Vol. 54.] GEITS AND SHALES IN NORTHERN ANGLESEY. 



379 



No. 448. — Alteration greater, and schistosity more strongly 

 marked. Contortion very intense, some of the micaceous folia being 

 twisted and sheared again and again within the length of ^ inch. 

 The quartz-seams are perfectly granular. Some of them extend 

 across the slide, others form short lenticles. There are no large 

 fragments in this specimen. The folia between the quartzose seams 

 are highly micaceous and chloritic, and embedded in them are 

 numerous minute bits of quartz, many of wbich are perfectly 

 angular. 



Pig. 2,— Schist from Pant-y-Glo {No. U8). 



X 45. 



[Alteration more advanced. A ' folium ' of chlorite, white mica, and clastic 

 quartz is seen to intervene between two folia of quartz-mosaic] 



No. 447. — Similar, but more micaceous. The minute quartz- 

 fragments are much less abundant, but still quite evident. (See 

 fig. 3, p. 380.) 



It is possible that the granular quartzose aggregates in these 

 slides contain some felspar, but the minuteness of the particles 

 renders the application of optical tests extremely difficult. Hardly 

 any fragmental felspar appears in the more schistose varieties. 



I am not prepared to offer a complete theory on the genesis of the 

 metamorphism. In a generalway, itis true that the alteration 

 has proceeded pari j)assu with the pressure. The most highly- 

 metamorphosed varieties of schist are contorted on both the large 



