1880.J Prof. Hughes, On the altered rocks of Anglesea. 843 



tlie character and arrangement of tlie materials of which the rocks 

 are composed and not likely to be of wide extent under any pro- 

 bable conditions ; so that it is no argument against the correlation 

 of rocks differing in this respect that if contemporaneous they 

 should be similarly affected. We observe also all combinations 

 of the results above described ; sometimes the rock is cleaved 

 between two beds that would not cleave ; sometimes the rock is 

 crumpled between two even-bedded masses, and generally the long 

 folds of the puckered rock more or less coincide with cleavage 

 planes in parts of the mass. 



Accompanying these phenomena are others of great importance 

 in considering the question of the nature of the changes which 

 have affected the Anglesea schists. Where in the folded rocks 

 we find beds of hard unyielding sandstone or limestone they 

 are often crushed and veined in all directions to such an extent 

 sometimes, that the w^hole mass becomes what, if we saw it in 

 lines and strings, we should call vein stuff. 



Obviously, we are not referring to ordinary precipitation of 

 layers of various minerals on the walls of a gaping fissure. We 

 are considering the cases in which the rocks are suffering from 

 compression and no open cavities exist. We find the whole mass 

 divided into angular portions retaining the original texture and 

 colour, and the dividing parts in every stage of alteration till 

 granular quartzite has passed into white or semi-transparent 

 quartz, and all the colour has been discharged. This may be well 

 seen in some of the grits near Garth ferry and on the coast north 

 of Dulas Bay, which are subordinate to the schists we are con- 

 sidering. 



The quantity of colouring matter in such a grit is very small 

 and is probably, chiefly a silicate of iron coating the grains. As 

 the process goes on, impurities are left out along the divisional 

 planes between the areas of crystallization first forming those 

 coloured films which occur along the smaller joint surfaces and 

 eventually carried away by the percolating water. So in the 

 limestones such as those seen between Amlwch and Llanpadrig, 

 it is a kind of indigenous vein structure which we first see in the 

 crushed rock. This we follow into the mass till none of the rock 

 exhibits its original character and the whole is a crystalline lime- 

 stone often more or less siliceous. Silica is apt to replace carbonate 

 of lime under such circumstances as e.g. the tabular flint lining 

 cracks and joints in the chalk ; the chert irj^egularly replacing 

 large masses of carboniferous limestone, and, I believe, the cherty 

 jaspery beds we often And in the Silurian and Cambrian, where, 

 from what we find in other undisturbed districts, we should expect 

 calcareous deposits. The flinty beds of the coast near Glanffynon 

 may perhaps be thus explained. 



