Vol. 52.] BASALT-PLATEATJX OF NORTH-WESTERN EUROPE. 335 



has no doubt frequently led to these lavas being mistaken for tuffs. 

 As I have recently found them to be much more plentiful than I 

 had supposed, a more detailed description of them seems to be 

 required. 



The banded character arises from marked distinctions in the 

 texture of different layers of a lava-sheet. In some cases these 

 distinctions arise from differences in the size of the crystals or in the 

 disposition of the component minerals of the rock ; in others, from 

 the varying number and size of the vesicles, which may be large or 

 abundantly crowded together in some layers, and small or only 

 sparsely developed in others. The structure thus points to original 

 conditions of the lava at the time of its emission, and may be regarded 

 as, to some extent, a kind of flow-structure on a large scale. 



Where the banding is due to differences of crystalline texture, 

 the constituent felspars, augites, and iron-ores may be seen even 

 with the naked eye as well-defined minerals along the prominent 

 surfaces of the harder ribs, while the broader intervening flutings 

 of finer material show the same minerals in minuter forms. The 

 alternating layers of coarser and finer crystallization lie, on the 

 whole, parallel with the upper and under surfaces of the sheets in 

 which they occur. But they likewise undulate like the streaky 

 lines in ordinary flow-structure. 



Banded structure of this type may be seen well developed in the 

 lower. parts of the basalt-plateaux throughout the Inner Hebrides 

 and the Faroe Islands. A specimen taken from the western end of 

 the island of Sanday, near Canna, which showed the structure by a 

 conspicuous parallel fluting on weathered surfaces, was sliced for 

 microscopical examination. My colleague, Mr. Alfred Harker, to 

 whom I am indebted for the notes on the microscopic characters of 

 rocks described in the present paper, has been kind enough to supply 

 me with the following observations regarding this slice : — 



4 In the slice [6660] 1 the banding becomes less conspicuous 

 under the microscope. The rock is of basaltic composition, and, 

 with reference to its micro -structure, might be styled a fine-grained 

 olivine-diabase or olivine -dolerite in some parts of the slice, an 

 olivine-basalt in others. It consists of abundant grains of olivine, 

 imperfect octahedra and shapeless granules of magnetite, little 

 simple or twinned prisms of labradorite, and a pale brown augite. 

 The last-named mineral is always the latest product of consolidation, 

 but it varies in habit, being sometimes in ophitic patches moulded 

 upon or enclosing the other minerals, sometimes in small granules 

 occupying the interstices between the felspars and other crystals. 

 The ophitic habit predominates in the slice, while the granulitic 

 comes in especially along certain bands. If the former be taken as 

 indicative of tranquil conditions, the latter of a certain amouut of 

 movement in the rock during the latest stages of its consolidation, 

 the banding, though not strictly a flow-structure, may be ascribed 



1 The figures within square brackets throughout this paper refer to the 

 numbers of the microscopic slides in the Geological Survey (Scotland) collection, 

 where I have deposited all those prepared from my specimens. 



