Yol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 635 



occupied with smaller lapilli. In some other specimens there is a 

 felspar-like material here and there in the lapilli which is biaxial and 

 may be secondary albite, and- the outer vesicles of a larger lapillus 

 sometimes contain smaller lapilli. 



A section cut from one of the spheroids (sp. gr. 2 - 45) shows a 

 similar structure, the only difference being that the brown lapilli 

 are not quite isotropic, and contain more skeleton-crystals of felspar 

 and crystallites. Another specimen consists of yellowish-green 

 lapilli, containing no crystals, and with a thin black border. The 

 vesicles are filled with calcite surrounded by a black border, or 

 with the black material only. Under a high power they often 

 consist of masses of globulites, and while sometimes free from 

 colour are in other cases stained by iron oxide. A section from a 

 bed of fine tuff near Shothouse Spring (sp. gr. 2*46) consists of small 

 lapilli in a cement of calcite. They are water-clear, yellow, brown, 

 or black with magnetite, and generally isotropic. Several of them 

 contain what may be possibly pseudomorphs of olivine. The 

 amygdaloids consist of fibrous material radially arranged. 



A specimen of the rock from the quarry (sp. gr. 2-66) consists 

 of a limestone containing amygdaloidal lapilli altered to calcite, 

 and bordered with iron oxide. The limestone contains haematite. 

 Of the dolerite, two specimens have been examined microscopically. 

 One (sp. gr. 2*78) contains pseudomorphs of serpentine after olivine, 

 fresh augite in small grains, and unaltered plagioclase. The other 

 is a similar rock but more decomposed, none of the minerals being 

 fresh (sp. gr. 2-61). 



Hopton, Outcrop 53. — This exposure is opposite Hopton Hall 

 grounds, on the road from Wirksworth to Carsington, where that 

 from Hopton Wood joins it. On a cursory examination the rock 

 might be taken for a brecciated limestone, due to the decomposition 

 of the calcite which forms the cementing-material of the lapilli. It 

 is described by Farey 1 as a ' brecciated toadstone,' and in the Survey 

 Memoir as "a very coarse brecciated ash, with beds of dolerite." 2 

 The fragments vary much in size, from 2 feet in length down to the 

 size of a pea and smaller. Some are more or less rounded, but the 

 majority are angular. Where the surface of the rock is weathered, 

 the larger angular pieces project and give it a rough appearance. 

 I have been unable to find any trace of a dolerite sheet. Wherever 

 larger pieces of rock are seen, they are found to be included blocks. 

 Sometimes a face of the rock extending for several yards will look 

 like a massive rock, but when broken into proves to be made up of 

 small fragments. The rock is exposed for about 200 yards along 

 the road to Hopton Wood, and the lower parts are made up of the 

 same material as the upper. The finer-grained parts in a hand- 

 specimen are seen to consist of a black fine-grained dolerite, with 

 felspars similar to those of the larger blocks, and of green lapilli. 



A thin section from a piece of the dark rock, which measures 

 2| x 2 x 2 inches (sp. gr. 2*68), contains olivine, augite, felspar, and 

 1 Op. jam cit. p. 278. 2 P. 24. 



