ANNlVEliSARY ADDRESS OF THE TEESIDKNT. I4I 



lower parts of the volcanic group, it is at the top that their chief 

 developniciit appears to occur. These rocks may be grouped together 

 as felsites (rhyolites). Some of them have been described by 

 Mr. llutley, who has found them to exhibit beautiful perlitic and 

 spherulitic structures *. That such rocks as these wore poured out 

 in a vitreous condition like obsidian or pitchstone cannot be doubted. 

 They probably play a much larger part in the structure of the 

 southern part at least of the volcanic area than the published maps 

 would suggest, and a detailed survey and petrographical study of 

 them would well reward the needful labour f . A fine series 

 of felsites is interbedded in the lower part of the Coniston Limestone, 

 and spreads out underneath it along the southern margin of the 

 volcanic district from the Shap granite south-westward for some 

 miles i. Between the valleys of the Sprint and Kent these 

 felsites (which farther east are said to be 700 feet thick) may be 

 seen interposed between the limestone and the fossiliferous cal- 

 careous shales below it, while from underneath the latter other 

 sheets rise up into the range of hills behind. Occasionally a distinct 

 nodular structure may be observed in the felsites, sometimes minute, 

 like an oolite, in other parts presenting large rounded balls which 

 have been more or less flattened and elongated in the direction of 

 cleavage. Some portions of the rocks have been so intensely 

 cleaved as to become a kind of fissile slate that might at first be 

 mistaken for felsitic tufi^. Short, extremely irregular, branching 

 veins of a fine cherty felsitic substance, which occasionally shows a 

 weU-marked flow-structure parallel to the walls, traverse certain 

 parts of the f elsite. 



The abundance and persistence of such thoroughly acid lavas along 

 the southern edge of the volcanic area where the youngest outflows 

 are found, is a fact of much interest and importance in the history 

 of the eruptions of this region. It harmonizes with the observations 

 made in Wales, where both in the Arenig and Bala groups a marked 

 increase in acidity is noticeable in the later volcanic products. At 

 the same time, as above mentioned, there is evidence also of the dis- 

 charge of more basic materials towards the close of the eruptions, 

 and even of the outflow of a lava approaching in character to basalt. 



According to the Geological-Survey maps, by far the largest part 



* 'The Felsitic Lavas of England and Wales,' Mem. Geol. Survey, 1885, 

 pp. 12-15. 



t Unfortunately these acid lavas are not distinguished from the others in the ■ 

 Geological-Survey maps. 



X ' Geology of Kendal, &c.,' Mem. Geol. Survey, Sheet 98 N.E., 2nd ed. p. 9. 



