124 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



obscure, he regards them as lava-flows interbedded among strata 

 of Bala age and occurring below the chief rhy elites of the 

 district. If this be their true position, they indicate the outflow 

 of much less highly siliceous lavas before the eruption of the acid 

 felsites. In the Snowdon area any such intermediate rocks which 

 may have been poured out before the time of the felsitic outflows 

 have been buried under these. 



The tufls of the Bala series in Caernarvonshire have not received 

 the same attention as the lavas. One of the first results of a more 

 careful study of them will be a considerable modification of the 

 j>ublished maps by a reduction of the area over which they have 

 been represented. They range from coarse volcanic breccias to 

 exceedingly fine compacted volcanic dust, which cannot easily be 

 distinguished, either in the field or under the microscope, from the 

 finer crushed forms of felsite. Among the oldest tuffs pieces 

 of dark blue shale as well as of felsite may be recognized, pointing 

 to the explosions by which the vents were drilled through the older 

 Silurian sediments already deposited and consolidated. Sometimes, 

 indeed, they recall the dark slate-tuffs of Cader Idris, like which 

 they are plentifully sprinked with kaolinized felspar crystals. 

 Among the beds of volcanic breccia intercalated between the lower 

 felsites of Snowdon are magnificent examples of the accumulation 

 of coarse volcanic detritus. The blocks of various felsites in them 

 are often a yard or more in diameter. Among the felsite fragments 

 smaller scattered pieces of andesitic rocks may be found. This 

 mixture of more basic materials appears to increase upwards, the 

 highest ashes containing detritus of andesitic lavas like those which 

 occur among them as flows. 



The tuff's in the upper part of Snowdon are well-bedded deposits 

 made up partly of volcanic detritus and partly of ordinary muddy 

 sediment *. Layers of blue shale or slate interstratified among them 

 indicate that the enfeebled volcanic activity marked by the fine 

 tuffs passed occasionally into a state of quiescence. As is well 

 known, numerous fossils characteristic of the Bala rocks occur in 

 these tuffs. The volcanic discharges are thus proved to have been 

 submarine and to have occurred during Bala time. 



I have already alluded to some of the probable vents from which 

 the lavas and tuffs were discharged, and to their position along a 

 line drawn from Penmaen-mawr into the peninsula of Lleyn. It 



* See the interesting account of these tuffs given by Sir A. Eamsay, Mem* 

 Geol. Survey, vol. iii. 2nd edit. p. 142. 



