330 Reports and Proceedings — Geological Societij of London. 



these fossils are obtained 40 feet below the upper surface of the 

 Gault seen in the section, it is clear that the whole of the Upper 

 Gault of Cambridge was not used up in the making of the 

 ' Cambridge Greensand ' ; and this fact, together with the northward 

 thinning of the Gault as it passes into the Red Chalk, necessitates 

 a modification in the view commonly held as to the origin of this 

 ' Greensand ' deposit. 



2. " On the Age of the Llyn Padarn Dykes." By James Vincent 

 Elsden, Esq., B.Sc, F.G.S. 



The paper produces evidence which seems to suggest that the 

 bulk of the greenstone dykes of this area belong to an earlier period 

 of eruption than has been generally assigned to them ; and there is 

 proof that some of them may even be older than the quartz-felsite 

 of the Llyn Padarn ridge. The greater part, if not actually of Bala 

 age, seem to have been intruded before the great post- Bala crush- 

 movements, which produced the folding of the Lower Cambrian 

 rocks of Llanberis, had entirely ceased. At the same time, the 

 evidence does not exclude the possibility that some of the intrusions 

 may be of later date. The evidence on which these conclusions 

 rest is based mainly on the signs which the intrusions exhibit 

 of having been considerably modified by earth-pressures, more 

 especially in those portions which protrude into Cambrian strata. 

 Petrographical considerations, also, make it impossible to separate 

 these rocks from the diabase sills of Bala age occurring farther to 

 the south and south-west of this area ; and there is a strong pre- 

 sumption that they represent the last residuum of the magma from 

 which Bala sills were derived. 



The north-western portions of the dykes, enclosed in the older 

 rocks of the Llyn Padarn ridge, are comparatively free from 

 dynamic metamorphism ; but when traced into the more yielding 

 Cambrian grits and slates, they become structurally deformed and 

 often so highly sheared as to be hardly recognizable as parts of 

 the same dyke. It is suggested that such highly sheai'ed greenstones 

 as occur in the ridge are of still older date. One section is described 

 in which a sheared greenstone is pierced by a tongue of felsite 

 about 2 inches wide and 2 feet long. The felsite is undistinguishable 

 from that of the rest of the ridge and on the borders of the green- 

 stone. Full petrographical descriptions of the minerals of the 

 rocks in their altered and unaltered state are given, the minerals 

 being taken in the order of their consolidation, and the rocks con- 

 sidered in the 'dynamic' or crush-zone of the sediments and in 

 the ' static ' or pressure-zone of the Llyn Padarn ridge. These 

 minerals are apatite ; iron-ores altered to sphene and leucoxene, 

 and to a mineral which is apparently perowskite ; felspars 

 belonging to the albite-anorthite series of one generation under- 

 going ' albitization,' and the formation of felspar-mosaic ; two 

 pyroxenes, one possibly rhombic and the other like raalacolite, 

 granulitized and associated with secondary albite in the crush-zones, 

 or passing into amphiboles and chlorites ; original amphiboles rare, 

 but common as actinolite, tremolite, and asbestos alteration-products 



