T46 PKOCEEDINGS or THE GEOLOGICAL SOCIETY. 



volcanic series far to the east, under Cross Fell, the volcanic tuffs 

 are intercalated among sedimentary strata like the Skiddaw Slates, 

 and containing the same fossils. Here, at the outer confines of the 

 volcanic district, the ejected materials evidently fell on the sea-floor 

 and mingled there with ordinary sediment *. 



One of the most marked points of contrast between the Cumbrian 

 and the Welsh volcanic districts is to be found in the great paucity 

 of sills in the former region. A few sheets of diorite and diabase 

 have been mapped, especially in the lower parts of the volcanic 

 group and in the underlying Skiddaw Slates. On the other hand, 

 dykes are in some parts of the district not unfrequent, and certainly 

 j)lay a much more prominent part here than they do in the Welsh 

 volcanic districts. The majority of them consist of felsites, quartz- 

 porphyries, diorites, and mica- traps. Eut there is reason to suspect 

 that where they are crowded together near the granite, as around 

 Shap Pells, they ought to be connected with the uprise of the post- 

 Silurian granitic magma rather than with the history of the volcanic 

 group. If this series of dykes be eliminated, there remain com- 

 paratively few that can with any confidence be associated with the 

 eruption of the Borrowdale rocks. 



(c) Scotland. — Llandeilo Period. 



It has been generally supposed that the Silurian rocks of Scotland 

 contain no contemporaneously erupted volcanic rocks. They have 

 been long known, indeed, to include many protrusions of igneous 

 material of various kinds, but these have been looked upon as 

 intrusive dykes and bosses, much younger than the strata among 

 which they are found. The recent work of the Geological Survey, 

 however, and more especially the numerous and careful traverses of 

 my friend and colleague Mr. Peach, have revealed the unlooked-for 

 and important fact that a large number of these supposed dykes are 

 really portions of a volcanic group brought up on the crests of anti- 

 clinal folds and laid bare by denudation. This group can be traced 

 for at least a hundred miles from north-east to south-west over a 

 belt of country sometimes thirty miles broad. Its original limits can- 

 not be ascertained, but they obviously exceeded those within which 

 the rocks can now be seen, Nevertheless the present boundaries 

 embrace an area of nearly 2000 square miles. This Palaeozoic 

 volcanic region is thus one of the most extensive in the British 



* See J. Gr. Goodchild in H. B. Woodward's ' Geology of England and 

 Wales,' p. 81. 



