210 me. h. ir. thomas on the [May 1 91 1,. 



flows and groups of flows: for rocks with much albite are often 

 to be fouud between flows of which the felspar is all labradorite. 

 Also the rocks rich in albite, as regards structures and the nature 

 and arrangement of the ferromagnesian minerals, are quite different 

 from the more basic rocks Avhich presumably would represent their 

 unaltered prototypes. Secondary albite, when it does occur in these 

 rocks as veins and infilling vesicles, is, as usual, water-clear and 

 quite different from the felspars of the phenocrysts, etc., which 

 show turbidity due to incipient decomposition. I am led, therefore, 

 to regard the chief mineralogical and chemical peculiarities of the 

 Skomer rocks as primary, and to consider the Series in part to be 

 rich in original soda and to present Pantellerian affinities. The 

 Series, as a whole, may be regarded as a curious interdigitation of 

 alkaline and subalkaline types. 



The accompanying diagram (fig. 13, p. 209) makes it appear 

 that the rocks represented are a mixture of types converging in 

 the basic direction, for the analyses when plotted seem to indicate 

 that the rocks belong to two distinct but overlapping series. 



The dolerites and basalts may be considered normal augite- 

 labradorite rocks; but all the other igneous rocks show mineralogical 

 and structural peculiarities, the most remarkable of which is the 

 widespread occurrence of albite and albite-oligoclase felspars in 

 intimate association with large porphyritic crystals of olivine, as 

 in the olivine-trachytes and marloesitcs, and with abundant augite 

 in the skomerites. 



Another peculiarity worthy of note is the occurrence of the 

 brown hornblende in the olivine-trachytes and marloesites. ' In 

 the more acid rocks the felspars are hardly ever zoned, but are 

 sometimes beautifully microperthitic — features which are common 

 in rocks of the alkaline class. There are no felspathoid minerals- 

 or zeolites. 



VIII. The Sequence in Time oe the vakious Kocx-Types. 



The sequence presented by the Skomer Series, as made out on 

 the mainland of Pembrokeshire, differs somewhat from that observ- 

 able on Skomer Island ; but the differences may be explained by 

 the eastward thinning of the series as a whole and the overlapping 

 of certain horizons by others. Considering the mainland, the 

 lowest member of the series is a group of rhyolites exposed at 

 Musclewick beyond the eastern edge of the map (PL XI), and 

 followed presumably by the mugearitic group of Wooltack Head. 

 This in turn is succeeded by the intermediate group of the northern 

 cliffs which underlies the main sedimentary group. The highest 

 rocks consist of olivine-basalts, and overlie the sediments. The 

 general succession, therefore, is from acid to basic. 



On Skomer Island a much greater thickness of volcanic rocks 

 may be studied. The lowest rocks visible are those of the rhyolitic 

 group of the mainland, which here reaches some 500 feet in thick- 

 ness. It is followed by an insignificant group of intermediate 



