ANNIVERSARY ADDRESS OF THE PRESIDENT. 1 39 



period. It begins probably not so far back as that of the Arenig 

 group of Merionethshire, aud its termination was perhaps coinci- 

 dent with the dying out of the SnoAvdonian volcanoes. But it con- 

 tains no record of a great break or interval of quiescence like that 

 which separated the Arenig from the Bala eruptions in Wales. 



The materials that form this enormous volcanic pile consist 

 entirely of lavas and ashes. No intercalations of ordinary sedi- 

 mentary material have been met with in it, save at the bottom and 

 at the top. The lower lavas, well seen among the hills to the 

 south of Keswick, were shown by Mr. Ward to be intermediate 

 between felsites and dolerites in regard to their silica percentage, 

 and he proposed for them the name of felsi-dolerites. They are 

 comprised in the group of the porphyrites or altered andesites. 

 From the analyses published by Mr. Ward, the amount of silica 

 appears to range up to about 60 per cent.* They are close-grained, 

 dull dark-grey to black rocks, breaking, where fresh, with a splintery 

 or conchoidal fracture, showing a few minute striated felspars, apt 

 to weather with a pale-brown or yellowish-grey crust, and some- 

 times strongly vesicular or amygdaloidal. They present many 

 external resemblances to some of the porphyrites of the Lower 

 Old Eed Sandstone of Scotland. A microscopic examination of 

 specimens collected by Dr. Hatch and myself from the hills to the 

 south of Keswick shows the rocks to be true andesites, composed of 

 a multitude of slender laths (sometimes large porphyritic crystals) 

 of felspar with a glassy brownish ground mass, and with some 

 chloritic material probably representing augite, but with no trace 

 of quartz f. 



The andesitic lavas are more particularly developed in the lower 

 and central part of the volcanic group. They rise into ranges of 

 craggy hills above the Skiddaw Slates, and form, with their 

 accompanying tuffs, the most rugged and lofty ground in the Lake 

 District. They extend even to the southern margin of the volcanic 

 area at one locality to the south-west of Coniston, where they may 

 be seen with their characteristic vesicular structure forming a 

 succession of distinct flows or beds, striking at the Coniston Limestone 

 which lies upon them with a decided, though probably very local, 



» Quart. Journ. Geol. Soc. vol. xxxi. (1875) p. 408, vol. xxxii. (1876) p. 24. 

 Geology of Northern Part of Lake District (Mem. Geol. Survey), p. 22. In a 

 subsequent series of analyses of the more basic lavas of Eycott Hill, the per- 

 centage of silica is given as from 51 to 53"3 and these rocks are compared with 

 dolerites (Monthly Microscopical Journ. 1877, p. 246). 



t These rocks were mapped as tuffs by Mr. Ward. 



