ANNIVERSARY ADDRESS OF THE PRESIDENT. lOI 



volcanic vent about 300 feet in diameter, which rises througli the 

 uppermost group of the Caithness flagstones. It is filled with a 

 coarse agglomerate consisting of a dull greenish diabase paste 

 crowded with blocks of diabase, sometimes three feet in diameter, 

 and others of red sandstone, flagstone, limestone, gneiss, and lumps 

 of black cleavable augite. ^ That this volcanic orifice was active 

 about the same time with those in the opposite island of Hoy may 

 be legitimately inferred. 



These northern volcanoes made their appearance in a district 

 where during the preceding Lower Old Red Sandstone period there 

 had been several widely separated groups of active volcanic vents. 

 So far as the fragmentary nature of the geological evidence permits 

 of an opinion, they seem to have broken out at the beginning or at 

 least at an early stage of the deposition of the Upper Old Eed 

 Sandstone, and to have become entirely extinct after the lavas of 

 Hoy were poured forth. No higher platform of volcanic materials 

 has been met with in that region. With these brief and limited 

 Orcadian explosions the long record of Old Eed Sandstone volcanic 

 activity comes to an end. 



YII. DEVONIAN. 



Throughout the wide region of Wales, where the Old Eed Sand- 

 stone is so massively developed, no trace of contemporaneous 

 volcanic activity has been detected in the many thousand feet of 

 red strata which intervene between the top of the Silurian and 

 the base of the Carboniferous system. The rapid and complete 

 change in the pre-Carboniferous type of sedimentary accumulations 

 from that of South Wales to that of Devonshire constitutes one of 

 the long-standing problems of British Geology. Among the many 

 contrasts which the Devonian rocks present to the Old Eed Sand- 

 stone of South Wales, not the least important is the abundant 

 evidence which the former include of vigorous, long-continued and 

 frequently renewed volcanic activity. Since the early and classic 

 researches of De la Beche,^ many geologists have studied these 

 igneous rocks ; but I would especially refer to the labours of our 

 lamented associate Mr. Champernowne, which, more than those of 



^ Trans. Eoy. Soc. Edin. vol. xxviii. (1878) p. 405. Since that paper appeared 

 my colleagues in the Survey have detected another smaller, but otherwise similar 

 neck on the same coast. 



^ Geol. Surv. ' Eeport on the Geology of Cornwall, Devon, and West 

 Somerset' (1839). p. 37 et seq^. 



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