134 MR J. D. FALCONER ON THP: 



Such occurrences undoubtedly indicate that these rocks have been subjected to some 

 slight extent to post- volcanic pneumatolytic action. 



Microscopically, the lavas show such a similarity in mineral composition, and such an 

 abundance of intermediate types between the better-marked varieties, that little more 

 than a general summary of their petrological characters can here be attempted. 



The Mineralogical Characters of the Lavas. 



(a) The Porphyritic Constituents. 



Felspar phenocrysts are rare. Only in the platy basalt overlying the Wardlaw 

 limestone are they found in any abundance, and there the scattered crystals appear to 

 belong mostly to anorthite. Xenocrystic felspars, more or less corroded, are fairly 

 frequent in the more compact lavas, and give extinction-angles for bytownite and 

 anorthite. Occasionally in felspathic types some of the lath-shaped felspars attain a 

 somewhat larger size than the others, but these can hardly be considered porphyritic 

 in the usual sense of the word. Phenocrysts of augite, always clear and undecomposed, 

 are very abundant in the finer-grained types. Sometimes they are sharply idiomorphic, 

 but very generally they are irregularly corroded and penetrated by the groundmass. 

 Polysomatic masses of augite are frequent, and these may have arisen in some cases by 

 the aggregation of imperfect crystals, and in others by the fracturing during eruption 

 of original augite phenocrysts. The colour is a pale brown or violet, frequently 

 deepening in tint on the margins, and rarely giving place to green in the interior. A 

 multiple zonary arrangement of tints is occasionally observed. Inclusions of magnetite 

 and groundmass are very abundant, and sometimes sharply confined to the interiors of 

 the crystals. The augites are frequently sensibly pleochroic, and simply or repeatedly 

 twinned on the orthopinacoid. Aggregations of small augite crystals, resembling those 

 formed during the resorption of quartz in basic rocks, are occasionally observed, and 

 probably represent original xenocrystic quartz. The worn remains of the quartz grain 

 may sometimes be observed in the centre of the aggregate. Olivine in pheno- 

 crysts is abundant throughout. Long rectangular crystals and pyramidal forms are 

 equally common, always more or less corroded and replaced by masses of serpentine, 

 magnetite, chalcedony, or calcite. The serpentine may be stained red or brown, 

 marginally or completely. Pseudomorphs of pleochroic iddingsite are fairly common. 

 Nodular masses of various minerals, sometimes as large as a marble and perhaps to 

 some extent xenocrystic, may be picked out of many of the lavas. These include 

 granular aggregates of rounded crystals of anorthite and schillerised augite as well as 

 intergrowths of augite and anorthite, and augite and olivine, usually much corroded 

 by the basaltic magma. 



(/>) The Groundmass. 



The groundmass is composed of small crystals of felspar, augite, magnetite, and 

 more rarely olivine, and a varying quantity of undifferentiated glassy base. The felspars 



