that they were of pre-Cambrian age. 



Mr. Harker found the paper of much interest from a purely 

 petrographical point of view, as well as for its bearing on the origin 

 of ancient gneisses. Banded structures are well known in many 

 basic plutonic masses, but the examples described in this paper are 

 more striking than any hitherto recorded. 



Mr. J. Hort Plater also spoke. 



Sir Archibald Geikie stated in reply that, so far as he knew, no 

 relation was observable between the breadth of the bands in the 

 gabbro and their basicity, nor at the locality referred to in the 

 paper was there any marked orientation of the crystals parallel to 

 the planes of banding. Among some of the basic sills of the 

 Western Isles, however, such orientation was strongly developed, 

 and he particularly cited a troctolite sheet in the Island of Bum, in 

 which the laminar arrangement was so conspicuous that the rock 

 might at first be mistaken for a schist. The observations recorded 

 in the paper did not seem to him to have any bearing on the age of 

 the rocks in the centre of Anglesey, referred to by Mr. Blake, 

 regarding which his opinion remained unchanged. 



