82 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



tion of its ferruginous minerals ; and from the contrast it thus 

 presents to the rest of the rock we may perhaps legitimately infer 

 that the disintegration took place before the outflow of the next 

 succeeding lava. If this inference be well founded, and it is 

 confirmed by other evidence which will be subsequently adduced, it 

 points to the probable lapse of considerable intervals of time between 

 some of the outflows of lava. 



But perhaps the most singular structure displayed by these lavas 

 is to be seen in the manner in which they are traversed by and 

 enclose portions of sandstone. Since I originally observed this 

 feature on the Ayrshire coast, near Turnberry Point, many years 

 ago, I have repeatedly met with it in the various volcanic districts 

 of the Lower Old Ked Sandstone across the whole of the Midland 

 Valley of Scotland. The first and natural inference which a cursory 

 examination of it sugo-ests is that the molten rock has caught up 

 and carried along pieces of already consolidated sandstone. But a 

 little further observation will show that the lines of stratification 

 in the sandstone, even in what appear to be detached fragments, are 

 marked by a general parallelism, and lie in the same general plane 

 with the surface of the bed of lava in which the sandy material is 

 enclosed. In a vertical section the sandstone is seen to occur 

 sometimes in narrow dykes with even, parallel walls, but more 

 usually in irregular twisting and branching veins, or even in lumps 

 which, though probably once connected with some of these veins, 

 now appear as if entirely detached from them. Frequently, indeed, 

 the nodular slaggy porphyrite and the sandstone are so mixed up 

 that the observer may hesitate whether to describe the mass as a 

 sandstone enclosing balls and blocks of lava, or as a scoriaceous 

 lava permeated with hardened sand. A close connexion may be 

 traced between these sandstone-enclosures and the beds of sandstone 

 interstratified between the successive lavas. We can follow the 

 sandy material downwards from these intercalated beds into the 

 porphyrites below them. On exposed upper surfaces of the por- 

 phyrite sheets an intricate reticulation of sandstone-veins may be 

 noticed, in each of which the stratification of the material runs 

 across the veins, showing sometimes distinct current-bedding, but 

 maintaining a general parallelism with the bedding of the volcanic 

 sheets and their fragmentary accompaniments. If we could remove 

 the sandstone-veinings and aggregates we should find the upper 

 surfaces of these igneous masses to present a singularly fissured and 

 slaggy appearance, reminding one of the rugged, rent, and clinker- 



