E. B. Tawney— Woodwardian Laboratory Notes. rast 
Boduan, is again a porphyry with greenish-grey ground; it is much 
like that of Moel Gwyn. 
The microscope shows that the separate felspars are chiefly plagio- 
clase ; some are so decomposed that they are doubtful; the alteration 
is that so often described: the smaller felspathic portions of the 
ground mosaic are also much attached, they do not show twinning. 
Hornblende is present, mostly changing to chlorite of chrome-green 
colour, but portions of the original crystal are preserved ; it occurs 
as separate crystals, and minuter grains in the ground, but 1s far less 
in quantity than the felspar. Quartz grains are abundantly scattered 
throughout the ground, which is minutely crystalline. Apatite is 
abundant in largish crystals, usually connected with the hornblende 
or chloritic areas, but sometimes included in felspar. One portion 
of augite was observed. Since the larger felspars are triclinic, the 
rock may be classed as quartz-diorite, or quartz-epidiorite of some 
authors. 
The relation of these Nevin rocks to the Cambrian shales which 
lie around them makes it extremely probable that they are intrusive ; 
indeed, the area of shales confined between two arms of the igneous 
rock at Pistyll shows evidence of the action of the latter in indura- 
tion, e.g. near Pen y nant and on the east side of Tany foel. 
In view of these circumstances, which moreover have been pre- 
viously indicated by Sir A. C. Ramsay, in the Survey Memoir [ed. 
2, p. 218], one cannot but adopt his conclusion, viz. that these rocks 
are intrusive in Cambrian beds mostly of the age Arenig to Bala. 
No reasons have been given by Dr. Hicks why the term Arvonian 
should be given to them, nor is the meaning of this correlation 
explained. 
Certainly the variety of rock in the same igneous mass is a little 
perplexing; this is specially noticeable in the Nevin tract, which is 
described generally by the Survey as syenitic-felspar-porphyry, but 
at Gwlfa quarry is called ‘‘a fine-grained diorite.” At one part 
quartz will be abundant, and at another time absent. Carn Boduan, 
Carn Madryn, Carn Fach, and séme other detached bosses are 
respectively fairly uniform throughout. 
Doubtless, the same speculation will occur to many, as is so well 
expressed by Sir A. CO. Ramsay [J.c. p. 220], that these dome-like 
masses are parts of deep-seated material, above which rose craters 
from which the lavas and ashes of adjacent parts of N. Wales were 
ejected. 
This variation in composition of these old rocks may further 
possibly be explained by their being supplied from time to time with 
melted matter of different constitution, which might become more or 
less fused together, though the pulsations of fresh material were 
added at considerable interval, and even after the crater had ceased 
its emissions at the surface from gradual decline of energy. 
(To be continued.) 
