52 STKATIGKAPHICAL GEOLOGY 



The whole Archaean complex of Anglesey was affected by very 

 powerful pre-Ordovician earth - movements. In some parts they 

 are closely folded and minutely corrugated, in other places so 

 sheared and torn that the harder and more resistant beds have 

 been broken up and are now merely a series of phacoids or lenti- 

 cular masses, floating as it were in a kind of matrix formed out of the 

 softer beds. Further, the movements induced varying degrees of 

 metamorphism, so that there are few of the rocks which have not 

 been converted into more or less crystalline schists. 



Under these circumstances and in the absence of fossils it has 

 not been found possible to establish anything like an order of 

 succession ; at the same time it is nearly certain that more than 

 one of the Archaean systems is present in the island. There is 

 even some reason to think that the central complex of gneissic 

 rocks is a fragment of a massif older than the great sedimentary 

 series, but its outer members are so welded to that series by a 

 common foliation that direct evidence can hardly be expected. 



The general sequence of events seems to have been as follows : 

 Upon an ancient floor, of which hardly anything is known, but 

 which was composed in part at any rate of plutonic rocks, a great 

 series of grits and shales, with quartzites and limestones, was laid 

 down. At some period during the deposition of these sediments 

 a volcanic episode occurred, and basic lavas with ellipsoidal structure 

 were poured out on the sea floor, after which the interstices of 

 the lava-flows were infiltrated with silica in the form of chert. 



After the accumulation of all these sediments and volcanic 

 materials, but before the beginning of Ordovician time, crust move- 

 ments on the most powerful scale supervened, causing the whole 

 complex to be crushed, folded, and sheared in a most complicated 

 manner. The rocks were in fact practically reduced to the con- 

 dition in which they are now found before the deposition of the 

 oldest local members of the Ordovician Series, and probably before 

 the commencement of Cambrian time. 



Carnarvonshire. Kocks similar to the sedimentary series of 

 Anglesey are found again on the western side of the Lleyn pro- 

 montory and form a long strip from Bardsey Island to Nevin, 

 with a width of from 1 to 2 miles. This district exhibits the same 

 series of schists and slates, of purple and green colours, with broken 

 bands of limestone and quartzite, and a similar association of basic 

 lavas and tuffs with jaspery infiltrations. According to Mr. J. F. 

 Blake there is rather a larger development of volcanic material 

 than in Anglesey, especially in the south-west where there is much 

 volcanic ash and some masses of coarse agglomerate. 



Two other small strips of pre-Cambrian rocks come to the 



