Tlie Ordovician Lavas of Mynydd Prescelly. 315 



Fig. 3. — Perhtic Soda-rhyolite, 200 yards N.W. of Mynydd-du, Eosebush. x 18. 



Fig. 4.— Soda-rhyolite, Craig-y-Cwm type, S.E. of old quarries, Craig-y-Cwm, 



N.E. of Eosebush. x 18. 



