ON MANOD AND THE MOELWYNS. 381 



suggested by ^fessra. Cole and Jennings * that the Tan-j'-grisiau 

 " syenite,'' '' witli its abundant quartz and its poorness in ferro- 

 magncsian silicates, is just such a rock as might have resulted had 

 more complete crystallization, under slower conditions of cooling, 

 taken place in the eurite of Cader Idris." 



Througli the kindness of Mr. L. AV. Fulcher, B.Sc, who has made 

 an analysis of the Tan-y-grisiau granitite, we are able to show that 

 that suggestion was justifiable. The silica percentage is rather 

 higher in the Tan-y-grisiau rock, but the two analyses present on 

 the whole a remarkable agreement : — 



Tan-y-grisiau Cader Idris 



Granitite. Knrite. 



SiO, 7502 72-79 



Fe,03 2-80 332 



Al.pg 1288 b3-77 



CaO 1-17 1-94 



MiiO -32 -62 



K,0 503 299 



Na.p 3-28 412 



HjO -60 Loss on ignition... TOS 



101-19 100-()3 



That the intrusion of the granite is subsequent to the hardening 

 and cleavage of the Treraadoc rocks is shown by the fact that 

 angular cleaved fragments of the latter lie, in all directions, in the 

 offshoots of the crystalline mass. As the cleavage of the Tremadoc 

 beds is similar to that of the overlying strata, it would seem probable 

 that the date of the intrusion of the granite is later than the period 

 of consolidation of the Llandeilo rocks, and that it has therefore 

 no immediate relation to the Arenig eruptions. 



Such questions as this can, however, only be decided by a wider 

 study of the country as a whole ; and for the present we prefer to 

 leave our record of observations in the Ffestiniog district as it stands, 

 without burdening it with theories that may be premature. 



Discussion. 



Prof. Hughes bore testimony to the excellent work being done 

 by Mr. Williams in the Ffestiniog district. He thought that pos- 

 sibly the extension of the syenitic mass at a small depth below the 

 surface would account for the metamorphism of the rocks so far 

 from the area where it is now exposed, and that the presence of 

 more felspathic material in the sediment had caused some beds to 

 lend themselves more readily to the kind of metamorphism noticed. 

 The earth-movements which resulted lu the Merioneth anticlinal 

 had crushed this unyielding mass of then solid intrusive rock among 

 the more yielding sedimentary strata so as to produce the disturb- 

 ance of the beds observed along its flanks. He had not himself 



* Quart. Journ. Geol. Sue. vol. xlv. (1889) p. 438. 



