The Northern Slopes of Cader ldris. 69 



could be accounted for. Analyses showed that there was an inter- 

 change of alkaline bases, soda going to the granite, and potash to 

 the diorite. Thus, some of the latter contained almost twice as 

 much potash as soda. 



The evidence collected seemed to prove that the schist-making 

 had taken place subsequently to consolidation ; but it was clear, 

 especially where the rock was heavily sheared, that the con- 

 stituents had been redissolved and reconstructed. Thus, as we 

 followed a diorite into the core of a shear-zone, we could see the 

 gradual disappearance of shear-planes and other mechanical effects, 

 as well as the progressive results of chemical synthesis. 



The secondary origin of the micas and of part of the felspar was 

 proved by the fact that they were moulded on decomposition-pro- 

 ducts, such as chlorite and epidote, and upon fragments of horn- 

 blende crystals, which had been crushed during the shearing, and 

 carried away from each other. The mineral changes here described 

 resulted from contact-action plus mechanical force. 



2. " The Northern Slopes of Cader ldris." By Grenville A. J. 

 Cole, Esq., E.G.S., and A. V. Jennings, Esq., E.L.S. 



Erom the publication of Mr. Aikin's paper in the Transactions 

 of the Geological Society in 1829 to the second edition of the Survey 

 Memoir on North Wales, the relations of the geological and physical 

 features of Cader ldris have been pointed out in some detail. The 

 present paper dealt with the nature of the eruptions that took place 

 in this area and the characters of their products at successive 

 stratigraphical horizons. The best exposures occur, as is well 

 known, upon the northern slopes. 



The lowest evidence of contemporaneous volcanic activity is to be 

 found at the Penrhyn-gwyn slate-quarry, where a somewhat coarse 

 bed of tuff, with slate-fragments and abundant felspar-crystals, 

 occurs above an andesitic sheet. Similar slate-tuffs are repeated up 

 to the base of the great cliff of Cader ldris, with intervening layers 

 of normal clayey sediment. On the whole, the tuffs and ashes 

 become more highly silicated as the upper levels are reached, and 

 they terminate on the southern slopes in beds with fragments of 

 perlitic and devitrified obsidian, such as are found under Craig-y- 

 Llam. On Mynydd-y-Gader the intrusive dolerites have altered the 

 ashes into hornstones; in places, moreover, they have become 

 jointed into distinct columns. Fragments of andesitic glass as well 

 as trachyte are recorded. 



The "pisolitic iron-ore" of the Arenig beds appears to have 

 resulted from the metamorphism of an oolitic limestone, as in the 

 case of the Cleveland ore described by Mr. Sorby, and that of 

 Northampton described by Prof. Judd. The grains still give 

 evidence under crossed nicols of their having been built up of 

 successively deposited concentric layers. The calcite so freely 

 developed in the hollows of the underlying rocks may have been 

 largely derived, during metamorphic action, from the destruction of 

 similar thin limestone-seams. No true lava-flows occur among 



