ANNIVERSARY ADDRESS OF THE PRESIDENT. I 2 1 



intraded rock is generally a dull yellowish or grey felsite. Among 

 the Eoxburghshire vents, on the other hand, where the injected 

 material is commonly olivine-basalt, it occasionally happens, as in 

 Rubers Law, that the uprise of the lava has almost entirely cleared 

 out or concealed the agglomerate, and in some of the bosses where 

 no agglomerate is now to be seen the basalt may have taken its 

 place. 



The largest and most interesting vents, however, are those which 

 occur within the limits of the Plateaux, where they are still sur- 

 rounded with lavas and tuffs that probably- came out of them. Of 

 these by far the most extensive and remarkable lies among the high 

 moorlands of Renfrewshire between Largs and Lochwinnoch, where 

 the ground rises to more than 1700 feet above the sea. This area 

 is unfortunately much obscured with drift and peat, so that the 

 limits of its rocks cannot be so satisfactorily traced as might be desired. 

 I think it probable that several successive vents have here been 

 opened close to each other, but their erupted ashes probably cannot 

 be distinguished. The tract measures about four miles in length by 

 rather more than two in breadth. Over this space the rocks exposed 

 at the surface are fine tuffs, breccias, and coarse agglomerates, 

 largely made up of felsitic material and pierced by innumerable 

 protrusions of various felsitic rocks in bosses and veins as well as 

 also by dykes of a more basic kind, such as dolerites and andesites. 

 Some of the tuffs present a curiously indurated condition ; and they 

 are frequently much decayed at the surface.^ 



Another interesting vent lies in the heart of the Campsie Fells, 

 where, instead of forming a prominence, it is marked by a great 

 hollow measuring about a mile in length and half a mile in 

 breadth. It is occupied mainly by a coarse tumultuous agglomerate, 

 like that of other necks in the same district, but with a matrix 

 rather more indurated, and assuming in certain parts a crystalline 

 texture, so as to be at first sight hardly distinguishable from some 

 of the surrounding porphy rites. Even in this altered condition, 

 however, its included fragments may be recognized, particularly 

 blocks of sandstone which have been hardened into quartzite. 

 Numerous small veins of pink and yellow felsite traverse the 

 agglomerate, and are found also cutting the bedded porphyrites 

 that encircle it. The induration of the materials filling this vent 



^ This tract of ground was mapped for the Geological Survey by Mr. R. L. 

 Jack, now in charge of the Geological Survey of Queensland. See Sheet 31, 

 Geol. Surv. Scotland. 



