634 MR. H. H. ARNOLD-BEMROSE ON THE MICROSCOPICAL [Nov. 1 894, 



Opposite the Mill the structure is distinctly spheroidal. Farey says 

 that it has a " boily and nodular texture " ' here and near Bakewell. 

 At the latter place the rock is a dolerite, but here it is a tuff. 

 The spheroids vary from 2 feet down to 1 inch in diameter. 

 Some of the larger ones have several coats or shells, and others 

 are divided into three or four pieces by irregular joints. At 

 first sight the rock might easily be taken for a basalt, but a closer 

 examination shows the spheroids to be composed of a grey rock 

 with green spots, which under the lens are seen to be amygdaloidal 

 lapilli. Prof. Bonney mentions a similar instance of spheroidal 

 structure in a volcanic ash in the Italian Tyrol, which at a short 

 distance might readily be mistaken for a decomposing basalt. 2 This 

 is the only occurrence that I have found of spheroidal tuff in the 

 county. There are spheroidal blocks in the tuff near Kniveton, 

 but these are very different (see outcrop 56, p. 638). 



Proceeding towards Winster the rock is more massive and often 

 laminated, contains pebbles of limestone, and also a small bed or 

 else a collection of blocks of dolerite. 



Near Shothouse Spring there is at least 10 feet of tuff exposed. 

 The layers vary much in coarseness and thickness, and there is no 

 spheroidal structure. The spring issues from the junction of the 

 tuff and the limestone above it, flowing along the top of the tuff- 

 bed. The latter is almost impervious to water, and when a speci- 

 men is dried it is seen to be made up of decomposed lapilli. About 

 100 yards farther north is a limestone-quarry. A bed in it thins 

 out from several feet in thickness, and consists of lumps of a reddish- 

 coloured limestone containing a few small lapilli. 



In a specimen from opposite the Mill (sp. gr. 2-64), the rock 

 consists almost entirely of lapilli. They vary in size and shape, 

 and are very tender and delicate, often having several branches. 

 In reflected light the lapilli and vesicles appear to be coated with 

 a porcelain-like grey material, which makes up the bulk of the 

 rock. In transmitted light a coffee-brown colour borders the larger 

 lapilli, coats the inside of some of the vesicles, and constitutes the 

 whole of the smaller lapilli. It is isotropic, and when magnified 600 

 diameters is seen to contain small globulites and is often bordered 

 by a thin layer of an almost black colour, probably magnetite (see 

 PI. XXIV. fig. 6). The inner portions of the larger lapilli. and the 

 vesicles consist of a light yellow material which has a feeble action 

 on polarized light. The structure is fibrous, and the fibres are often 

 in bundles, but it sometimes shows aggregate polarization, and is 

 often bordered by a band of globulites. The interior of some lapilli 

 consists of crystalline calcite and serpentine. Felspar occurs very 

 seldom. There are only a few lath-shaped sections, seldom twinned, 

 and all (except one of them) extinguish parallel or nearly parallel 

 with their length. There is a very little calcite filling some of 

 the space between the larger lapilli : the remainder of the space is 



1 ' Agriculture of Derbyshire, etc' vol. i. (1811) p. 278. 

 - ' On Columnar, Spheroidal, and Fissile Structure,' Quart. Journ. Geol. 

 Soc. vol. xxxii. (1876) p. 140. 



