38 DB.* STAMP A>D MR. WOOLDRIDGE ON THE [vol. lxxix,- 



peripheral area to the normal vesicle varies : in some cases it forms 

 a mere selvage, in others is observed a large circular patch of dark 

 material in which are set a number of rounded areas of calcite 

 and chlorite. It appears most likely that this effect is due to the 

 corrosive action of the liquid filling the vesicle on its walls. This 

 liquid may well have been alkaline, and the local leaching action,, 

 coupled with the replacement of lime by soda, doubtless con- 

 tributed to the calcite-infilling of the vesicles. 1 A conspicuous 

 feature is the tangential arrangement of felspar-laths on the outer 

 edge of the areas of dark spilite (PI. I, fig. 3). 



Before leaving the subject, we may remark that, if a corrosive 

 liquid be accepted as an adequate explanation of the dark spilite- 

 rims, then we may suspect that much of the murky alteration - 

 product of the ground-mass owes its origin to a similar action more 

 widely diffused. This gains support from the fact that the degree 

 of alteration bears no obvious relation to the degree of surface- 

 weathering of the rock. 



Cases in which the still fluid lava has ruptured a vesicle-wall,, 

 and partly infilled it, have been observed 3 (PI. I, fig. 4). 



We may note, in concluding the account of the normal spilites,. 

 that a slide showing the junction of a lava with associated chert 

 revealed a somewhat unexpected feature. Wisps and strings of 

 spilite are seen to be included in the chert, and scattered felspar T 

 laths occur throughout the latter. It seems clear from this that 

 the chert does not represent material deposited in the interspaces 

 of the pillows subsequent to cooling, but rather that it is sediment 

 which was involved in the rolling action of the pillows during 

 eruption. 



The lavas which form the centre of Car Cwin do not exhibit the- 

 ' pillow ' habit ; but they do not differ in microstructure from the 

 rocks described above. That they were once vesicular is apparent 

 on microscopical examination ; the fact is, however, to some extent 

 concealed by the extreme alteration which has taken place. In 

 most cases, these rocks are little more than a mass of secondary 

 minerals. One slide shows a fresher rock, and serves definitely to 

 link the massive spilites with the normal pillowy type. 



(4) The Tuff-Bands in the Hardened Mudstones. 



Tuff-bands of varying thickness occur throughout this division. 

 The majority are fine-grained black rocks weathering white (as so 

 clearly described by Murchison), and they are very similar to the 

 hornstone facies of the Upper Ashes ; for that reason, they will 

 not be separately described. 



Certain coarser bands occur, however, and these are of greater 

 interest. Two t3 r pes will be considered — 



i See J. F. N. Green, Proc. Geol. Assoc, vol. xxx (1'919) p. 177. 

 2 See A. Harker, ' The Tertiary Igneous Rocks of Skye ' Mem. Geol. Surv, 

 1904, pp. 331, 342, 399-401. 



