200 E. HILL AND T. G. BONNEY ON TIIE 



Highly altered Rocks, (5) Fragments from Breccias. Repeated 

 comparisons have been made between the less and more highly 

 altered varieties : and these again have been compared on the one 

 hand with sections of volcanic breccia from the Tyrol and Scotland, 

 and from the closely allied Borrowdale series of the Lake District, 

 and on the other with a considerable collection of lavas, ancient 

 and modern, from France, Germany, Italy, and Hungary. After 

 the greater part of the work was completed a series of devitrified 

 Pitchstones from the Wrekin was obtained, which was of great help 

 in deciding some doubtful points. 



(1) Quartzites. — Rocks mainly composed of indubitably rolled 

 fragments of quartz are rare, as is shown in Part I., in the Forest, 

 though grains of this mineral are, to a greater or less extent, pre- 

 sent in several varieties. Of the latter, however, we shall speak 

 further on ; at present we restrict ourselves to those rocks which 

 are evidently altered sandstones. Three sections have been ex- 

 amined — one from the Stable Quarry in Bradgate Park (p. 763*), 

 another from the quarry near Steward's-Hay Spring (p. 769), and 

 the third from near the Brande House (p. 760). 



These rocks all consist mainly of rounded and subangular grains 

 of tolerably clear quartz in a matrix of smaller quartz grains with 

 black dust (? magnetite) and a green mineral, which, on examina- 

 tion, seems to be probably partly viridite, partly a chloritoid 

 mineral. Besides the quartz there are a few other grains not easy to 

 determine ; some resemble a fine quartzite, some indurated slate, 

 some lapilli. 



In the Stable-Quarry specimen the quartz grains average about 

 •02 inch in diameter, and are not very clear f; the green mineral 

 sometimes seems to penetrate a slight distance into them. In that 

 from Steward's-Hay Spring the granules are smaller, a little more 

 angular, and clearer, and the black mineral predominates over the 

 green. In that from Brande House the grains are rather more 

 irregular in form than in the first, and there are more fragments of 

 other rocks, some certainly indurated slate. On the whole, micro- 

 scopic examination renders it probable that these three beds are on 

 the same horizon, as we have suggested in Part I. (p. 784). 



(2) Slaty Bocks. — These are very abundant at various horizons 

 in the Forest. As the structure of the roofing-slates does not seem 

 exceptional, we have not examined them microscopically, but have 

 selected one or two of the banded slates as likely to throw light 

 upon the origin of the materials, and of the highly indurated com- 

 pact slates in order to study their metamorphism. 



Banded Slate near Ulverscroft Mill. — Felstone-like variety de- 

 scribed in Part I. (p. 767). Ground-mass semitransparent, minutely 

 granulated ; darker parts earthy, and frequently showing a streaky 

 structure; lighter parts apparently comminuted felspar crystals, in form 



* These references throughout are to Part I. of this paper in vol. xxxiii. of this 

 Journal. 



t The numerous extremely minute dark microliths show an approach to a 

 banded arrangement in the several grains. 



