﻿Vol. 64.] ST. DAVId's-HEAD ' EOCK-SEEIES.' 293 



(3) The}' do not represent a composite intrusion, but simultaneous 

 intrusions of an imperfectly-rnixed magma. 



(4) There is no evidence of differentiation in situ, the facts 

 suggesting a common origin from a differentiated magma-basin. 



(5) The aplite-veins may represent the most acid phase of this 

 differentiated magma. 



(6) Petrographically the rocks are of considerable interest, as 

 exhibiting types not very commonly occurring in the British Isles. 

 They also afford unusual facilities for the study of both rhombic 

 and monoclinic pyroxenes, and appear to throw light upon the 

 origin of the sahlite-striation in the latter. 



(7) The probable age of the intrusions is not greater than that 

 of the earth-movements which folded the Arenig strata in this 

 district. 



(8) The observations recorded in the foregoing pages seem to 

 point to the conclusion that acid streaks and cores in basic igneous 

 rocks may not always be due to differentiation in situ. 



In conclusion, I desire to express my obligation to my son, 

 Mr. A. Y. Elsden, JB.Sc, F.I.C., for invaluable assistance in carrying 

 out the chemical work connected with this paper. 



EXPLANATION OF PLATES XXIX-XXXIL 



[All the figures are taken with a 1-inch objective, ordinary light.] 



Plate XXIX. 



Fig. 1. Bi tite-norite, Carn Llidi, showing rhombic and monoclinic pyroxene, 

 plagioclase and iron-ore. Ehombic pyroxene occupies the centre of 

 the field, and is mottled owing to alteration. (X 15 diameters.) See 

 p. 279. 



2. The same, showing biotite in the centre of the field, and in the lower 



right-hand corner. ( X 20 diameters.) See p. 288. 



3. Bronzite-diorite, Carn Llidi, showing a large bronzite-crystal on the 



right-hand margin, and augite developed into hornblende in the lower 

 left-hand corner. Tlie remainder of the field is occupied with augite, 

 plagioclase, and opaque iron-ore. (x 20 diameters.) See p. 282. 



4. Diabase, Carn Llidi, showing augite, plagioclase, and opaque iron-ore. 



The augites show basal striation, with patchy schillerization. ( X 15 



diameters.) See p. 282 

 6. Quai'tz-diabase, St. David's Head, showing a large augite-crystal in the 



lower right-hand corner, with a fringe of hornblende on the left 



margin. A large plagioclase-crystal occupies the top left portion, 



and tliere is some interstitial micropegmatite. (X 20 diameters.) 



See p. 281. 

 6. Soda-aplite, Pen Lledwen, showing albite, quartz, and a little iron-ore. 



(X 15 diameters.) See p. 283. 



Plate XXX. 



Fig. 1. Quartz-xenocrjst, with augite-fringe, from the marginal zone. (X 20 

 diameters.) See p. 276. 



2. Marginal zone a little f.'.rther in than fig. 1, showing the development 



of enstatite towards the left centre. (X 15 diameters.) See p. 276. 



3. Granophyric variety, St. David's Head, showing zoned felspar, fringed 



by micropegmatite. (X 20 diameters.) See p. 285. 



