542 MK. V. RUTLEY ON SOME OF THK 



specimen collected, which is merely a small surface-chip, is a brown 

 to dark purplish-grey rock, showing a brecciated appearance when 

 examined with a pocket-leDS. 



The section has been taken at a depth of about an inch from the 

 weathered surface and shows, under the microscope, that the rock 

 is in great part composed of fragments of rhyolite (devitrifjcd 

 obsidian) which exhibit a delicate, well-defined fluxion-banding. 

 This banding is frequently sinuous, but some of the fragments show 

 markings which approximate to damascene structure. 



The fragments are completely devitrified, displaying a micro- to 

 crypto-crystalline structure Avhen viewed between crossed nicols. 

 There can, I think, be no doubt that they are fragments of devi- 

 trified obsidian. 



With regard to the material in which they are embedded a more 

 guarded opinion should be given. It is somewhat darker in colour 

 than the rhyolite fragments and contains numerous little crystals 

 and fragments of crystals of felspar, some of which show the re- 

 peated twin-lamellation of plagioclase and, in one good example, the 

 extinction-angle clearly indicates labradorite. Occasionally small 

 grains of quartz may also be detected on employing convergent 

 light. A little magnetite is likewise present. 



This matrix, in which the larger rhyolite-fragments lie, also 

 contains much smaller rhyolitic fragments, practically mere dust. 

 The section is traversed by a network of very delicate fissures, which 

 are filled with quartz and which cut through the rhyolitic fragments 

 and the substance in which they are embedded. 



V. CoNCLtrSIOXS WITH EEGAED TO THE FeLSITES. 



Looking at all the evidence afforded by the felsites described in 

 this paper and by others collected at the same time but showing 

 less marked characters, it seems that we have, in the Caradoc area, 

 a great thickness of rhyolites, probably associated with tuffs, which, 

 if of fine texture, might easily escape recognition in the field or 

 afford no clear j)roof of their origin even under the microscope. The 

 rock of Bowdler's Chair appears, in part at least, to be an unques- 

 tionable rhyolite-tuff, and as the adjacent felsites seem closely to 

 resemble those of Caradoc, it may, I think, be assumed that the 

 Caradoc and Hope Eowdler masses form part and parcel of one 

 great series of lavas and tuff's, which, judging from their thickness, 

 must once have been continuous over a wide area. 



Eefore attempting to examine these rocks, I was aware that 

 Prof. Blake * and Dr. Callaway t had alluded to the presence of 

 rhyolites among them, but since their writings contained no de- 

 tailed description which afforded evidence that these rocks were 

 true rhyolites, further investigation seemed desirable. In the 



* ' On the Monian and Basal Cambrian Eocks of Shropshire,' Quart. Journ. 

 Geol. Soc. \o\. xlvi. (1890) p. 386. 



t 'On the Unconformities between the Keck Systems underlying the Cam- 

 brian Quartzite in Shropshire,' p. 1*20 of this volume. 



