A, R. Andrew—The Dolgelley Gold-belt. 169 
Again, the intrusion of the greenstone appears to be subsequent to 
the movement of the country which caused the formation of joint- 
planes. Some distance up the Bontddu stream, 200 yards above 
the St. David’s Gold Mill, a sill of greenstone is seen to end 
abruptly at a joint ; there is absolutely no trace of movement having 
taken place along this joint-plane, and of the sill being faulted off. 
It therefore seems that the joint-plane was there before the: green- 
stone was intruded, and was the determining factor in its abrupt 
termination. 
The general sequence of events in this district after deposition thus 
appears to have been—(1) folding and jointing, (2) intrusion of 
igneous rocks, (3) cleavage. In this district there is no evidence to 
show the actual geological date of origin of these separate phenomena. 
In the North Wales region generally, however, it is considered certain 
that the cleavage of the region was effected in the interval between 
Silurian and Carboniferous times. Inthe North Wales region, the 
igneous rocks found among the Ordovician beds are believed to have 
been intruded in late Ordovician or Silurian times, and the green- 
stones of the Dolgelley Gold-belt were probably intruded at- this 
time also. 
According to Ramsay, the sequence ot the movements in North 
Wales generally was—(1) intrusion, (2) folding, (8) cleavage. 
I think, however, that in the Dolgelley area the folding preceded 
the intrusion of the sills, but the point is not of great importance, 
as it is most probable that the two phenomena were closely associated, 
and took place at very nearly the same period. 
PETROLOGY. 
- Throughout the district in which the gold-bearing lodes of the 
Dolgelley Gold-belt occur, there are many intrusions of igneous 
rock, several of them of considerable magnitude. These intrusions 
occur in the form of dykes, of sills, and of more massive bosses. 
Dykes are by no means common and the only clearly defined examples 
occur in the country at the back of Bontddu. ‘The sills are found 
most frequently among the soft yielding shales. In the softest 
formation of all, the Clogau or Menevian, they are very numerous, 
and have such a considerable aggregate thickness that in places they 
‘fatten out’ the Clogau Beds by fully 50 per cent. ‘The sills 
constitute far and away the greatest proportion of the igneous rocks 
of the district; they range in thickness from tiny interbeddings 
6 inches wide, such as occur a short distance below the Vigra bridge, 
Bontddu, up to massive, almost laccolitic intrusions, 200 feet thick, 
such as form the conspicuous ridge at Llechfraith on the Clogau Hull. 
The sills continue quite evenly between the bedding planes for 
considerable distances before they die out. At times, however, they 
break across the bedding, and their intrusive and subsequent nature is 
thus clearly demonstrated. 
The metamorphic effect of these igneous intrusions upon the rocks 
with which they come in contact is not very considerable. The shales 
are baked into hard porcellanites, but only for a short distance 
(a maximum of a few feet) from the intrusions. On weathering, these, 
