﻿40 



PROF. S. H. REYNOLDS ON THE 



[vol. lxxii, 



granite and gneiss. A series of analyses is quoted of the actual 

 rocks, and these are paralleled in each case with a statement 

 giving the supposed composition of the rock when unaltered. 



The contrast between the environment of the rocks in the two 

 localities is so great as to show that the richness in potash cannot 

 be due to similar causes. Thus the Bristol rocks are everywhere 

 associated with thick masses of Carboniferous Limestone, and, if 

 there had been any transference of material, might have been 

 expected to have had the percentage of lime increased. No granite 

 or other rock rich in potash is known to occur nearer than Malvern 

 on the one hand, and Dartmoor and Lundy on the other. Hence, 

 any elimination of lime and addition of potash, such as Prof. 

 Termier describes, can scarcely be contemplated. 



3. The Milton-Hill rocks seem to bear a considerable resemblance 

 to the ciminites of Dr. H. S. Washington, 1 both rocks containing 

 orthoclase, pyroxene, and olivine, and being devoid of hornblende and 

 biotite. The resemblance is brought out by the analyses quoted 

 on p. 38, particularly as regards the excess of potash over soda, but 

 the percentage of silica in a ciminite is distinctly higher than in 

 the Milton-Hill rocks, and Dr. "Washington estimates that as much 

 as 22 - 8 per cent, of anorthite is present. 



4. The Bristol rocks resemble in some respects members of the 

 somewhat variable series from the Yellowstone Park, described by 

 Prof. J. P. Iddings as absarokites. 3 The two groups of rocks 

 agree in the occurrence of abundant olivine and augite, in asso- 

 ciation with orthoclase and a variable amount of more basic felspar. 

 The silica-percentage in each case is low, that of the alkalies 

 moderately high with potash higher than soda. Some of the 

 absarokites contain leucite, others quartz. The rock from Raven 

 Creek, of which the analysis is quoted (p. 38), seems to come 

 nearest the Bristol rocks ; it is fine-grained, and consists of augite, 

 serpentinized olivine, magnetite, and orthoclase, with no leucite 

 and only a very little lime-soda felspar. 



Despite the unusual association of minerals in some of the rocks 

 which are the subject of this paper, it does not seem possible to 

 avoid the conclusion that the acid felspars are original, and hence 

 that these rocks are comparable to certain of the Exeter traps, to 

 the ciminites of Dr. Washington, and to certain of the rocks 

 described by Prof. Iddings. 



It will probably be best to term them orthoclase-basalts. 



1 Journ. of Geology, vol. v (1897) p. 349. 



2 Ibid. vol. iii (1895) p. 938. 



