42 i Dr. C. Callaway — How to Work the ArcJicean Huclcs. 



semblance to greenstone. The Pebidian of Anglesey is very similar, 

 but in some patches the alteration has proceeded further. Near 

 Bangor, in Shropshire, and in Charnwood Forest, incipient meta- 

 morphism has also taken place in a greater or less degree. All these 

 rocks may fitly be classed with the " hypometamorphic," a term 

 used to express a partial change. 



The Dimetian series of St. Davids has undergone a much greater 

 change. The prevailing types are quartzite and a rock like granite, 

 but of metamorphio origin, being in fact a metamorphosed quartzo- 

 felspathic grit. In Twt Hill, Caernarvonshire, and in Central 

 Anglesey are similar granitoid bands. In the latter locality this 

 rock passes dow^n into schistose and gneissic strata (Fig. 1). The 

 alteration in these schists is complete. The rocks are thoroughly 

 crystalline and truly foliated. But comparing them with the Lauren- 

 tian or Lewisian gneiss, a clear difference is perceptible. In the 

 Anglesey group there are here and there indications of the original 

 fragmental structure. In the granitoid band are intercalated some 

 beds of felspathic breccia, and in the schists angular or rounded 

 grains of quartz may sometimes be detected. The crystallization of 

 these schists is also much more minute than in the Lewisian. The 

 metamorphism of the latter is intense, no trace of fragmental origin 

 being perceptible, while the constituent minerals occur in distinct 

 and frequently in large crystals. The Pebidian, the Dimetian (to 

 which the Anglesey gneissic series probably belongs), and the 

 Lewisian thus present three degrees of metamorphism, the partial, 

 the moderately complete, and the intense. Corresponding with 

 these degrees, it is to be observed that the Pebidian is certainly 

 newer than the Dimetian, and the Dimetian is probably newer than 

 the Lewisian. 



Between the Pebidian and Dimetian, Dr. Hicks has intercalated 

 a new group, the Arvonian, which he considers to present an in- 

 cipient gneissic structure, and to be intermediate in degree of altera- 

 tion between the oldest and youngest series. Should these views be 

 established, the hypothesis suggested in this section would receive 

 additional confirmation. 



In investigating the degree of alteration, it is important to bear in 

 mind, the influence of selective metamorphism. The forces which 

 alter rocks may select one bed, and convert it into a schist, while 

 strata below are comparatively unaltered. A rock which is mainly 

 composed of quartz can undergo little change, because quartz is a 

 simple mineral which cannot be decomposed by any of the chemical 

 processes which are at work in the earth's crust. A quartzose sand- 

 stone is metamorphosed into quartzite, which difiers chiefly in 

 induration and texture. But a felspathic shale may be changed into 

 something widely different. By the separation of a part of the 

 silica, the felspar may be converted into mica, and the mica and 

 silica (quartz) together would form mica-schist. If a portion of the 

 felspar remained undecomposed, the quartz, felspar, and mica would 

 constitute a gneiss or granitoid rock. Selective metamorphism may 

 therefore complicate our researches by changing parts of a younger 



