AKNIVERSAKr ADDRESS OF THE PRESIDENT. 85 



throws down the coalfield, some 6500 feet of lavas, tuffs, and con- 

 glomerates can be seen. There were thus, during the time of the 

 Lower Old Eed Sandstone, more than one volcano in Central Scot- 

 land which might be compared in bulk of ejected material to 

 Vesuvius . 



That the eruptions were mainly subaqueous is indicated, as I 

 have shown, by the intercalated bands of sandstone and conglo- 

 merate between the successive porphyrites, as these are traced away 

 from the centres of discharge, and likewise, even more impressively, 

 by the hardened sand which has been washed into former fissures 

 and cavities in the lavas. But that, in some cases, the volcanic cones 

 were built up above the surface of the lake may be legitimately 

 inferred from the remarkable volcanic conglomerates which occur, 

 more particularly in the great chain of the Ochil and Sidlaw Hills. 

 These thick accumulations of well-rounded and water-worn blocks 

 are interspersed between sheets of porphyrite, and are mainly 

 made up of porphyrite fragments. Impressive sections of them may 

 be seen along the Kincardineshire coast. The conglomerates are 

 sometimes so remarkably coarse, many of their blocks exceeding 

 2 feet in diameter, and so rudely bedded, that it is only by noting 

 the position of oblong boulders that one can make out the general 

 direction of the stratification. In their smooth rounded forms these 

 blocks resemble the materials of storm-beaches on an exposed coast. 

 The trituration of the porphyrite fragments has given rise to a 

 certain amount of green paste, which firmly wraps round the stones 

 and retains casts of them after they have dropped out. It is 

 further deserving of remark that while in some districts, as in the 

 central Ochils, the materials were entirely derived from the destruc- 

 tion of volcanic rocks, in others a large proportion of non-volcanic 

 materials is mingled with the debris of the porphyrites. South of 

 Stonehaven, for exami)le, large boulders of quartzite form a con- 

 spicuous feature in the conglomerates, of which in places they 

 make up quite half of the total constituents. There can be little 

 doubt, I think, that the materials of these coarse detrital accumu- 

 lations were gathered together as shingle-beaches, and were derived 

 in part from volcanic cones which had risen above the level of the 

 lake. They seem to suggest considerable degradation of these cones 

 by breaker-action, whereby blocks of rock a yard or more in dia- 

 meter could be rounded and smoothed. 



Another inference deducible from such couglomerates, and to 

 which I have already alluded, is that considerable intervals of time 



VOL. XLVT[T. g 



