PKE-CAHTBRIAN- ROCKS OF ST. DAVIDS. 301 



in the volcanic group- of the Lower Old Red Sandstone of the Pentland 

 Hills, near Edinburgh. In the latter area the volcanic accumula- 

 tions attain a depth of more than 5000 feet, and are composed of 

 successive sheets of basic lavas, with alternations of felsitic tuffs, in 

 which the proportion of silica ranges between 60 and 70 per cent. 

 Intrusive veins of felsite intersect these tuffs and porphyrites ; but no 

 case has there been observed of any such rock having been poured 

 out as a superficial lava- stream. 



Though the volcanic group of St. David's consists almost wholly 

 of volcanic materials, the tuffs contain evidence that ordinary sedi- 

 mentation was not entirely interrupted by the volcanic discharges. 

 Thus, in the Allan valley, west from the Cathedral, one of the 

 schistose tuffs is full of well-rounded pebbles of white quartz. 

 Occasional shaly bands indicate the deposit of mud with the tuffs. 



Seams of pale silky schist occur among the tuffs, similar in texture 

 and composition to bands that lie on various horizons among the 

 Cambrian sandstones and shales. These may originally have been 

 fine volcanic dust or mud. One of the most prevalent features, 

 indeed, among the finer varieties of tuff is the development in them 

 of a fine foliation, whereby they pass into silky schists, and might 

 be classed with the sericite schists of metamorpbic districts. This 

 structure will be described in connexion with the later changes 

 which the rocks of the district have undergone. 



Uppermost ZoneofScJiists, Shales, Siliceous Bands (Adinole, Eiesel- 

 scMefer. — In the first part of this paper (p. 279) reference has been 

 made to a remarkable band of strata lying between the volcanic group 

 and the quartz conglomerate, and serving, from its peculiar lithological 

 characters, as a convenient, because easily recognizable, horizon for 

 tracing out the structure of the district. It must be considered as 

 part of the volcanic group, but with an admixture of non-volcanic 

 sedimentary material. It probably indicates the resumption of 

 ordinary sedimentation as volcanic action became gradually feebler. 



The component rocks of this zone are fine tuffs, passing, on the 

 one hand, into fine grey shales, and, on the other, into pale schists. 

 but occasionally including bands of coarser tuff, seams of quartzose 

 sandstone or quartzite, and abundant siliceous aggregations. 



The schists are exceedingly fine, silvery, hydromica schists, 

 unctuous to the touch. They pass into fine tuffs and into shale ; 

 indeed, they must be regarded as a metamorphosed condition of 

 beds that were originally fine tuffs and shales. They vary very 

 much in their power of resisting disintegration, some portions 

 standing their ground well, as in Ramsey Sound, other parts decay- 

 ing into a soft, white, or yellow clay, as at Porth-lisky. 



One of the most conspicuous features of this zone in the St. 

 David's district is the remarkable abundance of its siliceous aggre- 

 gations. The material of which these consist varies considerably 

 in colour, texture, structure, and composition. In some instances 

 it occurs in bands having a finely granular texture like beds of 

 altered shale ; in others the bands are flinty and translucent. It 



