Vol. 50.] BANDED STRUCTURE OF SOME TERTIARY GABBROS. 649 



between the materials of the successive bands. Instead of being 

 separated by any sharp line of demarcation, they are welded into 

 each other by the mutual penetration of their component minerals. 

 Thus the felspars of the pale bands project among the augites and 

 magnetites of the darker bands, while the latter in turn are 

 enclosed among the felspars. 



No trace can be found here of any crushing and re-crystallization, 

 such as are familiar among the schistose rocks. The various 

 minerals, so far as can be judged by the eye, remain in their 

 original condition as they crystallized, save with such alteration as 

 weathering may have effected in them. There are no ' crush- 

 lines ' or ' shear-planes,' nor any evidence of mechanical defor- 

 mation. Even the puckering and plication just referred to is not 

 attended with any sensible effect on the minerals. 



In seeking among intrusive sheets of igneous rock for analogies 

 with this banded arrangement, we naturally turn to the familiar 

 forms of what is known as flow-structure. Some of the gabbros of 

 the Inner Hebrides exhibit that structure in the most striking 

 manner. In the mountain Allival in the Island of Rum, for 

 example, there lies near the base of the gabbros a sill of pale 

 troctolite, from 20 to 30 feet thick, in which the felspars are drawn 

 out parallel to the upper and under surfaces of the bed, in such a 

 manner as to impart to the rock a lamination which might 

 cursorily be mistaken for that of some variety of schist. 1 In the 

 banded gabbros of Druim an Eidhne, however, we have not detected 

 any sensible lineation of the individual crystals in the direction of 

 banding. Whatever may have been the conditions under which 

 this banding was produced, they would therefore seem to have 

 differed in some measure from those in which ordinary flow- 

 structure was produced. 



The occurrence of banded gabbros has already been described 

 from other regions, and further reference will be made to the 

 observations of previous authors in the second part of this paper. 

 But in these examples derived from rocks of great antiquity there 

 is often the difficulty of determining how far the banding has arisen 

 from some subsequent mechanical movement and re-crystallization. 

 The importance of the instances which we now cite from Skye 

 appears to us to lie in the fact that they undoubtedly reveal original 

 structures. 



No great terrestrial disturbances have affected the region since 

 older Tertiary times, when the gabbros of the Western Isles were 

 extruded. We are thus enabled to study the direct results of the 

 protrusion and consolidation of igneous masses unencumbered with 

 any of the doubt and difficulty which often attend the investigation 

 of pre-Cambrian and even of Palaeozoic eruptions. 



3. The Coarse Massive Gabbros. — These rocks, which form the 

 familiar type of gabbro, have their minerals indefinitely aggregated 

 in a granitic texture. They are sometimes exceedingly coarse, with 



1 Trans. Boy. Soc. Edin. vol. xxxv. pt. i. (1888) p. 123. 



