223 



to compact, sparingly porphyritic, ranging in colour from an epidote- 

 green to dull blackish-green and dark chocolate-brown. Some of them 

 are finely porphyritic from the presence of small glistening surfaces which 

 present the colour and lustre of hematite and yield its characteristic 

 streak." The alteration to which the rocks have been subjected makes 

 it difficult to determine their original characters with precision. They 

 must, however, have contained a variable amount of base together 

 with angite-granules, magnetite, felspar not now generally recognizable, 

 and olivine. Pseudomorphs of the latter mineral now occasionally give 

 a porphyritic character to the rocks. Dr. GEIKIE terms the rocks olivine- 

 diabases. The presence of base would remove the rocks from the diabases 

 of Professor ROSEXBUSCH and place them under the melaphyres. There 

 can be no doubt that they were originally olivine-basalts. 



The basic tuffs associated with the above rocks are usually purple, red 

 or dark green. They vary in texture from coarse breccias or agglomerates 

 to " fine silky schists in which the tuffaceous character is almost lost." 

 The following analyses, quoted from Dr. GEIKIE'S paper, show the similarity 

 in composition of the tuffs and lavas : 



I. II. III. IV. 



SiO., ... 51-25 ... 48-11 ... 45-92 ... 45-38 



A1 2 6 3 ... 20-41 ... 13-30 ... 18.16 ... 16-62 



Fe 2 3 ... 3-02 ... 3-70 ... 1-18 ... 4-06 



FeO ... 3-91 ... 8-10 ... 9.27 ... 8.63 



MnO ... 0-21 ... 1.43 ... 0.19 ... 0-14 



CaO ... 4-53 ... 8-48 ... 7-19 ... 8-19 



MgO ... 7-22 ... 9-51 ... 10-07 ... 9-41 



K 2 ... 2-93 ... 1-57 ... 1-78 ... 0-71 



Na 2 ... 1-82 ... 1.96 ... 2-12 ... 2-20 



l>ss on Ig. 5-02 .. 4-21 ... 4-22 ... 4-34 



Insol. 0-04 0-08 



100-32 100-37 100-14 99-76 



Sp. Gr. 2-84 2-92 ... 2-96 ... 2.99 



I. Purplish-red shaly tuff from below the olivine -diabase crag, Rhosson ( WILSON). 

 II. Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn and 

 Pen-y-foel (WILSON). 



III. OH vine -diabase, Rhosson (WILSON). 



IV. Olivine-diabase, Clegyr Foig (WILSON). 



The dykes and intrusive sheets of basic rock occur in all the other 

 rocks of the St. David's area. The dykes are especially abundant in the 

 Dimetian of Dr. HICKS. The rock of the dykes and sheets is much decom- 

 posed. It differs from that of the older lavas " in the abundance and fresh- 

 ness of its felspars, in the comparative scarcity of its augite and in the 

 absence of olivine " (GEIKIE"). This description implies that it is, or rather 

 was, an andesitic dolerite. 



