THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. VIII. 



No. VI— JUNE, 1911. 



OB.IG-mST^.I-, ARTICLES. 

 I. — On some British Pillow-lavas and ihe Rocks associated 



WITH THEM. 



By Henry Dewey and John Smith Flett. 



(By permission of the Director of H.M. Geological Survey.) 



{Concluded from the May Number, p. 209.) 



IF we take a rapid glance over the volcanic history of the British 

 • Isles we find, that eruptions having the spilitic facies have occurred 

 repeatedly, over a wide area and on a large scale. 



Pre- Cambrian. Among the Dalradian Schists of the West of Scotland 

 pillow-lavas were first recognized by Peach (30). They are well shown 

 on the shore at Tayvallich on the Sound of Jura, and extend thence 

 through Argyllshire past Loch Awe. Representatives of this series 

 are met with at Ardwell in Banffshire. The accompanying sediments 

 are the quartzites, black shales, and limestones of the Central High- 

 lands. Albite-diabases represent the intrusive phase, and there are 

 also keratophyres and soda-granites. The age of this series is uncertain, 

 but perhaps it is pre-Cambrian. 



In Anglesey and the Lleyn District of Carnarvonshire pillow-lavas 

 with variolitic structure have been described by Iiaisin (31) and 

 Greenly (32). "With them there are diabases and cherts, but no acid 

 rocks are known. These also are perhaps pre-Cambrian. 



Cambrian (?). Badiolarian cherts and pillow-lavas occur in a narrow 

 strip along the southern border of the Scottish Highlands extending 

 from Stonehaven into Arran. Diabases accompany them, but no acid 

 rocks, though serpentine and gabbro are found in lenticles, the relations 

 of which to the lavas &nd cherts are unknown. The sediments are 

 black shales, grits, and limestones, and Dr. Peach thinks it likely 

 that they will ultimately prove to be Upper Cambrian (33). 



Or&ovicimi. To this series belong the Arenig volcanics of the South 

 of Scotland and the Arenig or Llandilo pillow-lavas of South Cornwall. 

 In both places there are diabases, keratophyres, and soda-felsites or 

 soda-granites. 



About the same time there were eruptions in South Wales emitting 

 keratophyres, soda-felsite, variolite, and diabase, and Thomas (34) has 

 found a remarkable suite of rocks on Skomer Island that presents many 

 resemblances to the Arenig igneous rocks of South Scotland, though 

 only one or two flows of pillow-lava have been detected in it. They 

 include keratophyre, soda-trachyte and soda-felsite, diabase (not 

 decade v. — vol. vni. — no. vi. 16 



