Vol. 50.] STKTTCTT/RE OP CARBONIFEROUS DOI/EEIIES AND TUEES. 621 



130°, and 1 30°. In convergent light an optic axis is often seen to 

 be outside the field of view. Other specimens from the same out- 

 crop do not contain this mineral ; it is probably altered to calcite. 

 The rock also contains pseudomorphs of olivine, two generations of 

 felspar little altered, magnetite or ilmenite, some interstitial matter, 

 various alteration-products, but no traces of a monoclinic pyroxene. 



Sandy Dale, Outcrop 24. — Some sections of pyroxene in a large 

 group are yellow, and others pale green by transmitted light. The 

 former polarize in a bright yellow, and the latter in a dark grey tint. 

 The former are dichroic, giving two shades of yellow, and the latter 

 two of green . The former show in a convergent light the arm of a 

 biaxial figure and sometimes coloured rings, the latter are nearly 

 perpendicular to an optic axis. This mineral often occurs in groups 

 of two or more individuals : three slides out of the four contain it. 

 In the Sandy Dale rock are pseudomorphs of olivine, but there is 

 no augite, and it differs from the Staden Low rock in not always 

 having two generations of felspar. On the whole, the rhombic- 

 pyroxene is found more often in groups than in the specimens of 

 the Eyecott Hill rock which I examined. It possesses not only 

 a better crystalline outline, but behaves more often as a crystal, and 

 unlike an aggregate pseudomorph, the whole extinguishing together. 

 The cleavage is not so well marked, and the fibrous and confused 

 structures are present in a less degree, and only in the more altered 

 specimens. 



A small portion of the Sandy Dale rock was pounded and passed 

 through a sieve of 80, but was stopped by one of 120 meshes to the 

 inch. A number of fragments of a slightly dichroic green mineral 

 were thus obtained. One only gave a negative bisectrix, with a 

 small angle between the optic axes. This is probably bastite. 



The rhombic pyroxene also occurs in some specimens of the Cave 

 Dale rock, outcrop 2, whilst in others there is comparatively 

 fresh augite in grains. Small traces of it are also found in outcrops 

 23, 27, 28, 31, 37, and 48, and in 14 at the entrance to Chee 

 Tor Tunnel. 



We have, therefore, in these rocks a rhombic pyroxene with 

 cleavage poorly developed, which in some cases is altered to bastite. 



5. Augite. 



This occurs in large ophitic plates, large and small phenocrysts, 

 small irregular grains, and in prisms which give lath-shaped 

 sections. 



The ophitic plates vary in size from 7*5 mm. in length and from 

 5 mm. x 2*5 mm. downwards. The large plates enclose many felspars 

 and also olivine-crystals, which are more or less altered along the 

 cracks, though the augite appears quite fresh. This structure runs 

 throughout the slide, so that the whole of the augite has crystallized 

 last and formed the groundmass. Although these plates have no 

 crystalline boundary, cleavage-cracks are often well developed in 

 them. In some sections the sets of cracks are nearly at right 



