16 ME. H. C. SAEGEXT OX LOWEE CARBOXIFEBOrS [vol. Ixxiil, 



basalts. The phenocrvsts here are entii-ely replaced b}* alkali-felspar 

 enelosmg small patches of secondary calcite, which probably indicate 

 labradorite as the original felspar. They are twinned on the albite 

 law, and have extinction-angles indicating andesine or an acid 

 labradorite. Then* refractive index is higher than that of the 

 balsam. Messrs. E. B. Bailey & G. W. Grabham's observation^ 

 that, in a porphyritic basalt, the phenocrysts are alwa^^s attacked 

 before the less basic felspai's forming the ground-mass, is thus 

 confirmed here, as well as elscAvhere, in the Derbyshire rocks. 



The study of the vesicles affords matter of much interest. 

 Frequently the}' are lined with a very thin layer of a fibrous 

 green chloritic substance, the fibres standing out from the wall of 

 the cavity. The birefringence of this mineml is higher than that 

 of chlorites in general. The following minerals occur in the interior 

 of the vesicles, generally two or more together: quartz, chalcedony, 

 calcite, dolomite, albite, and chlorite. The sequence observed in 

 the deposition of these minerals varies in different vesicles. 

 Granular quai-tz frequently occurs in small grains or narrow strips 

 associated with the outermost chloritic lining, or itself forming the 

 actual lining of the cavity. When quartz and calcite are associated, 

 the quartz has generally separated out before calcite. llarely, the 

 reverse is the case, and dogtooth cr^'stals of an outer zone of calcite 

 are seen penetrating an inner zone of quartz. Dolomite is often 

 associated with calcite. Helminth is the most abundant form 

 of chlorite, and there is sometimes ])resent a yelloAvish-brown fibrous 

 mineral, pleochroic and with low birefringence, wdiich ma}' be near 

 delessite. Sometimes these chloritic minerals fill the entire vesicle, 

 or, in a composite amygdale, they may be formed at any stage 

 relatively to the other minerals in the vesicle. In the vesicles 

 described by Messrs. Bailey & Grabham ^ a regular sequence of 

 deposition occurs, namely: (a) albite, (h) chlorite, and (c) calcite. 



It is clear from the foregoing description that there is a facies of 

 lava-flows in Derbyshire presenting characters widely different from 

 those of the true basalts. That they have structural trachytic 

 affinities is obvious from the predominance of alkali-felspar con- 

 sisting of a mass of laths and microlites, sometimes exhibiting 

 fiuidal arrangement and often forming the bulk of the I'ock. On 

 the other hand, they are essentiall}' more basic than the trachytes, 

 as is shoAvn b}' their low silica-content, their abundant iron-ores, 

 and the presence of olivine. 



There is no need to emphasize the points of resemblance and 

 difference between these rocks and those of the spilitic and 

 mugearitic groups. Like the spilites, the Derbyshii'e rocks were 

 submarine flows in an area that was undergoing slow and prolonged 

 subsidence. Like both spilites and mugearites, their structure is 



^ ' Albitization of Basic Plagioclase Felspars ' [Geol, Mag. dec» 5, vol. vi 

 (1909) p. 251. 

 2 Ibid. p. 253. 



