204 ME. H. H. THOMAS OX THE [jSklayiCJII, 



Their boundaries are sharp, they are frequently zoned, and occa- 

 sionally include grains of augite (for instance, E 7480). Augite 

 is plentiful, and exists as subidiomorphic crystals, gloinero- 

 porphyritic groups, and sometimes (as in E 7026) as fairly large 

 idiomorphic crystals penetrated by, but not ophitically enclosing, 

 laths of felspar. The greater part of the augite exists as small 

 granules involved in the plexus of felspar-laths which form the 

 bulk of the ground-mass. These felspars usually are perfectly fresh : 

 they average 0*3 or 0*4 mm. in length, and are generally twinned 

 according to the Carlsbad law. They may show repeated albite- 

 lamellation if ; their width allows of it. Their high refractive 

 indices and wide extinction indicate a labradorite about Ab^n — 

 that is, slightly more acid than that of the phenocrysts. 



The olivine usually exists as small idiomorphic crystals in the 

 ground-mass, about 0-2 mm. long ; but occasionally a single partly 

 absorbed individual may reach 2 mm. in length (fig. 11, A, p. 203). 

 This mineral is always represented by pseudcmorphs, either in 

 limonitic iron-ores, or in chlorite, serpentine, calcite, iddingsite, and 

 the green pleochroic mineral mentioned above (p. 199). 



The iron -ores are well defined, and, from the number of regular 

 sections, they would appear to be magnetite or titanomagnetite, 

 probably the latter. There is little or no residual matter, but in 

 some of the rocks a certain amount of glassy base seems to be 

 represented by interstitial chlorite which is devoid of the other 

 rock-constituents. The vesicles never contain zeolites, but are 

 rilled with calcite, secondary silica, chlorite or epidote, and often 

 lined with sphene (for instance, E 7079). Between the porphyritic 

 and the extremely fluidal types of basalts is a set of rocks 

 (for instance, E 7058, 7480, etc.) which exhibit practically no 

 porphyritic felspars, the porphyritic individuals being represented 

 by a few laths of larger size. All the augite is granular, and the 

 olivine pseudomorphs are of small dimensions. 



The extremely fluidal rocks, of which there are many examples, 

 are identical in composition with those just described, but differ 

 in the felspar-laths and plates being slightly smaller in size and 

 arranged in parallel streams. In a few of these rocks (as, for 

 instance, E 7049) the felspars are near to andesine in composition, 

 and thus they show affinities with the mugearites. These rocks 

 are prevalent at the Smalls, and might be classed as mugearitic 

 basalts. 



An analysis (Analysis XIII, p. 205) of a typical non-porphyritic 

 Skomer basalt shows that these rocks are somewhat less basic than 

 the average olivine-basalt, the silica being about 6 per cent, higher. 

 Also there is a slight falling-off in the percentage of lime, magnesia, 

 and iron, with a correspondingly small increase in the alkalies : 

 pointing to a labradorite -felspar of less basic composition, less 

 augite, and less iron-ore. However, the rocks are sufficiently basic 

 to be classed with the basalts, and show no tendency to pass into 

 rocks which might with any consistency be termed augite-andesites. 

 According to the classification of Whitman Cross, Iddings, etc., the 



