Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 623 



nine under 5 mm. Some of these are broken and corroded, and 

 some have zonal structure. They belong to the first generation, 

 and give lath-shaped or tabular sections. The majority of the smaller 

 felspars give lath-shaped sections. They are often twinned on the 

 albite plan. I have measured a large number of extinction-angles 

 of adjacent lamellae, and have obtained about fifty in which the 

 extinctions take place symmetrically with reference to the trace 

 of the plane separating the lamellae. These sections are out of the 

 zone perpendicular to the brachypinacoid (010). The felspars 

 occurring in the ophitic augite collected from seven localities give 

 the following angles of extinction for adjacent lamellae : — 



10° . 10° . 13° . 13° . 20° . 20° . 25° . 27° . 25° . 27° . 28° . 27° . 30° . 32° 

 10° . 11° . 13° . 17° . 20° . 22° . 25° . 28° . 30° . 29° . 30° . 33° . 35° . 37* 



All, except the first four, may be referred to labradorite or perhaps 

 to anorthite, and the two last to a plagioclase intermediate between 

 labradorite and anorthite. According to this test, therefore, the 

 majority of the felspars in the ophitic olivine-dolerites of Derbyshire 

 may be labradorite ; this agrees with the determinations of Schilling 

 for similar rocks of the Harz, though in the Derbyshire rocks some 

 of the felspars are probably bytownite. In a lath-shaped section 

 one individual extinguishes at an angle of 32°, and the other has 

 zonal structure, the outer portion extinguishing at an angle of 35°, 

 the inner at an angle of 45°, and the intermediate portions at 

 angles between 35° and 45°. This may be referred to anorthite. 



In the rocks with granular augite, from 16 slides I have obtained 

 26 cases of symmetrical extinction. In 22 of them the extinction- 

 angle between two adjacent lamellae varies from 42° up to 61°. 

 The maximum for labradorite is 62° 30'. Three others give angles 

 of 63°, 68°, and 70°, and may be referred to bytownite or anorthite ; 

 and one gives an angle of 75° or extinctions referred to the trace 

 of (010) the angles 35° and 37°, and may be referred to anorthite. 

 Several cleavage -flakes from a specimen near Tideswell Dale Quarry 

 (outcrop 17) in convergent light show an optic axis just outside the 

 field of view. They may be referred to bytownite. These results 

 confirm the view expressed by Teall 1 that the prevailing felspars in 

 the basic division of the normal plagioclase-rocks belong to the 

 labradorite- anorthite group. 



7. Structure of the Lavas. 



There are only eleven thin sections from eight outcrops which 

 contain no certain traces of olivine. These are from specimens of 

 rock which has suffered a certain amount of decomposition. In 

 fresher specimens from the same outcrops, olivine or its pseudomorphs 

 are found. In only one of them, namely, outcrop 15, have I seen no 

 traces of olivine. The rock is much weathered, and I have little 

 doubt that a less altered specimen would be found to contain olivine- 



1 ' British Petrography,' 1888, p. 147. 



