British Association Reports. 559 
the line of junction of the Lower Cambrian, or Harlech Grits, and 
the black slates which form the mass of the ‘ Lower Lingula Flags.’ 
Some major lines had been laid down in the mountain country to 
the east and south, and very accurately ; but the faults along which 
the precious metals were won, and those which under the term of 
slides and cross-courses interrupted their work, were still for the 
most part to be put in. 
Having laid down all the faults he could trace, the author found 
that they became systematic upon the map ;.and he was at length 
enabled to predict their occurrence by the contour of the ground, and 
to verify them by natural sections, with which the country abounds. 
The author traversed a good deal of the ground from the Clogan 
mines on the west to Cefn Coch and Cwmheisian on the north-east ; 
but finding that the faults were very numerous and complicated, he 
soon determined to contract the area of close observation to that in 
which his employer was most interested ; and while he has maps of 
the fault lines observed all along the boundary, those only of Tyd- 
dyngwladis, Cwmheisian, and Gwynfynydd are laid down in detail. 
He had no map to begin with except the Ordnance one-inch, and 
had to enlarge the Survey map four times its size during the course 
of his work. It is not even yet on a large enough scale. 
The first thing was to correct the boundary line between the 
Cambrian and the Lingula Flags; and this could only be done by 
ascertaining the succession of the beds of the latter. 
By this means it was found that the tendency of the main lines 
of fault—those which follow more or less accurately the trend or 
strike of the beds—was to repeat the series of beds; therefore there 
must have been ipheaval and collapse of the Lower Cambrian nu- 
cleus. These great faults are joined by oblique and cross faults of 
no great regularity, but no doubt reducible to system. 
Another set of faults of less dimensions, along the original 
strike of the country, and probably due to strain produced during that 
upheaval. 
Along some of these faults and a third set at oblique angles 
(NE. and SW.) to them, the principal gold veins occur; while in 
one, which from its greater throw of the beds has probably been a — 
second time faulted, a considerable vein of lead is found. They cer- 
tainly cut off and shift the east and west veins. 
These are crossed by a series of NW. and SE. faults, which in 
the Tyddyngwladis and Cwmheisian district and Dolfrwynog mines 
run parallel, and contain both gold and silver-lead. 
And lastly, the whole are traversed by an important set of 
faults which shift all the others; and are undoubtedly the last, 
namely, true N. and S. faults, which run through the whole country 
—far away into the hills of Diphwys, and in fact as far as I was able 
to examine. 
They give the features to the ground—steep cliffs, river courses, 
ledges, ravines, bluffs ; all are defined by these master faults, which 
though seldom of any very great amount in the actual mining region 
eastward, become greater to the westward, and are, I suspect, 
