part 3] XENOLiTHic minor inteusions in mull. 235 



Where vitreous, the rock consists of small porphyritic crystals 

 of labradorite and less frequent uniaxial augite (usually pseudo- 

 morphed in serpentine or calcite) in a brownish glass traversed in 

 all directions by acicular crystals of augite and skeletal growths of 

 magnetite. In portions of the rock that have undergone greater 

 devitrification the ground-mass carries the same phenocrysts and 

 acicular augite, but may be seen to contain alkali-felspar and 

 quartz. The central and greater portion of the acid interior is a 

 grey rock, locally vitreous and often showing the ' sheath-and-core ' 

 structure characteristic of many of the Mull pitchstones.i The 

 acid portions varj^ from rhyolite to felsite, but are always very 

 closely related to rocks classed with the extremely acid types of 

 inninmorite. They consist of small porphyritic crystals of acid 

 labradorite in a glassy base which, like that of the inninmorite, is 

 traversed by blade-like crystals of a greenish augite. Even in the 

 glassy portions (PL VI, fig. 1) small devitrified areas are of 

 frequent occurrence, and are composed of felspar-crystals, of less 

 basic composition than the phenocrysts, which are often grouped in 

 a radial or subvariolitic manner. 



When more completely devitrified (PI. VI, fig. 2) the rock is 

 lighter in colour; the base contains little glass, its place being 

 taken by a feathery felspathic mass which is still traversed by the 

 acicular crystals of augite, but holds (in addition to acid plagio- 

 clase) some orthoclase and a little free quartz. 



The analysis of a felsite closely related to inninmorite, made by 

 Mr. F. E,. Ennos, of the Government Laboratory, and tabulated in 

 col. 1 of the analyses on p. 236, although from another locality, 

 may be taken to represent closely the composition of the acid 

 interior of the Kudh' a'Chromain sill. 



The normal stony basic portion (PI. VI, fig. 4) consists of some- 

 what elongated irregularly-formed crystals of aluminous augite, 

 which tend to assume a prismatic habit, and felspar, with which 

 the augite often has subophitic relationship. 



The felspar is a basic labradorite zoned with plagioclase of com- 

 position varjdng to that of oligoclase. These two minerals make 

 the bulk of the rock, and together give rise either to a sub-variolitic 

 or to an intersertal structure. The interstitial matter is devitrified, 

 and consists of acid plagioclase and quartz. It contains a relatively 

 small amount of magnetite, and abundant apatite in the form of 

 slender needles. 



In the more vitreous portion (PI. VI, fig. 3), in which cognate 

 xenoliths appear to be the more prevalent, the rock has a definitely 

 intersertal structure. 



The varieties met with in the basic margins are all closely 

 related. Their composition is clearly on the border-line between 

 that of basalt and augite-andesite ; and this, combined with a 

 frequently well-marked intersertal structure, warrants their inclu- 

 sion among the tholeiites. In Mull it has been found that 55 per 



^ E. M. Anderson & E. G. Eadley, op. supra cit. pp. 210, 211. 



