344 Pt'of. Hughes, On the altered rocks of Anglesea. [Feb. 23, 



Tlie anthracitic bands and films in some of the older rocks 

 probably represent the bituminous colouring of limestones and 

 other rocks got rid of during the process of crystallization, and 

 left along joints or clayey bands coinciding with the bedding. 



When on a small scale nodules, or on a large scale masses, of 

 hard rock lying in compressible shale are subjected to contortion, 

 the shale is squeezed out over the harder masses producing a kind 

 of fault all round between the harder and softer rock, and giving 

 rise to slickensides and similar phenomena, and often mineral 

 changes are set up along the parts thus more crushed. 



In the case of a crumpled and gnarled shale in which there 

 are thin laminae of harder and softer beds, this unequal yielding 

 must produce similar results and a kind of slickensides must 

 pervade the whole mass. The beds will readily split along the 

 lines of shale exhibiting shiny slickensided faces, especially if there 

 be a good deal of mica, or talc, or other magnesian minerals in the 

 shale. A minute veiny structure will be very common in the more 

 crushed portions. 



Hard rocks crushed up and veined will be apt to become 

 serpentinous, and if it happens to be a limestone probably masses 

 of Ophicalcite will represent it. 



Such I take it is the origin of the mica schists and associated 

 rocks in the four great masses of Anglesea. They are simply 

 mechanically altered shales with subordinate hard rocks here and 

 there, and were probably derived largely from volcanic material. 

 They consist of finer and coarser laminae of more or less siliceous 

 felspathic mud, and are in nowise to be regarded as true mica 

 schists. 



There are two other kinds of mica rock of very different 

 origin. 



1. A rock made up almost entirely of mica fragments such 

 as may be found associated with other granite debris. For on 

 the destruction of a granite we have either an arkose, where all 

 the various ingredients are again thrown together so that the 

 sedimentary rock derived from the granite resembles it in com- 

 position — or we may have the quartz and perhaps the felspar 

 carried to one place, and the mica transported a little further and 

 allowed to settle by itself. Probably we have some beds of this 

 kind between Amlwch and Dulas. 



2. There are rocks in which there has been an entire re- 

 arrangement of the component minerals by chemical action, 

 different from that referred to above under the head of veining. 

 In that a mineral was taken away or a mineral was introduced 

 along definite lines by percolating water carrying Avith it something 

 which had an affinit}' for the transported mineral or its solvent, 

 but the main mass remained passive, received these gradual 



