550 Prof. T. 0. Bonney — North-Western Chamwood Forest. 



brownish red in colour with reflected light, of another which 

 similarly viewed is an earthy grey, sometimes almost white (when 

 it may be leucoxene), and of a minute clear mineral, possibly 

 representing a pyroxene. In this ground-mass larger grains are 

 scattered, also replaced by alteration products, probably once an iron 

 oxide, perhaps ilmenite, and a minute mineral, giving bright tints 

 with crossed nicols, sometimes apparently replacing a felspar. 1 One 

 or two imperfect phenocrystals of that mineral appear in the slice, 

 with a grain of quartz ; they are probably, it is almost certainly, 

 derivative. The secondary changes make it difficult to give a name 

 to this rock, but I think it probably a rather basic porphyrite, and 

 that in chemical composition it does not differ so much from the 

 other two as its structure might suggest. 



The microscopic structure of the ordinary Bardon Hill rock was 

 described in our papers of 1880 and 1891, so that I need only add 

 that, as stated above, I am now more impressed than formerly by the 

 close resemblance of its matrix, and even of most of the included 

 fragments, to that of the Peldar and Sharpley porphyroids. The 

 fragments, indeed, sometimes exhibit a fairly marked fluxion 

 structure, but the ground-mass of even these seems, with crossed 

 nicols, identical with that which surrounds them. Yet to the naked 

 eye they stand out quite plainly. For instance, in some trial holes 

 above the highest working on Bardon Hill, a variety was obtained 

 which is locally called the ' sultana rock ', because reddish spots 

 about as large as those raisins are scattered in the usual dull green 

 ground. But under the microscope we find it very difficult to make 

 out where the one begins and the other ends. 



The rock outcropping on the summit of Bar.don Hill so much 

 resembles an agglomerate that I formerly felt no doubt of its 

 pyroclastic origin, but my work in the pits lower down made another 

 examination necessary, in which I was aided by Mr. Rastall. The 

 presence of fragments is indubitable, for they project on weathered 

 surfaces from about a quarter to half an inch, subangular to rather 

 rounded in shape, and in diameter from less than an inch to about 

 four iuches. They were not, however, quite so large or so numerous 

 as my memory had suggested, being perhaps most abundant in an 

 outcrop, about forty yards from the tower on its eastern side. These 

 fragments, so far as I could see, represented both the ' purple 

 porphyritic ' rock and another fairly common in the breccia quarried 

 below. 2 The outcropping masses assume in weathering a rather 

 rounded outline, and one part, a few yards S.S.E. of the tower, 

 faintly suggests a stratification, dipping roughly N.N.E. at an angle 

 rather less than 45°. My friend admitted that a pyroclastic origin 

 seemed to be the more natural interpretation, but after visiting one 

 or two of the pits below he felt obliged to favour the fluxion-breccia 

 hypothesis. A slice from the matrix of a specimen, collected on this 

 occasion; proves that, if we allow for the effects of weathering, it is 

 indistinguishable from the specimens obtained lower down the hill. 



1 In that case it may be a zeolite, but Mr. Bastall suggests to me that some 

 grains more resemble dolomite. 



2 A pale red with dull green spots. 



