Prof. T. G. Bonney — North-Western Chamwood Forest. 553 



To conclude : my recent studies in this part of Charnwood Forest 

 have impressed me more strongly than hefore with the close 

 resemblance of the ground-mass in the porphyroids, the Bardon rock, 

 ordinary and brecciated, and the brecciated rock in the middle part 

 of Ratchet Hill. The fragments also, which occur in the indubitably 

 pyroclastic rocks of the Maplewell series, differ but little from these. 

 They have led me to think that I erred in considering this Bardon 

 rock pyroclastic, to which I may add that of part of Ratchet Hill and 

 perhaps a little more in the north-western district. While the 

 microscope in some cases reveals structures which might well be 

 interpreted as indicative of a pyroclastic origin, these often prove on 

 further examination to be more probably consequences of some kind of 

 flow-brecciation. Besides this, increased experience of the microscopic 

 structure in the ground-mass of the above-named rocks, and in those, 

 such as lavas, where the origin cannot be doubted, have shown me 

 that they can hardly be due to the alteration of volcanic dusts, as 

 I once supposed possible, but that they agree, as said above, with 

 those characteristic of a rather glassy lava, either in the strict sense of 

 that term or as a shallow intrusive. How far their present structure 

 is original and how far due to subsequent devitrification I cannot at 

 present feel quite certain. The devitrification is not so conspicuous 

 as in the old rhyolites of the "Wrekin, Pontesford, the Bangor- 

 Llanberis district, St. David's and Boulay Bay (Jersey), and I incline 

 to regarding the microlithic felspar laths mentioned above as original. 

 So the Sharpley rock and its allies may very well have had a vitreous 

 base, but this point must be left for further study. 1 Other volcanic 

 districts, such as Auvergne, the Siebengebirge, and the Phlegrsean 

 Fields, prove that lavas varying considerably in texture and chemical 

 composition can be ejected from orifices in one and the same district, 

 and the Lipari Islands show us obsidians, in close proximity to 

 sub-crystalline rhyolites and andesites, and in one case even to rocks 

 which are almost basalts. Indeed, my specimens from these islands, 

 in which subsequent devitrification is improbable, show wider 

 differences than I have observed in Charnwood. 



1 I write this after spending a long time over rock slices from other districts 

 in my collection, and comparing those from Charnwood with others, like them 

 very ancient, in which the structure was less ' blurred ' by the formation of 

 secondary minerals, or in which any subsequent change was improbable. 

 There must, no doubt, be some micromineralogical secondary change in the 

 ground-mass, but the main question is, how far that ' spotty ' structure of 

 the Peldar rock, which disappears with crossed nicols, is a consequence of this. 

 This disappearance favours an affirmative answer, but I find a generally similar 

 structure exhibited by slices in a (purchased) collection of Hungary rocks, 

 where it must, I think, be primary, and it occurs in a slightly more pronounced 

 state at the Lower Quarry, Enderby (near junctions with a sedimentary rock 

 considered by Mr. A. J. Lowe to be Stockingford Shales), and in the two 

 quarries between Narborough and Croft. But in a slice labelled Thonstein 

 porphyr, Mohorn, near Freiberg, I find a ' spotted ' structure which mostly 

 disappears with crossed nicols, and in rock from the Wrekin district, which 

 must have been devitrified, I find indication of a very clear separation of the 

 more felspathic from the more quartzose part, which may be secondary. For 

 a discussion of devitrification, see Quart. Journ. Geol. Soc, vol. lix, p. 440, 

 1903. So at present I prefer to regard the question as still unsettled. 



