•"3-1:0 MR. F. KUTLEV OX SOME OF THE 



IV. The Pelsitic Series. 



The Felsitic Series in the Caradoc area is a very important one, 

 but extremely difficult to work out under the microscope, since the 

 structures characteristic of rhyolites are, in most cases, obscure, so 

 obscure, in fact, that, unless exceptionally thin sections are ex- 

 amined, they may often completely baffle detection. After careful 

 examination, however, it has been possible to recognize not only 

 spherulitic structure and occasionally bands of spherulites, but also 

 perlitic structure. The latter is very obscure in the best examples, 

 but is sufficiently marked to prove that the structure is present. 



It may be fairly well seen in a section made from a specimen 

 collected a little above Caradoc Coppice, near the southern end of 

 the hill and on its north-western llank. Very faint indications of 

 the structure were first seen in this and in one or two sections from 

 other spots in the neighbourhood. The sections were then reduced 

 in thickness, and, in the thinnest portions of them, perlitic structure 

 was found to be unquestionably present although still obscure. In 

 ordinary transmitted light it is less easy to detect than between 

 crossed nicols, and in the latter case it is rendered more apparent 

 by a rapid rotation either of the section or, if a Dick microscope be 

 used, of the nicols. The reason of this appears to be that the 

 crystalline grains which lie in or along the perlitic fissures are, as a 

 rule, slightly larger than those which constitute the main mass of 

 the rock and that, although the optical orientation of the different 

 grains along any one perlitic fissure is very diverse, yet on rapid 

 rotation either of the section or of the crossed nicols the maximum 

 illumination of one grain in the series is so quickl}- followed by the 

 maximum illumination of each succeeding grain that the retina 

 retains these impressions sufficiently long to receive the general 

 impression of a narrow ring or of a number of narrow rings more 

 brilliantly illuminated than the remainder of the section. Without 

 having recourse to rotation these rings can, however, still be seen 

 (PL XIX. fig. 5). 



Spherulites are somewhat plentiful in these rhyolitic rocks. They 

 are usually small, but not difficult to detect even under low powers. 

 For the most part they are irregularly distributed, but in some of 

 the sections examined they are more closely massed and occasionally 

 coalesce in irregular bands. 



Setting aside the devitrification which these rocks have experienced, 

 we have their exact counterparts in many spherulitic obsidians of 

 comparatively recent date, notably in those of the Yellowstone 

 District, especially in some which occur near the Madison Hiver. 



Ordinary fluxion-banding appears to be very poorly represented in 

 the Caradoc rhyolites. Evidence of such structure has been better 

 seen on the ground than under the microscope. On the S.E. flank 

 of Caradoc, a little to the south of the Camp and about half way up 

 the hill, there is, for instance, an outcrop of felsite, on the weathered 

 surface of which there is a well-marked banding visible. At this 



