part 3] METAMORPHISM IN THE MONA COMPLEX. 341 



that the hornblende-schist in contact with them is nearly always 

 much more basic than it is elsewhere. Judging by its coloration 

 one would estimate that it is often twice as basic. In the same 

 rock there are many bands of the compact siliceous schist which is 

 regarded ('G. of A.' p. 121) as of adinolic nature; but these have 

 no dark encasements, reaction-rims, indeed, being sometimes 

 developed. Thus, where the siliceous inclusion is a xenolith, the 

 hornblende-schist is either unaffected, or slightly robbed of basic 

 matter : where the siliceous inclusion is a segregation, it is robbed 

 of acid matter. 



V. The Age of the Gneisses. 



Clastic oligoclase. — This felspar has been found in the 

 New Harbour Beds, the Church Bay Tuffs, and the Gwna Beds. 

 But, in the Mona Complex, authigenic oligoclase is known only in 

 the gneisses, where it is a persistent feature. 



Characters in the different regions. — The process of 

 development of the basic gneisses (pp. 343-49) was the same, even 

 down to minute details, in the Middle as in the Aethwy Region. 

 This indicates that these rocks are not mere modified forms of local 

 intrusions, but that they belong to a widespread gneissic formation. 



Relations to the hornfels. — Mica-hornfels of*the Coedana 

 Granite occurs within a yard or two of gneiss at Gwyndy (' G. of 

 A.' pp. 162, 334). A slide cut by J. F. Blake [E 10680] has now, 

 by comparison of maps, been found to be from this place. It is 

 quite unfoliated, with large porphyroblasts of intergrown muscovite 

 and biotite, and it contains orthoclase, which indicates its indepen- 

 dence of the Gneisses. Moreover, 217 yards away to the north- 

 east, compact cryptociystalline hornfels occurs only 50 yards 

 from gneiss. Mica-hornfels (' G. of A.' pp. 162, 322, 333) has now 

 been found on the north side of the main Holyhead road, between 

 Caer-glaw and the 9th mile-post, only 30 yards from typical gneiss, 

 and not separated therefrom by the Treban Fault. 



Relations to the Gwna Beds. — Re-examination* of the 

 three granitoid masses of Mynydd Wylfa ('G. of A.' p. 307) 

 reveals that the southernmost has a strong lenticular foliation ; 

 also, that along its southern margin there are a few feet of basic 

 gneiss, heavily crushed, but i"etaining survivals of banding, pegma- 

 titic seams, and ciystalline junction with the granitoid rock, as 

 well as a 1-foot band of a coarse acid gneiss. No doubt, therefore, 

 need remain that these rocks belong to the Gneisses. The larger 

 of the two northern masses (' G. of A.' fig. 139) can be seen to 

 overlie Gwna Beds wherein anamorphism is almost imperceptible. 1 



Relations to the Fydlyn Beds. — On the view of the 

 general succession which has been adopted, the Fydlyn Beds might 



1 No Fydlyn Beds could be found, so they are evidently (p. 339) cut out. 



