﻿36 



PROF. S. H. REYNOLDS ON THE 



[vol. lxxii, 



iniddle of the mass had specific gravities of 2 - 72 and 2 - 74, while 

 that of a specimen from the eastern end was 2 - 67. The chemical 

 analyses quoted on p. 38 (though the percentage of magnesia and 

 soda is somewhat low for a hasalt, while that of the potash is high) 

 do not hear out the idea of a spilitic relationship, the soda especially 

 being far too scanty. The occurrence of well-terminated pseudo- 

 morphs in carbonate after olivine in certain parts of the Spring- 

 Cove lava is also inconsistent with a spilitic relationship. The 

 felspars are, in every case, too much weathered for the extinction- 

 angle to be ascertained with accuracy, but they appear to extinguish 

 straight. In none of the nine slices examined could any recog- 

 nizable pyroxene be detected. Much brown glassy material is 

 present in the ground-mass. 



Mikton Hill. — The westernmost exposure is that mentioned 

 by Dr. Strahan 1 as occurring 160 to 190 3 r ards north of Milton- 

 Hill Lodge* at the eastern margin of Weston Woods. Here a 

 compact, black, only slightly vesicular rock (97) occurs in the 

 path along the edge of Furze Close ; but the exposure is so bad 

 that it may easily be overlooked. The specific gravity (2 - 78) is 

 higher than that of any other rock of Milton Hill. The percent- 

 age of silica and alkalies (see Table I, p. 38) is that of a normal 

 basalt ; and the agreement between this rock and those in Goblin' 

 Combe is close, both chemically and mineralogically. It has a 

 rather fine-grained ground-mass, composed of felspar -laths with 

 an extinction-angle of 10° to 15°, magnetite and granulitic augite, 

 with phenocrysts of fresh augite and of idiomorphic olivine replaced 

 by green serpentine. The chemical characters and extinction-angle 

 suggest that the felspar is probably andesine. The rock differs 

 considerably from any other seen on Milton Hill. 



The next exposure lies north-north-east of Milton-Hill Lodge at 

 the point 98 in the sketch-map (fig. 6, p. 30), and has already been 

 mentioned as having been made in 1911 in the course of erection 

 of a cistern. No trace of this exposure is now to be seen. The 

 rock here is brown, considerably weathered, and rather strongly 

 amygdaloidal. In a hand-specimen it bears a general resemblance 

 to that from Limeridge Wood, Tickenham, which it also resembles 

 as regards specific gravity (2"66 to 2-68) and silica-percentage 

 (42-21). Of alkalies Mr. Kadley found 2'89 per cent, of potash 

 and 0 - 68 of soda. The resemblance to the Limeridge-Wood 

 rock is further brought out in a thin section, particularly as 

 regards the relatively-large size of the felspar-laths in comparison 

 with those in the Spring-Cove rock and that of the cross-roads, 

 Milton Hill. The felspars are too much weathered for anything 

 to be made out with regard to the extinction-angle. Olivine in 

 idiomorphic crystals, mainly replaced by a very pale serpentine, 

 is abundant. 



1 ' Geology of the South Wales Coalfield : Part III— The Country around 

 Cardiff' 2nd ed. Mem. Geol. Surv. 1912, p. 29. 



