ANNIVERSARY ADDRESS OF THE PRESIDENT. I 49 



posed mainly of fragments of microlithic rocks, perhaps andesites, 

 with detached crystals of felspars. Others present, not only felspars, 

 but largo well-formed crystals of augite similar to those of modern 

 volcanic tuffs. There are likewise bosses of gabbro and other 

 intrusive rocks. A more thorough examination of this tract will 

 doubtless throw much light on the history of this volcanic episode. 



The other part of the region where relics of some of the eruj)tive 

 vents may exist is the district south of Girvan. Various bosses and 

 sheets of eruptive rock rise there through the lavas and agglomerates, 

 some of which may mark the sites of volcanic orifices. The sheets 

 of gabbro and serpentine running in a north-east and south-west 

 direction with the general strike of the rocks remind us of the later 

 basic sills of the AVelsh volcanic districts. But this ground will be 

 revised in the light of recent researches, and more definite informa- 

 tion will doubtless then be obtained regarding its structure. 



The geological horizon of this extensive volcanic group in the 

 Lower-Silurian series of the south of Scotland can be fi,xed with 

 precision. On the coast of Aj^shire, at Bennane Head, near Bal- 

 lantrae, Professor Laj) worth found in some hardened black shales a 

 group of graptolites which indicate an unmistakable Arenig horizon*. 

 These fossils lie below the volcanic group, which is thus not older 

 than the Arenig period. The upper limit is admirably defined in 

 many sections in the eastern part of the area, by the superposition 

 of the Glenkiln black shales upon it. These shales are distinguished 

 by an assemblage of Llandeilo graptolites. The volcanic eruptions 

 are thus shown to have occurred within the Llandeilo period. But 

 they do not appear to have entirely ceased till after the close of 

 that period, for in at least one locality, that of Hartfell, a mode- 

 rately coarse volcanic agglomerate occurs in the heart of the so-called 

 '* barren mudstones " of the Hartfell black-shale group which, from 

 its graptolites, is placed on the horizon of the Bala rocks. 



Some interesting facts have likewise been observed by Mr. Peach 

 and his colleagues regarding the usual upward limit of the volcanic 

 group. Between the Glenkiln black shales and the volcanic sheets 

 below them alternations of green, grey, or red shaly mudstones and 

 flinty greywackes are interleaved with fine tuffs, and are specially 

 marked by the occurrence in them of nodules and bands of black 

 and reddish chert. This latter substance, on being submitted to 

 Dr. Hinde, was found by him to yield twenty-three new species of 

 Radiolaria belonging to twelve genera, of which half are new. It 



* Geol. Mag. 1889, p. 22. 



