624 ME. H. H. ARNOLD-BEMROSE ON THE MICROSCOPICAL [Nov. 1 894, 



pseudomorphs. Plagioclase occurs in every thin section examined. 

 Augite has been found in all the sections, except No. 30. In some 

 of them is the rhombic pyroxene, and in others are secondary 

 calcite, serpentine, and chloritie aggregates which probably have 

 replaced the augite. Magnetite or ilmenite occurs in nearly all the 

 sections. The rock consists essentially of olivine, augite, plagioclase, 

 and magnetite or ilmenite. 



There are three main types of structure, olivine-dolerite, ophitic 

 olivine-dolerite, and divine-basalt. 



The olivine-dolerite occurs most frequently. It consists of augite 

 in grains, olivine in idiomorphic crystals, plagioclase giving lath- 

 shaped and tabular sections, and magnetite or ilmenite in rods and 

 grains. The Tideswell Dale Kock figured in TealPs ' British Petro- 

 graphy,' pi. ix. fig. 2, well illustrates this type, except that the 

 olivine is in many cases much less altered than in the figure. In 

 the least-altered specimens there is, as a rule, not a large amount 

 of interstitial matter. In outcrops 9, 56, 57 the groundmass some- 

 times consists of a small felt of felspar-laths often giving parallel 

 extinction, and similar to the microfelsitic base of the Tynemouth 

 dolerite figured by Teall. 1 



The ophitic olivine-dolerite consists of augite in ophitic plates 

 forming the groundmass, in which are embedded the idiomorphic 

 olivine, the plagioclase often giving large lath-shaped sections, and 

 magnetite or ilmenite. PI. x. in Teall's ' British Petrography ' illus- 

 trates this structure. The minerals in the least-altered specimens 

 of the Derbyshire rock are quite as well preserved as those in the 

 Scottish Tertiary dolerites. This structure occurs only in out- 

 crops 4, 6, 7 b, 39, 44, 45, 60, and Black Hillock 105. In only 

 one of these, namely 39, have I found the granular augite, and 

 in no specimen both the ophitic and granular augite together — 

 if we except a few cases in which some granules of augite are 

 penetrated by one or more felspars. When the ophitic augite 

 occurs, it generally forms the whole groundmass in all specimens 

 which have been collected from the outcrop. 



The olivine-basalt contains olivine and large augite-phenocrysts. 

 (In outcrop 14 the augite is often larger than the olivine, and 

 sometimes encloses a small crystal of it.) The phenocrysts of olivine 

 and augite lie in a groundmass of small felspar-laths, of augite in 

 small phenocrysts, grains, and prisms which give lath-shaped sections, 

 and of magnetite or ilmenite. There is little interstitial matter 

 present. The rock is in a very good state of preservation, all the 

 minerals being quite fresh, except that the olivine is sometimes 

 slightly altered along the cracks. The rock is a typical olivine- 

 basalt. A thin section containing augite-prisms and magnetite is 

 similar to one of the Dudley basalt figured in 'British Petrography,' 

 pi. xxiv. fig. 2, except that the Derbyshire rock contains olivine. I 

 have found this basalt only in outcrops 14 and 19, and in each 

 case its specific gravity is greater than that of the olivine-dolerite 



1 ' British Petrography,' 1888, pi. xii. 



