Vol. 54.] GKITS AND SHALES IN NORTHERN ANGLESEY. 377 



The change in the quartz consists chiefly in the replacement of 

 the angular fragments by granular particles fitting into each other 

 with foliate interlocking margins, as in a normal schist. This 

 alteration sometimes takes place, even where there are but slight 

 traces of pressure. The particles and fragments have evidently 

 been softened by heat, and more or less compressed, the angularities 

 being thus obliterated, and the grains moulded into each other and 

 welded together. This change, of course, implies contact between 

 the fragments. Where portions of the matrix remain between 

 them the welding is less complete. Sometimes angular particles of 

 clear mineral are entirely immersed in a soft matrix of mica or 

 chlorite, and still retain their sharp outlines even in the most crys- 

 talline of the schists. This feature is of especial interest, since it 

 furnishes us with evidence of an original clastic structure where 

 we do not usually expect to find it. 



In places were the rock has been intensely crushed and sheared, 

 patches of fragmental quartz and felspar have been rolled out into 

 distinct folia, sharply differentiated from the micaceous and chloritic 

 matrix, which then ceases to exist qua matrix, and becomes a 

 normal constituent in a true foliated schist. 



The matrix, even in the least altered varieties of grit and shale, 

 has undergone some alteration. It consists chiefly of a green 

 mineral of the chlorite family, or of a minute felt-work of trans- 

 parent mica, or of a mixture of the two. There is in some slides a 

 moderate quantity of opaque dusty matter, presumably iron-oxide. 

 Occasionally epidote is present. 



The progressive change in the chlorite and the mica takes the 

 form of enlargement. The streaks and blotches of the former grow 

 larger ; the films of the latter become larger, thicker, and more 

 distinct, polarizing more and more clearly in the brilliant colours of 

 a white mica. 



The best microscopic specimens for the study of this question are 

 from the quarries at Pant-y-Glo. Four slides have been cut. In 

 all of them we can see that the quartz and felspar have assumed to 

 a greater or less extent the mosaic structure, yet all the slides 

 display distinct traces of a fragmental origin, wherever the matrix 

 has kept the clear particles from touching each other. These are 

 the two points which are chiefly kept in view in the following 

 descriptions. 



No. 450.^ — More than three-fourths of this slide consists of 

 quartz. Next come, in the order of their abundance, chlorite, 

 epidote, opacite, and felspar. The quartz presents the chief points 

 of interest. Large irregular fragments of it are scattered through- 

 out the specimen, and all of them are angular and well-defined, 

 but occasionally they pass, at one side, into granular aggregates. 

 There is also present a great deal of minute quartz in the granular 

 condition, the particles interlocking with sinuate contours, forming 



' Thft mimbe'd Jire those of the slides pr.^jerved in my cabinet. 



