ON THE CRYSTALLINE ROCKS OF ANGLESEY. 283 



Coinposition and Origin of the Crystalline RocJcs of Anglesei/. — Third 

 Report of the Committee, consisting of Mr, A, Harker (Chairman), 

 Mr. E. Greenly (Secretary), Mr. J. Lomas, Ur. C. A. Matley, 

 and Professor K. J. P. Orton. 



The analyses contained in this Report, like those of last year, have 

 been drawn up by Mr. John Owen Hughes, B.Sc, who contributes a 

 chemical note upon them. They have all been executed by himself. 



In almost every case they have been chosen for the purpose of 

 throwing light on difficult questions of the origin of highly altered rocks, 

 and a discussion of them would lead far into complicated metamorphic 

 questions, to which justice could not be done apart from microscopic 

 structures and detailed field relations. In this place, therefore, only the 

 nature of the rock and the bearings of the analysis will be given, 

 sufficiently to make its objects clear. 



Two more rocks belonging to the zone of hornfels in Central 

 Anglesey have been analysed. When one more, a very important one 

 that is now in progress, is completed, that particular line of investiga- 

 tion will be at an end for the present, a typical selection having been 

 examined. 



The remaining rocks belong to other groups, and have been selected 

 partly to aid field-work actually in progress at the time, partly because 

 the specimens had long ago been collected for analytical purposes, and it 

 seemed advisable to get them examined and out of hand. 



The next line of investigation it is proposed to take up systematically 

 is that of the composition of the pillowy diabase and jasper group, with 

 the metamorphic rocks associated with them, a question of great import- 

 ance upon which some chemical work has been done already. 



The numbers, as before, are those of the slides in the Secretary's 

 cabinet. 



No. 184a. Banded Horn/els. 



Ang. 12 S.E. ; l,200yit. E.N.E. Tal Eiddem. 



I. II. 



K.p .... 4-05 4-04 



Na^O .... 1-49 1-65 



This is a banded hornfels from near Gwalchmai. The object of the 

 analysis is, as in the case of those in last year's report, to aid in deciding 

 whether these rocks are igneous or sedimentary in origin. 



No. 205a. Oarnet Mica Schist. 

 Ang. 13 S.W. ; N.E. nd Llyn Bendref. 



I. IL 



K.,0 . . . .2-59 2-66 



Nap .... 2-44 259 



Owing to the small amount of material available, only the alkalies 

 could be estimated in the above rock. 



This is a garnetiferous mica schist, also from near Gwalchmai, 

 belonging to a zone into which the hornfels appears to pass. 



