1848.] JUKES AND SELWYN ON THE STRUCTURE OF N. WALES. 301 
tween Barmouth and Dolgelly the dip is S.E. at about 60°. Along 
the Trawsfynnydd road the dip is E. at about 25°. About Trawsfyn- 
nydd the dip is N. at 25° or 30°; and between Trawsfynnydd and 
Harlech the dip is N.W. at angles varying from 10° to 50°. Round 
this rudely bastion-shaped mass sweep the rocks of group B, dipping 
S.E. about Cader Idris, E.S.E. in the neighbourhood of Aran, E. in 
Arenig Fawr, and then suddenly curving to N.E. and N. about Arenig 
Bach and Festiniog, to the west of which latter place the dip is N.W. 
for several miles. Outside of this curved tract again, and taking a 
wider sweep, lies the Bala group (C), with the same general inclina- 
tions; dipping E.S.E. from Dinas Mowddwy to the Dee near Bala, 
and thence to the Clwyd three miles north of Bettws Gwerful Goch, 
and N. along its northern boundary about Cerrig y Druidion and 
Yspytty Ifan. The change however from one dip to the other is 
much more sudden, or at least more sensible in these beds than in 
the lower groups, and can be well seen on the part of the Clwyd above- 
mentioned. This sudden change is produced by a broad, low, but 
strongly pronounced anticlinal flexure of the rocks, called by Pro- 
fessor Sedgwick ‘‘ The great Merioneth Anticlinal,” which more or less 
apparent in the lower groups, causes the trap rocks about the Arenigs 
to project three miles north-east of their mean boundary up to the 
hills of Carnedd Filiast, and spreads the rocks of the Bala group in 
a broken and distorted arch over all the country mcluded by a line 
running from Bala to the head of the Clwyd, and thence by Cerrig y 
Druidion to Yspytty Ifan. To the north-west of this tract are seve- 
ral smaller but sharper anticlinal and synclinal curves (likewise de- 
seribed by Professor Sedgwick), the axes of which run nearly north- 
east and south-west. A synclinal line traverses the valley of Yspytty 
Ifan, an anticlmal that of Penmachno, and another synclinal that of 
Dolwyddelan. As these flexures sink or become less towards the 
north-east (their axes not being horizontal, but inclined in that direc- 
tion), the outcrop of any bed forms curved lines, Vandyke-fashion, 
across the country, by following which lines the Bala beds have been 
traced into the valley of Dolwyddelan. 
- Besides these flexures, the country is traversed by many large dis- 
locations, which have been traced whenever the character of the rocks 
is sufficiently distinct to allow of it. Near Bala, by means of the 
ash beds underlying the limestone, many large faults have been laid 
down, and others of still greater magnitude indicated ; and it has 
been found that the supposed three or four bands of limestone are all 
broken parts of one bed, each piece ending suddenly both ways, and 
having at the proper distance below it a piece of ash of the same shape 
and dimensions, likewise ending suddenly in each direction. The 
limes joining the ends of these pieces likewise are nearly parallel, 
running about N. by W. and 8. by E. In this way the limestones of 
Y Gelli-grin, Rhiwlas, Llwyn-y-ci, Penmaen, and EKglwys Anne, were 
found to be fragments of the same bed, drifting as it were across the 
country, half a mile apart, to the northward of their apparent strike ; 
still farther north of this tract the dislocations are so enormous that 
they are no longer traceable in the Bala group, but the boundary of 
