BASALT-GLASS OF THE WESTERN ISLES OF SCOTLAND. 



EXPLANATION OF THE PLATES. 



463 



Plate XIII. 



Fig. 1. Portion of selvage to the basalt-dyke of the Beal, near Portree, 



Skye, natural size, showing the finely columnar structure exhibited 



by the basalt-glass of this locality. The most glassy portion of 



the rock, forming the outside of the dyke, lies to the left of the 



figure. 



Fig. 2. Slice of the basalt-glass of Lamlash, Arran, magnified two diameters, 



and exhibiting the marked porphyritic character of the rock. The 



glass is perfectly black and opaque, as is always the case except in 



sections of extreme thinness ; and the enclosed crystals are those 



of felspar, augite, and olivine, the latter much decomposed. 



[Figs. 3, 4, 5, 6 illustrate the corroded, rounded, and fractured 



condition of the crystals contained in the basalt-glasses and 



the magma basalts which they accompany in the Western Isles 



of Scotland.] 



Fig. 3 is a felspar crystal from the basalt-glass of the Beal in Skye. X 15 



diameters. 

 Fig. 4. An augite crystal in the basalt of the dyke to which the last-men- 

 tioned rock forms a selvage. X 15 diameters. 

 Fig. 5. A greatly corroded crystal of felspar from the same rock as fig. 4. 



X 15 diameters. 

 Fig. 6. A group of felspar crystals from the basalt-glass at Lamlash, show- 

 ing rounding and fracturing. X 10 diameters. 



[Figs. 7 and 8 illustrate the characters of the magma-basalt 

 dykes which pass into basalt-glass in the Western Isles of 

 Scotland.] 

 Fig. 7 is taken from the centre of the dyke at the Beal, near Portree, in 

 Skye, and is an ordinary basalt with a large proportion of glassy, 

 base. 

 Fig. 8, taken from near the outside of the same dyke, contains much more 

 glassy material, the substance of the felspar being almost wholly 

 uncrystallized. The passage from the rock shown in fig. 7 to 

 that represented in fig. 8, and again into the glass illustrated in 

 Plate XIV. fig. 1, is of the most insensible character. 



Plate XIV. 

 [On this Plate are placed for comparison drawings made from exceedingly 

 thin sections of five of the varieties of basalt-glass of the Western Isles of Scot- 

 land, viewed with a magnifying-power of about 500 diameters. Beside them is 

 placed, for comparison, an example of the clear brown glass of Hawaii, 

 viewed with the same objective.] 



Fig. 1 is the basalt-glass of the Beal, in Skye. In it the magnetite dust is 

 simply collected into cloudy patches (cumulites), leaving clearer 

 spaces of the dark brown glass between them. 



Fig. 2 is the basalt-glass of Lamlash, Arran. In this rock the minute 

 crystallites are collected into linear series forming beaded rods 

 similar to the structures which have been called " margarites." 



Fig. 3, the basalt-glass of Screpidale, in Raasay, exhibits a more perfect 

 separation of the crystallites of magnetite, the skeleton crystals 

 thus formed resembling, except in their much smaller size, the 

 forms found in many iron slags. Around each skeleton crystal 

 an area of colourless glass is produced by the abstraction of the 

 iron oxides. 



Fig. 4. In this basalt-glass, from Some in the Isle of Mull, the separation 

 of the magnetite in the form of skeleton crystals is more complete, 

 and the forms of some of the transparent crystals are beginning 

 io appear. 



