TRANSACTIONS OF SECTION C. 755 



tion of Lehmann that this structure is essentially due to mechanical deformation 

 has heen widely accepted, and has undoubtedly been of great service in the inves- 

 tigations of the last ten years among pre-Cambrian rocks. That the foliation of 

 many gneisses has arisen from the efiects of enormous compression can no longer be 

 disputed. But among these rocks other structures occur which cannot be satisfactorily 

 80 explained. In the granulitic gneisses, where the folia are thin, and where over 

 considerable spaces a marked uniformity of lithological character prevails, crushing 

 and recrystallisation have no doubt played a chief part in the production of the 

 gneissic structure. But in the coarsely banded varieties, where thick layers of 

 different chemical and mineralogical composition alternate irregularly with each 

 other, mechanical deformation seems to be wholly inadequate to account for this 

 arrangement. The author stated that he had pointed out some years ago that a 

 close analogy might be traced between this banded character and certain structures 

 to be observed in the deeper portions of large intrusive bosses. lie had since then 

 had opportunities of repeating and extending his observations, which had led him 

 to the belief that the coarsely banded arrangement in the ancient gneisses was not 

 due to any subsequent crushing and recrystallisation, but was a structure developed 

 in the original, massive, or eruptive rock before its final consolidation. In the 

 deeper-seated parts of intrusive bosses he had noticed that the component minerals 

 had sometimes been segregated in parallel bands, each of which was marked by 

 the predominance of one of them, and that the minerals had there crystallised in 

 much larger forms than in the main body of the rock. Layers of felspar, pyroxene, 

 olivine, and iron ores had in this way been separated out, and could be traced in 

 alternate parallel bands for distances of many yards, sometimes even exhibiting 

 puckered, folded, and inverted structures. Such segregations were so like the 

 hornblendic, felspathic, quartzose, and pyroxenic bands of many gneisses that 

 the observer could hardly at first believe that they were not portions of some 

 ancient rock enclosed within the eruptive boss. He could soon convince himself, 

 however, that they were really integral parts of the general mass. Not only is 

 the banded structure of the gneisses perfectly reproduced in the bosses, but another 

 equally characteristic structure, that of the pegmatite veins, is likewise simulated. 

 Occasionally veins of this nature composed of the same minerals as the boss, but 

 aggregated in different proportions, may be seen, not only in the main amorphous 

 mass of the rock, but even traversing the segregated bands. So closely does this 

 association of structures resemble that of the old gneisses as to impress the convic- 

 tion on the observer that it probably represents the origin of some of the most 

 conspicuous features in these rocks. Illustrations of the structures described may 

 be found in eruptive bosses of Palaeozoic age, but the best examples which the 

 author has seen occur among the Tertiary gabbros of the Western Isles. 



I 



4. On the Fittings in Pebbles from the Trias. 

 By Professor W. J. Sollas, D.Sc, F.B.S. 



The singular indentations in the pebbles of the pebble beds of the Trias have 

 been variously attributed to solution and pressure, and in limestone pebbles Sorby 

 has conclusively shown how both have shared in their formation. No one, how- 

 ever, appears to have suggested the influence of slight movements as a powerful 

 adjunct to pressure ; and yet earth tremors are of such constant occurrence that 

 slight movements must exist. How great may be the influence of these is proved 

 by the incised bones of the great Irish deer, which have made sharp and deep cuts 

 into each other wherever they have happened to lie in contact, and this although 

 only under the pressure of a peat bog. Still better illustrations are afforded by 

 some pebbles, to which my attention has been directed by my colleague Mr. 

 MacHenry, These are from an ancient beach over which the tram line passes at 

 Tritonville, Sandymount: they are covered with impressions essentially similar to 

 those on the Trias pebbles, a result of the perpetual jarring produced by the pass- 

 ing trams. It is obvious that under the great pressure to which the Trias pebble 

 beds have been exposed the slightest trembling at points of contact would produce 

 similar or even more marked effects. 



