528 H. HICKS ON THE PRE-CAMBRIAN 
its II. 
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SG Dee halle cil nae edt ont 76°90 75°48 
gl Oia eNotes ic eh 11:60 9°60 
d DEN Obs ab pnatt Ba hie Metter A 0-71 1:73 
VE) aires atalialcly ucl ss ae 4 2:56 4°26 
CaO cer ere er 0:85 0:98 
INES OM eas ec cree ene ace 0-59 1°32 
OO Weare ayers vere oi tare ose Om 0-56 
1S EK O bees ie gn aie eh icra 5°62 5:09 
100°38 100-40 
An analysis of a fragment of the Dimetian from St. David’s, 
made by Dr. Blyth, of Queen’s College, Cork, was published in the 
paper by Prof. Harkness and myself in the Journal of the Geological 
Society in 1871, p. 8387. The chemical composition proved to be as 
follows, and it may be said to be still nearer in some respects to 
that given of the so-called ‘ adinole” concretion, by M. Renard :— 
laCaster cee cnn nase cetera OTe 78 
PAN UINTILIMEL ores, Me te eee eee 16°5 
time. Sodas and Irons). 5:5 
100:0 
I maintain that there is ample evidence to show that much of the 
material in the Pebidian breccias must have come from the denuda- 
tion, or the breaking-up by volcanic forces, of some such rocks as 
the Dimetian, and of those I class under the name Arvonian ; 
therefore it is probable that the Pebidian breccias and porcellanites 
at Nun’s Chapel and elsewhere owe their origin in great part to 
such sources. Still these so-called concretions do not now show on 
microscopical examination that this was actually the case with them. 
This may be due in part. to subsequent change. ‘The fact that their 
chemical constitution, however, is so nearly the same as that of 
the Dimetian is a highly interesting one,. but not conclusive on 
the point. That they are true fragments, however, of some pre- 
existing rocks, and not. concretions, is clear beyond dispute, from 
microscopical (Notes 17 & 18) and.chemical evidence*. Notes 
nos. 43-48 relate to specimens of the finer conglomerates of the 
Cambrian from the West Creek, Nun’s Chapel.. They show that the 
matrix is largely made up of a dirty quartz like that of the Dimetian, 
and of fragments of rocks like the underlying porcellanites (41), of 
felsites, of quartz-schists, and of quartzites, &c. 
* Prof. Frazer calls attention also to the close chemical relationship between 
the so-called ‘‘ Ash and tuffs” and the Dimetian. Further examinations in 
this direction may lead to important results bearing on the origin of the 
Dimetian. 
