ANNIVERSARr ADDRESS OF THE PRESIDENT. 69 



miles. In this long narrow basin the remarkable volcanic history 

 was enacted of which I now proceed to give some account. 



The Lower Old Red Sandstone of Central Scotland may be con- 

 veniently divided into three great groups, each of which marks a 

 distinct epoch in the history of the basin wherein they were succes- 

 sively accumulated. The lowest of these groups indicates a time of 

 quiet sedimentation during which the basin was defined by plication 

 of the terrestrial crust, and when by the same subterranean move- 

 ments some parts of the floor of the lake were pushed upward above 

 water and were then denuded and buried. The middle group consists 

 mainly of volcanic rocks. It points to the existence of lines of 

 active volcanic cones situated along the centre of the lake. The 

 uppermost group records the extinction of volcanic action and the 

 gradual obliteration of the lake, partly by the pouring of sediment 

 into it, and, partly no doubt, by the continued terrestrial movements 

 which had originally produced the basin. 



Confining our attention for the present to the records of the middle 

 group, we find evidence of at least eight distinct clusters of volcanic 

 vents ranged along the length of the basin for a distance of some 

 150 miles. The independence of these volcanic districts may be 

 inferred from the following facts : — 1st. The actual vents of dis- 

 charge may in some cases be recognized. 2nd. Even when these 

 vents have been buried, we may often observe, as we approach 

 their probable sites, a marked increase in the thickness of the vol- 

 canic accumulations as well as a great development of agglomerates 

 and tuffs. 3rd. Traced in opposite directions the volcanic materials 

 are found to thin away or even to disappear. Those from one centre 

 of discharge may be observed now and then to overlap those 

 from another, but the two series remain distinct. The follow- 

 ing summary may suffice to indicate the position of the eight volcanic 

 centres and the distribution of the ejected material around them : — 



Isb. Beginning at the north-eastern end of the area, we first en- 

 counter a series of volcanic rocks which attain their maximum 

 thickness in Porfarshire around the town of Montrose, whence they 

 thin away towards the north-east and south-west. The main vents 

 probably lay somewhere to the east of the present coast, under the 

 floor of the North Sea. Admirable sections of the lavas of this 

 district may be examined along the shore to the north and south of 

 Lunau Bay, likewise along the cliff's a few miles south of Stonehaven. 



2nd. The volcanoes which poured out the masses of material that 

 now form the chain of the Ochil and Sidlaw Hills appear to have 



VOL. XLVIII. / 



