88 ME. E. H. WOETH OX THE [vol. lxXV,. 



igneous ' rock at LXXYI, S.E. 59 «, in the L. & S.W. Railway 



quariy on the north side of the vein. It is in contact with the 

 south side of a helt of ' dark igneous ' rock on South Down at 

 LXXYI, S.E. 76, and occurs freely as inclusions in the same 

 rock. The radiolarian beds have not been carefully traced in 

 the field, but that from 76 would appear to follow the general 

 strike, and it has been found a little west of LXXYI, S.E. 11, 

 on the Redaven. All these cherts so far examined show the finely 

 laminated character referred to by G. J. Hinde & Howard Fox, in 

 their paper on * Radiolarian Rocks of the Lower Culm Measures/ 1 

 The radiolaria are best preserved in those specimens which come 

 from contacts with or inclusions in the * dark igneous ' rock, and 

 the recognizable examples would appear to belong to the genus 

 Cenosphcera (PL Y, fig. 6). Casts of radiolaria also occur at 

 LXXYI, S.E. 56, at the contact with the northernmost exposure 

 of ' dark igneous ' rock in the L. & S.AY. Railway quarry. 



Y. The Igneous Rocks. 



The felsite with micropegmatite phenocrysts. — This 

 in the hand-specimen is buff-coloured (pale lead-coloured when 

 (mite fresh), usually dotted with small grey specks. It is readily 

 discriminated, by its texture and colour combined, from all the 

 other rocks of the locality. It does not appear to correspond 

 precisely with any of the felsites described as from Sourton Tors 

 by C. A. McMahon (5), although his No. 16 may be this rock 

 altered by contact with the later igneous rocks of the Tors. The 

 best exposure is at LXXYI, S.E. 25, on the eastern bank of the 

 AYest Okement, and about 200 yards north of Elushcombe foot. 

 At 25 the rock comes into contact with the 'dark igneous' series, but 

 south of 25 there is a considerable mass which is wholly unaltered 

 from this cause. The ground-mass is cryptocrystalline, with local 

 variations in texture, and it is sparsely sprinkled with irregular flakes 

 of green mica, much of which is now wholly chloritized. Euhedral 

 and subhedral felspar occur [is phenocrysts : some is devoid of 

 all twinning, but more is twinned in an irregular and impersistent 

 manner in pericline lamellae. The whole of the felspar has a 

 refractive index lower than that of quartz. 



The quartz frequently exhibits very neat rhombic sections, 

 and rarely falls below subhedral outline ; inclusions of the ground- 

 mass are frequent, sometimes flask-shaped and communicating with 

 the outside by the neck of the flask. Xeedles of rutile occur in a 

 few of the crystals, and all show cavities, usually empty so far as 

 fluid is concerned, occasionally in the form of negative crystals. 



The phenocrysts of micropegmatite are more or less irregular ; 

 but in most of them the dominant mineral, be it quartz or 

 felspar, lias at least some external crystal faces (PI. YI, fig. 7). 



It should have been noted that the quartz shows a well-marked 



1 Q. J. C4. S. vol. li (1895) p. 632. 



