Vol. 67.] SKOMEE VOLCANIC SERIES. 201 



increase in the bisilicates. The percentage of phosphoric acid is 

 much lower. The alkalies are fairly high, and, considering the 

 potash as present in the albite-moleculc, their percentages would 

 indicate the presence of 48 per cent, of albitc. 



The apatite content is only 0*3 per cent, in the least altered rock. 



(/) The Mugearites. 



The mugearites form a well-defined group which, although con- 

 stant in structural characters, presents some variation in chemical 

 composition. In this group are placed typical mugearites with 

 oligoclase as the dominant felspar, but there are others in which, 

 the felspars may be more acid or more basic. Those with more 

 acid felspars pass in the direction of the keratophyres and soda- 

 trachytes, and those with more basic felspars in the direction of 

 the fluidal olivine-basalts (p. 203). 



All these rocks are generally but slightly vesicular, though some 

 contain vesicles measuring up to 4 or 5 mm. in diameter (E 7078). 

 They are dark grey, streaked with red, and show a finely crys- 

 talline structure. They weather to a pale grey or buff, and their 

 surfaces are often red-stained. The true mugearites, as developed 

 in the Skomer Series, consist of a mass of oligoclase-microlites,. 

 arranged with well-defined flow-structure, and a few laths of some- 

 what larger size with their axes parallel to those of the microlites 

 (fig. 9, B, p. 197). The laths average 0'3 millimetre in length, 

 and have somewhat indefinite terminations. 



The ferromagnesian minerals consist of numerous green serpen- 

 tinous pseudomorphs (well shaped) after olivine, and small idio- 

 morphic crystals of a strongly coloured augite. Augite also exists 

 as minute grains between the microlitic felspars. Iron-ores are 

 abundant, and from the number of regular sections appear to be 

 magnetite ; but the prevalence of granules of secondary sphene 

 would point to the presence of the titaniferous variety of that 

 mineral. 



Typical samples were collected from the crags north-east of 

 the Limekiln, Skomer Island (E 7022, 7023), and from the crags 

 200 yards south of the flagstaff (E 7107). 



In many rocks of the mugearitic series both augite and olivine 

 are present in about equal proportions, but in others either olivine 

 or augite may be developed in excess. In a rock (E 7030) from 

 South Haven north of the Seal Hole, olivine alone is represented by 

 small idiomorphic crystals (pseudomorphs of calcite) with their 

 axes parallel to the lines of flow ; also a few of the felspar pheno- 

 crysts are more basic than those of the ground-mass. In the acid 

 direction, by the diminution of the ferromagnesian constituents, 

 they pass into rocks characterized by a few augite and olivine- 

 phenocrysts, but otherwise allied to the keratophyres (for instance, 

 E 7059). The felspars at the same time become more acid than 

 typical oligoclase. In the basic direction, if the augite and olivine 

 increase in amount, the felspars approach labradorite, but are 



