232 



PROF. T. G. BONNET ON THE MICROSCOPIC 



(4) Charnwood. 



The slaty rocks of Anglesey are nearer in their lithological 

 characters to the Charnwood slates than to any other formation with 

 which I am acquainted. The resemblance was very marked even 

 in the field ; but in the case of a homogeneous rock like slate, the 

 microscope is of special value, and Prof. Bonney's examinations 

 (Notes 42, 43, 44, pp. 234, 235) are strongly confirmatory of my 

 opinion. 



Conclusions. 



1. In Anglesey there are two Archaean groups, the Slaty and the 

 Gneissic. 



2. The Slaty series is composed of slates, shales, hornstones, grits, 

 conglomerates, limestones, and chloritic schists, in which no definite 

 order has been positively ascertained. The Gneissic series is divided 

 into five groups, in which the following descending order is in- 

 variable, viz. granitoidite, chloritic and hornblendic schists, grey 

 gneiss, quartz-schist, and halleflinta. 



3. The Slaty series is occasionally foliated, but is usually in a 

 partially altered state : the Gneissic group is thoroughly meta- 

 morphosed. 



4. The Slaty series has closer lithological affinities with the St.- 

 David's volcanic group, the Charnwood rocks, and the Lilleshall 

 series than with the Bangor group. 



5. The Slaty series is undoubtedly Pebidian : the Gneissic series 

 may with some probability be referred to the Dimetian. 



APPENDIX. 



Notes on the Microscopic Structure of some Anglesey Eoce:s. 

 By Prof. T. G. Bonnet, M.A., F.R.S., Sec. G.S. 



In the following notes I shall endeavour to avoid repetition by 

 grouping together those specimens which in some important cha- 

 racteristics have a general resemblance ono to another. I may 

 add that, as the purpose of my examination was petrological rather 

 than mineralogical, I have not felt bound to spend much time in 

 endeavouring to ascertain the exact species of some microlithic 

 minerals which are accidentally present in one or two of the slides. 



A. Quartz-Schist Group. 

 Nos. 25-29, 31-34. 



This group consists of a series of schistose rocks, in general highly 

 metamorphosed, in which quartz is the most important constituent, 

 associated with micaceous or chloritic minerals, ferrite or opacite, 

 and more or less felspar. 



The fragmental origin of most of these specimens is indubitable ; 

 but with regard to numbers 25 and 27 it is less easy to be quite 

 sure. Their structure comes nearer to that of some microcrystal- 

 line felsites : but still I believe that I am right in ranking these also 

 among the altered clastic rocks. 



25. (Gwalchmai, p. 218.) Contains numerous grains of quartz of 



