Vol. 54.] GRITS AND SHA.LES IS NORTHERN ANGLESEY. 381 



lY. Summary. 



(1) The crystalline schists of the region south-west of Amlwch 

 have been formed out of grits and shales. 



(2) The metamorphism is chiefly seen in the coalescence of 

 angular fragments of quartz and felspar to form granular mosaic, 

 and in the enlargement of the chloritic and micaceous films con- 

 stituting the matrix of the fragments. 



(3) The metamorphism is most intense where the rock has been 

 subjected to the greatest pressure, but it has proceeded to a less 

 advanced phase where pressure has been apparently wanting, and 

 the causes of the metamorphism are therefore not completely 

 ascertained. 



Discussion. 



Prof. BoNNEY said that his opinion, expressed to the Author, was 

 simply founded on the slices sent to him, for he did not know the 

 district ; that as metamorphism was dependent partly on the com^ 

 position, partly on the environment of rocks, it was possible that 

 rocks considerably changed might occur with those rocks which were 

 slightly metamorphosed. At the same time, allowance must be made 

 for the illusory effect of thrust- faults. 



Prof, Hughes said that he had many years ago brought before 

 the Societv reasons for believing that the beds now described by 

 Dr. Callaway were sedimentary deposits altered by crush. In the 

 shaly portions differential movements between the particles had 

 produced silky slates. In the gritty portions, where the rock was 

 of an unyielding character, the grains, which were only minute 

 pebbles, indented one another. Where the shaly beds were thin 

 and were contained between unyielding rocks, the freedom of vertica 

 or other compensation-movements was restricted, and puckering 

 instead of cleavage resulted. He thought that some of the beds 

 contained much volcanic material which lent itself in different 

 manner and degree to metamorphism, whether that was due chiefly 

 to pressure or partly also to the heat evolved by the movements. 

 He had also drawn attention to the fossils which were found in 

 schists, similar in character to, if not of exactly the same age as, 

 those which occurred on the northern coast of the island. 



The Author said that he could not agree with the last speaker 

 that slickensides were concerned with the metamorphism, or that 

 the softer rocks were more highly altered than the harder — in 

 fact, it was the more gritty bands that had undergone the greatest 

 change 



