1880.] Prof. Hughes, On the altered rocks of Anglesea. 3 -to 



changes from without, and we generally get a simpler rock as the 

 result. In the case we are now considering, the whole mass is 

 started off on a kind of mutual exchange system and the materials 

 already there are resorted, giving as the result segregations of 

 complex minerals such as occur in gneiss, &c. In these we get 

 mica schists whea the mica has come into prominence by crystal- 

 lization after the deposition of the material of which the rock is 

 composed. This is the only rock to which the term mica schist 

 should be applied. A sedimentary mica rock splitting along bedding 

 planes is a shale. A sedimentary mica rock splitting along cleavage 

 planes is a slate. But a mica rock which we have reason to be- 

 lieve owes its present character to metamorphism, i.e. to a mineral 

 rearrangement of the whole mass, and which splits along the mica 

 plates, but in which we cannot say whether they coincide in their 

 arrangement with bedding or cleavage, may be properly called 

 a mica schist. 



We will now apply these observations in greater detail to the 

 explanation of the Anglesea rocks, omitting for the present all 

 criticism of the boundary lines, which with a new reading of 

 the rocks would have to be somewhat modified. It will be seen 

 on reference to the survey map^ that there is a central axis of 

 granitic rocks represented as running approximately N.E. and S.W. 

 from near Llanfaelog to near Llanfihangeltre'rbeirdd. This consists 

 of mica schists, hornblende schists, gneiss, and a granitoid rock 

 made up of a crystalline aggregate of quartz, orthoclase felspar 

 and less commonly mica. It is often succeeded by a grey felsitoid 

 rock for which some, till we know all about its origin, prefer to 

 use another vague term borrowed from the Swedes, calling it 

 Halleflinta. The felsitic rocks seem to overlie the granitic, but 

 there are passage beds of varying thickness between them. I 

 think there is sufficient evidence to show that these felsitic rocks 

 are sometimes metamorphosed into the granitoid rock. There are 

 rapid alternations of the two kinds of rock and one shades off into 

 the other in a manner which shows that it is not difference of 

 composition, or different rates of cooling, or anything but a slow 

 segregation of the quartz, felspar, and other minerals that has 

 produced the change. Similar examples were pointed out by Dr 

 Callaway in the Wrekin, where the change seems to- have taken 

 place all round a mass from the joint surfaces so that a block of 

 halleflinta has a granitoid outside. We can attribute such change to 

 chemical action only, and the process that would best explain 

 the phenomena observed seems to me to be somewhat like that 

 described by Prof Liveing, Proc. Camh. Phil. Soc. Vol. III. p. 75. 



Further, it will be seen that there are four great masses of 

 schist coloured pink on the map : the first lying along the N. shore 



1 Sheet 78. 



