250 Proceedings of the Royal Physical Society. 



weathered face of the rock where the contrast between the 

 core and the external sheath is to be seen. Their dull finely 

 crystalline aspect is found to arise mainly from a change in 

 the vitreous magma, whereby the enclosed crystalline con- 

 stituents are allowed to stand out more prominently. In 

 general arrangement and relative proportion of ingTcdients 

 the rock does not differ in any marked degree from the 

 glassy portion ; but, unfortunately, it is considerably decayed. 

 The felspars are somewhat dull and granular. The augite is 

 plentifully surrounded with diffused " viridite," and the mag- 

 netite has no doubt furnished most of the limonite which 

 gives a prevalent yellowish hue to the rock. 



The most singular feature, however, in the internal struc- 

 ture of these ribs is the disappearance of the abundant and 

 characteristic glassy base. At first, indeed, when a thin 

 slice is examined under the microscope, the numerous pale 

 brown interspaces which it presents are naturally taken 

 by the observer for the usual glassy substance, though in a 

 somewhat modified form. They even retain what evidently 

 represents the fluxion structure as in the portion drawn in 

 fig. 3 of PI. VI. But they are no longer passive in polar- 

 ised light. They have all acquired a crystalline structure, 

 indicated by abundant points of light in the dark field of the 

 crossed prisms. They show under a magnifying power of 

 400 diameters, traces of globulites and dart-like microliths, 

 with amorphous granules and shred-like filaments. 



How far this change in the base is an original one or 

 due to subsequent weathering, the specimens in my posses- 

 sion do not enable me to decide. In ordinary columnar 

 basalt the sutures between the prisms offer convenient 

 channels for the passage of percolating water, and it is evident 

 that where this percolation takes place, the outer parts of the 

 prisms may be more altered than their inner parts. In such 

 cases, however, the greater degree of alteration is usually 

 evident in a more oxidised crumbling aspect. But in this 

 Eskdale dyke it is these inter-columnar parts which stand 

 up most prominently, while the vitreous cores are hollowed 

 out. 



The Felspars. — The great majority of the clear prisms so 



