342 KEY. E. HILL AND PEOF. T. G. BONNET ON THE 



about on the same horizon ; and though the former are much less 

 uniform than the latter, containing well-marked agglomeratic beds, 

 yet parts may be found even here very closely corresponding "with 

 the typical Peldar-Tor rock. On that hill fragments seem to have 

 become rare and small; but the crystals of quartz and felspar are 

 rather larger. 



This rock of High Sharpley is one presenting several difficulties. 

 Its ground-mass is of a purplish grey colour, and is compact, much 

 like a felsite, containing numerous crystals (often fairly perfect) 

 of quartz and felspar, sometimes as much as ^ inch in diameter, 

 but generally rather less. It has a distinct though imperfect 

 cleavage, the surfaces being wavy (doubtless owing to the resistance 

 of the included minerals). These surfaces have a very faint satiny 

 lustre, giving the rock at first sight a slightly schistose aspect. 

 Cleaved f elsites do, indeed, sometimes look rather schistose ; but the 

 latter structure is usually very local ; while the thickness and extent 

 of this Sharpley rock is considerable, and its character uniform*. 

 Purther, there is in its aspect something hardly to be described in 

 words, which to the practised eye suggests a doubt of its igneous 

 origin. Here and there we note a very faint indication of a frag- 

 mental structure ; and at the western end of the southern ridge, and 

 again about halfway along it, the rock becomes unquestionably 

 clastic, containing in an ashy matrix fragments of a purplish por- 

 phyritic felsite, which is itself extremely like the typical Sharpley 

 rockf. 



Eight microscopic slides have been prepared from different parts 

 of the ridges of High Sharpley, and four from other outcrops in the vi- 

 cinity. The ground-mass of these is often, at first sight, remarkably 

 like that of an acid lava, as it consists of a transparent base, crowded 

 with microliths of opacite and ferrite, with epidote and (?) sericite, 

 generally rather irregular in form. The first and second are so 

 arranged as to give the slide a more or less granular structure, 

 which in some cases is well defined, and then the ground-mass is 

 more translucent. In one or two of the slides a vague indication 

 of fragmental structure can be discerned. In this ground-mass are 

 scattered crystalline grains of quartz and felspar (small and large), 

 of iron peroxide, and of epidote and viridite replacing some other 

 mineral. With crossed Nicols the rather granular aspect of the slide 

 nearly or quite disappears, and the field resembles a glass crowded 

 with inumerable microliths, mostly, if not all, felspar, often rather 



* We hare received from a friend a partial analysis of this rock, which may 

 give a general idea of its composition : — 



SiOo =6805 



FeAJ 



2X3 I 26-23 



CaO : 1-28 



MgO 110 



Alkalies&c 3-34 



10000 



t The same rock is found on Eatchet Hill and elsewhere : see p. 344. 



