ANNIVERSARr ADDRESS OF THE PRESIDENT. II5 



Dykes are associated with many of the vents, while the sills which 

 may mark the latest manifestations of volcanic energy, though not 

 developed on so large a scale as among the Cambrian and Silurian 

 volcanoes, can nevertheless he distinctly recognized. 



1. Bedded Lavas and Tuffs. — The successive sheets of lava in 

 a plateau usually form thin and widespread beds which are only 

 occasionally separated by intercalations of tuff or of red marl. 

 Tliey are generally marked off from each other by the slaggy upper 

 and under portions of the successive flows, and this structure gives 

 a marked bedded aspect to the escarpments, as in the Campsie and 

 Largs Hills, or still more conspicuously in Little Cumbrae and the 

 southern end of Bute. Considerable diversity of structure may be 

 noticed among these sheets. Some present a compact jointed centre 

 passing up and down into the slaggy material just referred to ; 

 others have assumed a vesicular character throughout, and now^ 

 appear as marked amygdaloids. 



In each plateau the lavas may be observed to increase in total thick- 

 ness in certain directions. Thus in the great tableland above Largs 

 they attain a depth of more than 1500 feet, rapidly thinning away 

 towards the south. In the Campsie Fells they are thickest towards 

 the west. In the Solway plateau they attain a maximum develop- 

 ment about Birrenswark, whence they diminish in bulk towards the 

 north-east and south-west. In some cases, within or close to the 

 area of greatest thickness the chief visible vents of discharge are 

 to bs found. 



The remarkable trachyte-lavas of the Garlton plateau constitute 

 a group by themselves, and display some marked differences in their 

 mode of occurrence from the more prevalent type of lavas. They 

 fail to show that terrace arrangement in successive, approximately 

 parallel sheets which, though not always exhibited by the porphy- 

 rites, is eminently characteristic of them. Yet from some points of 

 view, particularly from the westward, a succession of steep escarp- 

 ments and longer dip-slopes can here and there be detected among 

 the Garlton Hills, while there can be no doubt that the trachytes 

 lie as a great cake above a lower platform of more basic lavas. 



The volcanic history of this area, which is remarkably clear, 

 differs sufficiently from that of the other Plateaux to deserve sepa- 

 rate mention. At first, about the close of the deposition of the red 

 sandstone group of the Caiciferous Sandstones, from a number of 

 vents a large discharge of ashes and blocks of rock took place. 

 These fragmentary materials, mingled here and there with the 



