BASALT-GLASS OP THE WESTERN ISLES OF SCOTLAND. 459 



greater or less extent, porphyritic in structure. Basalt-lavas con- 

 sist, as is well known, of a number of perfectly formed crystals of 

 augite, olivine, felspar, and magnetite, entangled in a mass of un- 

 crystallized material. These included crystals have probably been 

 formed at great depths below the surface. After the extrusion of 

 the lava, and as it slowly cools, crystals of the same minerals 

 become more or less completely developed in the entangling magma. 

 Careful study of such rocks, as Levy and Fouque have so well 

 shown, enables us, in most cases, to clearly distinguish those crystals 

 which have been formed at considerable depths from such as have 

 separated from the magma, near or at the surface, during the 

 cooling of the lava. 



The crystals formed at great depths in the earth are usually 

 larger and more perfectly developed than those separating from the 

 glassy magma during its cooling, and they often contain liquid- 

 cavities and enclose other crystals. Moreover they are frequently 

 broken and rounded at their edges, and have suffered great corrosion 

 by being partially melted-up and having their substance absorbed into 

 the glassy magma. This glassy magma often sends prolongations 

 of its substance into such crystals, which are, in deed, sometimes 

 completely honey-combed by these extensions of the glassy mass *. 



The facts described point to the conclusion that crystals formed 

 under great pressure in the midst of a fused magma may, on the 

 relief of that pressure, be attacked and dissolved by the magma in 

 which they were originally developed. 



Many of the basalt-glasses of the Western Isles of Scotland 

 furnish examples of porphyritically imbedded crystals, these being, 

 in all cases, of the same kind as are found in the basalts with which 

 the glasses are connected. Olivine and -magnetite are the most 

 abundant of these porphyritically imbedded crystals ; but augite and 

 sometimes felspar also occur (PI. XIII. fig. 2). 



The basalt-glass of the Beal, near Portree in Skye, exhibits very 

 interesting examples of olivine and felspar crystals, the latter much 

 corroded and eaten into by the glassy magma (PL XIII. fig. 3). The 

 rock of Lamlash contains large and well- developed crystals of both 

 felspar and augite, much broken and rounded (PL XIII. fig. 6). 

 The other varieties in the Scotch area appear to contain only por- 

 phyritic crystals of olivine, much decomposed ; and in some cases 

 these are apparently rare. 



All the basalts which have vitreous selvages appear to be of the 

 class which contains a large amount of glassy residuum between the 

 crystals ; and some of them may in fact be classed as magma basalts. 

 In certain cases, indeed, they appear to have little or no in- 

 dividualized felspar, and may be grouped with the Limburgites of 

 Rosenbusch. 



The most perfectly vitreous type of these rocks, which is ex- 

 emplified by the extreme edges of the selvages in the Beal dykes, 

 exhibits only the merest embryonic crystallites scattered through 



* Similar instances of corrosion are familiar among the porphyritic crystals 

 occurring in acid rocks such as rhyolites, pitchstones, and obsidians. 



