

Vol. 50.J STRUCTURE OP CARBONIFEROUS DOLERITES AND TUFFS. 625 



with granular augite which makes up the remainder of the outcrops 

 (except the tuff underlying No. 19). 



In some places and outcrops the rock is so much altered that it 

 might be called an olivine-diabase, or diabase-mandelstein. Even 

 in the same outcrop it may be a dolerite little altered in one place 

 and an olivine-diabase in another, while a further stage of de- 

 composition results in a more or less granular, clayey material 

 containing spheroids of diabase. I have described it as an olivine- 

 dolerite in the Table (p. 608). 



Part II. — The Fragmental Roces or Tuffs. 



The tuffs cover a much more extensive area than had been pre- 

 viously supposed. Two outcrops, namely, those at Ashover and 

 at Hopton, are mentioned in the Geological Survey memoir. In 

 addition to these I have found eleven others. I am not aware that 

 any of them have been microscopically described, and for this 

 reason and because in most cases they have undergone so little alte- 

 ration a detailed description of them is here given. Speaking 

 generally, petrographers have not bestowed so much consideration 

 on the fragmental igneous rocks as they have on the massive ones. 

 In the field, it is sometimes difficult to distinguish a tuff from a 

 decomposed amygdaloidal dolerite. A method that I have found 

 very useful is to carry a small file for filing the edge of the specimen. 

 After it is thus prepared and wetted, it is easy with the aid of a 

 lens to make out the lapilli in a fragmental rock. 



The tuffs occur in all parts of the district, north, south, east, 

 west, and centre, and may be divided into a northern group and a 

 southern group, like the lavas. The northern group includes out- 

 crops 1, 7 a, 8 b, 16, 18, 19, 34, and the southern outcrops 39, 46, 

 53, 54, 56, 58. In 19 and 39 the layers of tuff are succeeded 

 immediately by a lava-flow. In 7 a and 8 & we have a lava-flow 

 and tuff-beds separated by bands of limestone. In 7 b the lava is 

 uppermost, while in 8 b it is the lower bed. 



Castleton, Outcrop 1. — This is seen in a field behind Goose Hill 

 Hall, towards Speedwell Cavern. It is triangular in shape, and 

 forms a ridge about 80 feet in length. The outcrop is almost 

 entirely covered with grass. The rock is of two kinds, one soft and 

 loose in texture, made up of fragments : a tuff ; the other, probably 

 blocks in this tuff, of a light-grey colour, hard, and weathering very 

 much like limestone. With the aid of a lens, small felspar-crystals 

 may be seen. 



Two specimens of the compact rock were examined (sp. gr. 2 - 67 

 and 2-66). Large calcite-pseudomorphs after olivine occur. They 

 often contain portions of the base with small magnetite-grains, and 

 in one case two felspar-laths. The felspar occurs in a few rhombic 

 sections, also in large and small lath-shaped sections, often ragged 

 or forked at the ends, and in skeleton-crystals. The laths generally 

 have parallel extinction, and the mineral is partly or wholly altered 



