ANNIVERSARY ADDRESS OF THE PRESIDENT. l6l 



seen the lavas in one or two localities on the ground, but have not 

 yet had an opportunity of properly examining the district. A 

 small collection of specimens from these lavas is exhibited in the 

 rock-collection of the Jermyn Street Museum. Externally, as seen 

 in the quarries and natural exposures, some of these rocks present 

 the closest resemblance to those of the Permian basins of Ayrshire 

 and Nithsdale. They show considerable differences of texture 

 even within the same mass, some portions being dull fine-grained 

 purplish-red rocks, with scattered pseudomorphs of haematite and a 

 few porphyritic felspars, other parts passing into an exceedingly 

 coarse amygdaloid. Here and there I observed near Exeter veins 

 ol;' fine hard sandstone in some of the dark lavas, like those which I 

 have referred to as occurring characteristically in porphyrites of 

 the Old Red Sandstone, likewise in the Permian lavas of Scotland. 



Erom the specimens in the Jermyn Street Museum it would 

 appear that these volcanic rocks include more acid varieties then 

 have been met with in the Permian series of Scotland. Dr. Hatch, 

 after a microscopical examination of them, informs me that while 

 some are olivine-basalts, containing ferruginous pseudomorphs after 

 olivine (Raddon Court, Pocombe, and near Eudlake), others are 

 true andesites (Ide, Kellerton Park) and even mica- trachytes 

 (Copplestone, near Knowle Hill). By some of the older writers the 

 existence of quartz-porphyries is also mentioned.^ 



That these lavas were contemporaneously erupted early in the 

 period of deposition of the red sandstones was clearly perceived and 

 stated by De la Beche. He recognized the amygdaloids as slaggy 

 lavas, and saw that the volcanic breccias and tuffs are interleaved 

 with the sandstones. But he appears also to have detected some of 

 the probable vents from which these materials were ejected. He 

 thought the chief centre of activity lay at Kellerton Park, while in 

 other localities he found the bosses of igneous rock '^ to descend in 

 mass downwards, as if filling up some crater or fissure through which 

 these rocks had been vomited." ^ He speaks also of " quartziferous 



1 See De la Beehe ' Report,' pp. 203, 204. My colleague, Mr. Ussher, has 

 recently found close to the Thurlestone outlier of conglomerate near Kings- 

 bridge, Devonshire, a small boss of quartz-porphyry which rises through and 

 alters the Devonian rocks. The actual junction of this mass with the conglo- 

 merate is not seen, nor have any fragments of the porphyry been noticed 

 among the pebbles. Mr. Ussher informs me that in the quarry the visible 

 exposure of the acid rock is surrounded and covered by mica-porphyrite. 

 probably andesite. 



2 O^.cit.^. 201. 



