+-----------------------------+--------+--------+--------+ | | I. | II. | III. | +-----------------------------+--------+--------+--------+ |SiO_{2} | 51·25 | 48·11 | 61·54 | |Al_{2}O_{3} | 20·41 | 13·30 | 16·30 | |Fe_{2}O_{3} | 3·02 | 3·70 | 4·40 | |FeO | 3·91 | 8·10 | 3·66 | |MnO | 0·21 | 1·43 | 0·32 | |CaO | 4·53 | 8·48 | 3·08 | |MgO | 7·22 | 9·51 | 2·99 | |K_{2}O | 2·93 | 1·57 | 1·62 | |Na_{2}O | 1·82 | 1·96 | 2·81 | |H_{2}O and Loss on Ignition. | 5·02 | 4·21 | 2·99 | |Total. | 100·32 | 100·37 | 99·71 | |Specific Gravity. | 2·84 | 2·92 | ... | +-----------------------------+--------+--------+--------+ I. Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson. Analysis by Mr. J. S. Grant Wilson. II. Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn and Pen-y-foel. Analysis by Mr. Wilson. III. Greenish shaly finely granular tuff, from road-side, north of Board Schools, St. David's. Analysis by Prof. A. Renard of Ghent. Although the majority of the tuffs are more or less basic, they frequently contain evidence in the form of small felsitic lapilli that acid lavas were present in the eruptive vents, while the pale tuffs show that at the time of their discharge it was these acid lavas and not the diabases that were blown out by the explosions. Appended are three analyses of the acid tuffs (Nos. IV. V. and VI.). +-----------------------------+--------+--------+--------+ | | IV. | V. | VI. | +-----------------------------+--------+--------+--------+ |SiO_{2} | 80·59 | 73·42 | 72·63 | |Al_{2}O_{3} | 11·29 | 12·09 | 16·23 | |Fe_{2}O_{3} | 0·28 | 0·91 | 2·70 | |FeO | 1·41 | 3·13 | 0·48 | |MnO | trace | 0·25 | ... | |CaO | 0·52 | 2·94 | 0·18 | |MgO | 0·95 | 1·12 | 1·36 | |K_{2}O | 2·98 | 1·67 | 3·35 | |Na_{2}O | 0·72 | 3·88 | 0·15 | |H_{2}O and Loss on Ignition | 1·96 | 1·28 | 3·00 | |Total. |100·70 | 100·69 | 100·12 | |Specific Gravity. | 2·55 | 2·74 | ... | +-----------------------------+--------+--------+--------+ IV. Greenish felsitic breccia, Clegyr Hill; angular fragments of various felsites in a greenish base. Analysis by Mr. J. S. Grant Wilson.