ON fHE CRYSTALLINE ROCKS OF AN&LfcgfiY. 117 



No. 528 A. Clegyr Summit] Llanrhyddtdd. 



I. It. 



Si0 2 59-61 59-63 



AL,0 3 1963 19-56 



Fe,0 3 2-62 270 



Feb 3-61 3'56 



CaO 2-85 2-80 



MgO 2-01 2-09 



KjO 3-60 3-52 



NajO 356 3-47 



HL.0 (at 110°) 0-24 022 



H.,0 (above 110°) 2-51 254 



100-24 100-09 



This is from the least altered portion of a large and remarkably 

 uniform formation that occupies a great part of the west of Anglesey, 

 and is very conspicuous along the western coast. It is finely clastic, 

 but with rather peculiar structures, both on the large and the micro- 

 scopic scale, and has been analysed with the view to determining 

 whether it ought to be regarded as an ordinary sediment or as volcanic 

 tuff. The analysis was undertaken at the especial suggestion of Dr. 

 Home, who examined the coast sections with the secretary in the 

 spring. It will be seen at once that the rock is not an ordinary sedi- 

 ment. Incidentally, it is likely that this rock, now that its composition 

 is known, will throw considerable light on the perplexing rocks that 

 were analysed in the first years of the work of this Committee. 



Dolomite in Carboniferous Limestone, 600 yards N. of Tros y Marian, 



Penmon. 



I. II. 



Residues insoluble in 20 per cent. HC1 1-27 1-23 



AU.O,+Fe s 3 



CaO . 



MgO . 



5 CO a . 



1-94 196 



32-14 3218 



18-33 18-42 



45-96 4605 



Per cent, of CaC0 3 57'39 5746 



99-64 9984 



57-39 5746 



MgC0 3 38-34 3842 



This rock is closely allied to the one whose analysis was published 

 last year. It completes a series from the Carboniferous. 



No. G65 A. Diorite on margin of Picrite, Llaneilian. 



I. II. 



Si0 2 48-87 48-76 



This silica percentage was taken to obtain an idea of the variations 

 of the series of intrusions to which the well-known hornblende-picrites 

 belong. A similar rock from another district was given in the report 

 for 1908. 



Mr. Hughes is now in London, and, by the kindness of the Director 

 of the Geological Survey, is carrying on his work for some time in 

 the laboratory at Jermyn Street, in order to be able, while doing so, to 



