Yol. 55.] GEOLOGY OP NORTHERN ANGLESEY. 675 



strata comprising conglomerates, grits, shales, and ironstone — all or 

 most of which are of Llandeilo age. 



(3) The whole region has been extensively affected by powerful 

 earth-movements, mainly acting from the north, which have con- 

 torted, overfolded, dislocated, cleaved, crushed, shattered, and altered 

 the strata. Thrust-planes of the Highland type occur, and the 

 north coast exhibits extensive tracts of crush-conglomerates. 



(4) The isolated masses of limestone and quartzite that are to be 

 found in the Northern Complex are portions of bedded strata which 

 have been broken up by movement. 



(5) The green strata and the rocks of the Northern Complex are 

 older than the Llandeilo, and were metamorphosed to their present 

 extent before Upper LlaDdeilo times. Whether these rocks are pre- 

 Cambrian, Cambrian, or even Arenig, ISTorthern Anglesey as yet affords 

 no direct proof, but the available evidence points strongly to the 

 view that the bulk of the strata are of pre-Cambrian age. 



(6) The strata yield little evidence of contemporaneous volcanic 

 activity. 



In conclusion I have to express my indebtedness to Prof. Lapworth 

 and Miss E. M. B. Wood for their examination and determiDation of 

 the graptolites, and to the former also for various suggestions ; also 

 to Mr. JB. N. Peach for identifying and forwarding notes on the lime- 

 stone fossils. Pinall}', my thanks are due to Prof. Watts for the 

 trouble that he has taken in examining a large number of rock- 

 sections and contributing the following appendix. 



VII. Appendix. — On the Microscopic Study of some of the Eoce:s of 

 Northern Anglesey. By Prof. W. W. Watts, M.A., Sec.G.S. 



The following notes are based on the examination of about 100 

 slides made from rocks collected by Mr. Matley during his work in 

 Northern Anglesey, and examined by me at his request. These 

 rocks may be classed under the following heads : — 



(1) The Green Series. 



(2) The Ordovician Rucks. 



(3) The Quartzites and ' Quartz-knobs.' 



(4) The Results of Earth-movement ; Crush-Conglomerates. 



(1) The Green Series. 



These rocks usually show their original clastic structure dis- 

 tinctly, although they are often considerably altered. They vary 

 from coarse quartzose grits, through finer grits, to close-grained 

 phyllites. The fragments consist of quartz-grains, clastic mica, and 

 felspar, the first constituent being present in greatest quantity. 

 Secondary chlorite and sericite are usually developed, the latter 

 becoming more abundant as the evidences of earth-movement in- 

 crease, and certain bands in the fine grits are schists consisting of 



