The St. David' s-H'ead Rock-Series. 579 
imperfectly-mixed magma. There is no evidence of differentiation 
in situ, but the facts suggest a common origin from a differentiated 
magma-basin. The aplite-veins may represent the most acid phase 
of the differentiated magma. 
Petrographically the rocks are of considerable interest, as 
exhibiting types not very commonly occurring in the British 
Islands. 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 of the latter. Rhombic pyroxene 
generally crystallized earlier than the monoclinic pyroxene, but 
sometimes these relations are reversed, and often the two forms are 
crystallographieally intergrown, sometimes as twins. There are 
two distinct varieties of augite, distinguished by the presence or 
absence of a basal striatum. The relation of these two types lends 
support to the perthitic theory, that there is an ultra-microscopic 
erystallographic intergrowth of rhombic and monoclinic pyroxene. 
The probable age of the intrusions is not greater than that of the 
earth-movements which folded the Arenig Shales in this district. 
The observations recorded in the paper seem to point to the 
conclusion that acid streaks and cores in basic igneous rocks may 
not always be due to differentiation in situ. 
February 19th.— Sir Archibald Geikie, K.C.B., D.C.L., Sc.D., 
Sec.R.S., President, in the Chair. 
The following communications were read : — 
1. ' The Two Earth-Movements of Colonsav.' Bv William Bourke 
Wright, B.A., F.G.S. 
The supposed Torridonian rocks of Colonsav exhibit in their 
folding and cleavage the effects of two movements analogous in 
their results to those proved by Mr. Ciough in the Cowal district 
of Argyll. Not only the planes of the first or slaty cleavage, but 
also the quartz-veins formed along them have been folded by the 
second movement, and may be observed to be crossed at considerable 
angles by the cleavage produced during this second movement. An 
extensive series of lamprophyre-dykes, obviously later than the 
first cleavage, are found to be folded and cleaved by the second 
movement. Moreover, some of these dykes traverse and are chilled 
against a mass of syenite, which can also be proved to be later than 
the first cleavage. The distinctness of these two movements is, 
therefore, considered to be completely established. The second 
cleavage being of the nature of strain-slip, its development along 
the axial planes of the folds is of interest and is brierly discussed. 
2. ' Sotes on the Biver Wev.' Bv Henry Burv, M.A , F.L S 
F.G.S. 
The part of the Biver Wey within the Wealden area is divided 
