84 PKOCEEDINGS OF THE GEOLOGICAL SOCIETr. 



thin away southwards. Each sheet consists of different layers of 

 material — some of it in angular chips, some in fine compacted 

 dust — pointing to many distinct explosions in the accumulation of a 

 single sheet. These pale rocks are strongly marked off from the 

 dark and much heavier diabases and porphyrites between which 

 they are intercalated. As we trace them southwards fragments of 

 porphyrite become increasingly abundant in them ; but near the 

 vent they are remarkably homogeneous and felsitic in character. 

 Indeed, I have sometimes thought it possible that certain more 

 flinty homogeneous parts in these thick bands might be actually 

 remnants of lava-streams. The rocks weather so deeply into their 

 mass that it is often difficult to procure tolerably fresh specimens ; 

 moreover, even when a fresh fracture can be obtained it is some- 

 times hardly possible to discriminate between the compacted vol- 

 canic dust and the same material still existing as a mass of felsite. 

 If not true superficial lavas, some of the felsite-like portions may 

 perhaps be intrusive sills or veins. 



That these acid tuffs were erupted from the vent of the Braid 

 Hills, which is still largely filled with similar materials, may be 

 regarded as tolerably certain. The much more basic lavas inter- 

 calated between them in like manner thicken towards the north 

 and thin away southwards. They are thickest at the north end, on 

 the brink of the vent ; and but for their very diff'erent composition, 

 there would be no hesitation in looking upon both lavas and tuff's 

 as having been ejected from the same orifice. I shall return to 

 this question in describing the structure and materials of the Eraid 

 Hills. 



The amount of volcanic material ejected from the more important 

 vents of ' Lake Caledonia ' was much greater than the height of the 

 present hills would lead us to suppose. The rocks have been tilted 

 into positions much more inclined than those which they originally 

 occupied, so that to measure their actual thickness we must take a 

 line approximately perpendicular to the dip. In this way we 

 ascertain that the accumulated mass of lavas and tuffs immediately 

 outside the vent at the north end of the Pentland Hills must be at 

 least 7000 feet thick, for the base of the series is concealed under 

 the unconformable overlap of the Lower Carboniferous Sandstones, 

 while the top is cut off by a fault which brings down the Carbo- 

 niferous formations against the eastern flank of the hills. Probably 

 not less voluminous is the pile of ejected material in the Ochil Hills, 

 where, though the base of the whole is concealed by the fault which 



