ANNIVERSAKT ADDRESS OF THE PRESIDENT. 1 1 7 



and give place to tuffs. During the discharge of the fragmentary 

 materials over that ground no lava seems to have flowed out for a 

 long period. Ordinary sediment, however, mingled with the volcanic 

 detritus, and there were even pauses in the eruptions when layers 

 of ironstone were deposited, together with thin impure limestone 

 that enclosed shells of Productus gig aniens .^ 



Where the highest members of the volcanic series can be seen 

 passing conformably under the overlying Carboniferous strata they 

 are generally found to be mainly composed of fine tuffs, showing 

 that the last feeble efforts of the plateau-volcanoes consisted in the 

 discharge of showers of ashes. These materials became mingled with 

 a gradually increasing proportion of ordinary mechanical sediment 

 which finally overspread and buried the volcanic tracts of ground, 

 as these slowly sank in the general subsidence of the region. There 

 is thus an insensible passage from the volcanic detritus into the 

 fossiliferous shales and limestones which are succeeded by the Main 

 or Hurlet Limestone. Examples of this gradation may be seen in 

 many natural sections along the flanks of the Ayrshire plateau from 

 above Kilbirnie to Strathavon. 



It is still possible to fix in some quarters the limits beyond which 

 neither the lavas nor the tuffs extended, and thus partially to map 

 out the original areas of the Plateaux. We can see, for example, 

 that in certain directions the Carboniferous formations can be 

 followed continuously downward below the Main Limestone without 

 the intervention of any volcanic material, or with only a slight 

 intermixture of fine volcanic lapilli, such as might have been carried 

 by a strong wind from some neighbouring active vents. By this 

 kind of evidence and by the proved thinning-out of the materials of 

 the plateau we can demonstrate that in the north of Ayrshire the 

 southern limits of the great volcanic bank did not pass beyond a line 

 drawn from near Ardrossan to Galston. We can show, too, that 

 the lavas of the Campsie Fells ended off about a mile beyond Stirling 

 before they reached the line of the Ochil heights, and that the 

 coulees which flowed from the Sol way vents did not quite join with 

 those from the Berwickshire volcanoes. 



Moreover, evidence enough remains to enable us to form a toler- 

 ably clear conception of the original average slopes of the surface 

 of some of the plateaux. Thus in the great escarpment above Largs 

 and the high ground eastward to Kilbirnie the volcanic series must 



^ Explanation of Sheet 22, Geol. Sury. Scotland, p. 12. 

 VOL. XLVIII. t 



