I TO TROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



group, T found a coarse agglomerate with blocks of felsitic lavas, 

 sometimes three feet across. This gradual increase of acidity in the 

 lapilli of the tuffs finds an interesting confirmation in the contem- 

 poraneous lava-sheets to which I shall afterwards allude. 



One of the most noticeable features in the tuffs of this volcanic 

 group is the great abundance of entire and broken crystals dispersed 

 through them. These crystals have certainly not been formed ^?^ 

 situ^ but were discharged from the vents as part of the volcanic dust. 

 They usually consist of felspar which, at least in the southern portion 

 of the district, appears generally to be plagioclase. Frequent reference 

 to these crystals as evidence of volcanic explosions may be found in 

 the publications of the Survey. Nowhere can they be better seen 

 than in the black slate-tuffs of Cader Idiis. They are there white,, 

 more or less kaolinized, and as they lie dispersed through the black 

 base they give the rock a most deceiDtive resemblance to some dark 

 ])orphyry. On llhobell Pawr, on the other hand, large crystals, 

 broken and entire, of hornblende are abundantly scattered through 

 much of the tuff. 



In the central parts of the district thick bands of ashes were 

 mapped by the Survey and described as coiisisting almost wholly 

 of volcanic materials, but containing occasional thin bands of 

 slate which suffice to mark pauses in the eruptions, when ordi- 

 nary sediment was strewn over the sea-bottom. In the Cader 

 Idris ground, on the other hand, interstratifications of non-volcanic 

 material are of such frequent recurrence as to show that there, 

 instead of constant and vigorous discharges accumulating a vast 

 pile of ashes, the eruptions followed each other after intervals of 

 sufficient duration to allow of the usual dark sediment spreading for 

 a depth of many feet over the sea-bottom. One of the most inter- 

 esting deposits of these interludes of quiescence is that of the pisolitic 

 ironstone and its accompanying strata on the north front of Cader 

 Idris. A coarse pumiceous conglomerate with large slag-like blocks 

 of andesite and other rocks seen near Llyn y Gadr, passes upward 

 into a fine bluish grit and shale, among which lies the bed of pisolitic 

 (or rather oolitic) ironstone which is so widely diffused over Xorth 

 Wales. The finely oolitic structure of this band is obviously original, 

 but the substance was probably deposited as carbonate of limennder 

 quiet conditions of precipitation. The presence of numerous small 

 Lingulce in the rock shows that molluscan life flourished on the spot 

 at the time. The iron exists in the ore mainly as magnetite, the 

 original calcite or aragonite having been first replaced by carbonate 



