314 Transactions. 



labradorite enclose all these minerals and abundant prisms of apatite in 

 poecilitic fashion. The chief mineralogical difference from the olivine- 

 dolerites lies in the greater relative abundance of hornblende and apatite 

 and the lesser amount of olivine. The resemblance to the lamprophyres 

 lies in the small size and elongate prismatic habit of the ferro-magnesians. 



Two rocks are of a very unusual type. Augite (partially uralitized), 

 brown hornblende, iron-ore, and an acid plagioclase are the principal con- 

 stituents. Apatite and sphene occur in lesser amount. Chlorite, epidote, 

 calcite, leucoxene, pyrite, and sericite are abundant as secondary products. 

 The order of consolidation appears to have been apatite, sphene, iron-ore, 

 augite, hornblende, and feldspar. The most marked feature of the structure 

 is the perfect idiomorphism of the brown hornblende towards the feldspar, 

 a,lthough it encloses augite, iron-ore, and apatite abundantly. The texture 

 is coarse, like that of the average dolerites. 



The feldspars are seldom twinned, and are very sericitic. Since, how- 

 ever, they have indices less than both balsam and cement, and are optically 

 positive, they must be referred to albite or oligoclase-albite. The rocks 

 show some superficial resemblances to the Cornish minverites, but differ 

 in the absence of basic plagioclase in addition to the albite. A re-examina- 

 tion, based on fresher material, is necessary before they can be correctly 

 named 



The Spessartites. 



The spessartites, according to Rosenbusch, bear the same relationship 

 to vogesites as kersantites bear to minettes — i.e., they contain plagioclase 

 instead of, or in addition to, orthoclase. Camptonites are distinguished 

 by containing barkevicitic hornblende and aegerine borders to the augite. 

 Moreover, the olivine of camptonites is never pilitized. 



A considerable number of the rocks from the Dee Gorge are hornblende 

 lamprophyres. Like most rocks of this class, they are considerably altered, 

 and in consequence the determination of the feldspars presents considerable 

 difficulty. Orthoclase has not been certainly determined, while the plagio- 

 clase ranges from albite to a semibasic species. Owing to the lack of the 

 distinctively camptonitic characters, and the presence of pilite in some of 

 the rocks with lamprophyric affinities, it is safer to class the hornblende 

 lamprophyres with the spessartites. 



No two of the rocks examined agree in all respects, and the degree of varia- 

 tion is considerable. The presence of phenocrysts and needles of brown 

 hornblende is the most constant character. Feldspar occasionally occurs 

 as phenocrysts, more or less sericitized, and is abundant as small prisms 

 or microlites in the groundmass. Titano-magnetite also occurs in two 

 generations, and often shows a marginal alteration into sphene. Augite 

 is not always present, but is sometimes abundant in large and small crystals. 

 Biotite occurs only in the groundmass. Apatite is fairly abundant, some- 

 times in prisms of sufficient size to justify the name of phenocrysts. Calcite, 

 chlorite, and epidote are abundant. 



There are considerable variations in grain-size, but the groundmass is 

 always holocrystalline, the feldspars being sometimes in short prisms and 

 sometimes in radial aggregates. Some of the rocks vary rapidly in texture 

 from place to place, and appear to consist of dark parts veined by clearer 

 material. 



Acid Rocks. 



Only three specimens of the acid rocks have been studied, and of these 

 one is too near the junction of an argillite inclusion to show its normal 



