14 Mfi. H. C. SAEGEXT OX LOWEE CAEBOXIFEEOUS [vol. IxXlii, 



Dr. Bemrose has noted that, hi spechnens from three outcrops 

 examined hy him, 



' the ground-mass sometimes consists of a small felt of felspar-laths often 

 ^ving parallel extinction/ ^ 



It is interesting to contrast these rocks with those at the other 

 ■end of the series, the darker rocks which are of normal basaltic or 

 doleritic tj^pe. It is unnecessary to describe these in detail, since 

 Dr. Bemrose has already done so ^ ; but a point of importance 

 bearing, as I conceive, on the evolution of the whole series, is their 

 general freshness as comjDared with the rocks above described. They 

 are sometimes beautifully fresh and well preserved, the principal 

 sign of alteration being slight serpentinization of the olivine. 



A more detailed description of a few of the more noteworthy 

 rocks Avith spilitic affinities may now be given. This will be mainly 

 confined to the felspathic content, since the general description of 

 the other constituents given above holds good for all. 



In the rock fi'om Ball Eye, Via G-ellia [1], the felspar occurs in 

 two generations : large lath-shaped phenocrysts and a ground-mass 

 of microlites. Many of the phenocrysts are tAvinned on the albite 

 law, and show extinction-angles which point to andesine-labradorite. 

 Carlsbad twins with simultaneous or unsymmetrical extinctions, 

 and untwinned crystals extinguishing straight, also occur in sub- 

 ordinate quantity. The microlites of the ground-mass appear to 

 be dominantly oligoclase. Very little, if any, replacement by 

 secondary felspar has taken place here. 



The Salter's Lane and Masson rocks [2 & 3] are both outcrops 

 of the Upper Lava of the Matlock area. The former was collected 

 in situ ; the latter was obtained from the waste -heaps of the Seven 

 Rakes Mine, now closed. The rock is sub-porphyritic, the felspar- 

 content consisting of a few^ small phenocrysts and a mass of laths 

 of fairly uniform size. The phenocrysts are altered throughout 

 to a water-clear fels])ar- substance associated with yjatchesof calcite. 

 The laths are sometimes similarly altered ; but often the secondary 

 felspar is seen on the margins only, the interior forming a spongy 

 core of chlorite and calcite. Vestiges of albite-twinning occur in 

 some of the laths, and the secondary calcite suggests that labradorite 

 was origmally present. The seconilar}' fels]:>ar has a lower refractive 

 index than the balsam, and in view of the analyses (Nos. VI & 

 VII, p. 22) it must be assumed to be orthoclase. 



The lava-flow on Knot Low, Millers Dale [8], is about 120 feet 

 thick. At the summit the rock below the zone of atmospheric 

 weathering is a fresh basalt ; at the base it is of spilitic character, 

 and is altered throughout. It is non-porphja-itic ; the felspars are 

 all lath-shaped, frequently untwinned, and sometimes twinned on 



Q. J. G. S. vol. 1 (1894) p. 624. 



2 Ibid. pp. 611-2 



