Ixxii PEOCEEDIXGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxiil, 



particular, the more basic members of the series are much more in 

 evidence, as also the invasion of these by the later acid rocks with 

 resultant intermingling and hybridism. 



Time does not permit me to do more than mention the Archaean 

 crystalline rocks of North- Western Ireland, of Central Anglesey, 

 of Malvern and the Wrekin, and of the English Channel tract. I 

 pass on to the Dalradian Series, belonging on the more generally 

 accepted view to some time between the Lewisian crust-movements 

 and those to Avhich the ' Older G-ranites ' of the South-Eastern 

 Highlands were related, but on Mr. Bari-ow's reading older than 

 the one great Archaean system of mountain -building in that 

 region. Whatever ma}^ be their chronological relations, it appears 

 that these ancient sediments, now^ so highly disturbed, were laid 

 down in a basin wdiich was undergoing a slow subsidence but no 

 movement of the natrure of folding. This is to be inferred from 

 the extension of the series and the persistence of the same types of 

 sediment over wide areas. Contemporaneous igneous action is 

 indicated, on no great scale, but of very distinctive type. Specially 

 characteristic are the spilite-lavas in the Loch Aw^e group of Knap- 

 dale, wdth their associated intrusions of albite-dolerite and soda- 

 felsite. If the spilites seen in North Glen Sannox (Arran) are in 

 their original relations, which I see no reason to doubt, they afford 

 evidence of similar eruptions at a low^er horizon. The 'pillow-lavas ' 

 and associated volcanic rocks of County Tja'one were likewise 

 referred b}'- Sir Archibald Geikie to a Dalradian age, and, although 

 he has retracted this opinion, the materials for a final judgment 

 are still to be gathered. Probably to this age, and certainly to a 

 very early pre- Cambrian epoch, belong the ' Green Kocks ' studied 

 by Mr. Greenly in Anglesey and by Dr. Matley on Bardsey and the 

 coast of Lleyn. These reproduce the same characteristic types, 

 both effusive and intrusive. 



The oldest volcanic rocks in Britain have, then, a common petro- 

 graphical facies, and with them are associated intrusive rocks also 

 of certain well-defined types. The chief characteristic of the whole 

 is a richness in sodic felspars. Mr. H. Dewey & Dr. J. S. Flett have 

 shown that the same association of i-ock-types recm's in the British 

 area at several later epochs, and they consider that it is peculiar to 

 ' districts that have undergone a long- continued and gentle subsidence, with 

 few or slight upward movements, and no important faulting.' 



In several parts of Wales and the English Midlands we have 

 evidence of igneous activity at a much later period of pre- Cambrian 



