part 1] SPILTTES IX DERBYSHIEE. > 13 



serpentinous character, often with radial and spherulitic structures. 

 Iron-ores, both magnetite and ihuenite, are generally abundant. 



But, amid the general Avreck of the other constituents, tlie felspars 

 stand out conspicuously as much less altered. They retain their 

 crystal-form, though their edges are always ragged and corroded. 

 Their interiors, when original, are clouded witli alteration -products ; 

 but not seldom they are quite fresh and watei'-clear, apparentl}^ as 

 the result of a replacement of the original felspar-substance by 

 secondary material. 



In all the localities named above the felspathic element pre- 

 dominates, and not infrequently forms the bulk of the rock. It 

 occurs as phenocrysts, laths, or microlites, and sometimes in allo- 

 triomorphic interstitial fragments. In general, porph^^ritic struc- 

 ture is not very noticeable, and it is often absent. In a few 

 localities large felspar-phenocrysts abound [1; 13]. ^ In many of 

 the thin sections the felspar-content is seen to consist wholly of 

 laths of fairly uniform size, and in a few it is almost entirel}^ 

 microlitic [9; 11; 16]. The species, too, show a rather wide range. 

 Oligoclase appears to be always present among the laths and 

 microlites, and sometimes it is the dominant felspar. Its identity 

 is known b}^ the extinction-angles which, in sections normal to 

 the albite-lamellse, range from zero to 6° ; and by the refractive 

 index which, when it can be compared with that of the balsam, is 

 seen to be about the same, or a very little higher. More often than 

 not, comparison with the index of the balsam is obscured by 

 alteration -products in the felspar. Subordinately, a more basic 

 pltigioclase occurs among the laths, and this may generally be 

 referred to andesine or to an acid labradorite. When phenocrj^sts 

 of original composition are present, they are often more basic 

 than the felspar of the ground-mass, and belong to the andesine- 

 labradorite series. 



In addition to plagioclase there is undoubtedly a considerable 

 proportion of potash-felspar, which is assumed to be orthoclase 

 and is often of a secondary character, in these rocks. Sometimes, 

 when a large amount of replacement of the original laths has 

 taken place, it becomes the dominant species [2 ; 3]. This view 

 is borne out by both tlie microscopic examination and the analj'-ses. 

 Carlsbad twins extinguishing simultaneously when parallel with 

 the cross-wires occur in both phenocrysts and laths, and nntwinned 

 crystals extinguishing straight are seen in many of the sections. 

 Very low refractive indices, lower than that of the balsam, may 

 be seen in many of these individuals. Shapeless fragments of 

 felspar-substance, probably orthoclase, also occur interstitially in 

 some of the sections. 



Zoned felspar-crystals are not uncommon, a thin outer zone less 

 basic than the interior and with straight extinction being often seen 

 on the laths. 



^ Numbers in square brackets refer to the localities enumerated in the list 

 on p. 12. 



