302 J. J. Harris Teall—Effect of Earth-Movements. 
a large scale in North Wales, where, if we leave out of account 
certain smaller undulations, and neglect the effect of faults, we may 
describe the rocks as arranged in two great synclinal folds, separated 
by an anticlinal fold. Thus, beginning in the N.W., we have first 
of all the great synclinal roll, along the axis of which lie the 
mountains Moel Hebog, Snowdon, and Glyder Fawr; then, following 
this, the Merionethshire anticlinal (see Memoirs of the Geological 
Survey, vol. iii.); and following this again the Berwyn synclinal. 
It is worthy of note that the anticlinal axis of Merionethshire is not 
horizontal, but slightly inclined to the N.E., so that the mountains 
of Cader Idris, the Arans, and the Arenigs, which are all formed 
by the outcrop of the harder igneous rocks of Llandeilo age, do not 
trend in a N.E. and 8.W. direction, but bend round towards the N., 
following of course the strike of the beds. The same igneous beds 
about Ffestiniog run in a N.H. and §.W. direction. The effect of 
the earth movements here referred to may be traced over the whole 
of the area now occupied by Cambrian formations, the strike of the 
beds in all districts varying little from a N.H. and §.W. direction. 
Where igneous rocks are intercalated with the sedimentary deposits, 
the prevalent strike may be observed at once by a glance at the 
geological map, as in N. Wales, S.W. Wales, and the 8.E. of Ireland. 
In other districts, though not indicated by any feature on the maps, 
it may readily be detected in the field. There can be little doubt 
that the Cambrian strata were formed over all the area now occupied 
by the British Isles, and that the earth movements which have 
affected the Cambrian rocks extended also over the whole of the 
district. To these earth movements then we are indebted for the 
present physical structure, and, to a certain extent, though only 
indirectly, for the scenery also of our mountainous regions. With 
regard to the age of these disturbances, I have already stated that 
they took place before the Silurian period. This is borne out by 
the fact that in Wales the Silurian rocks rest unconformably on 
the Cambrian, and do not seem to have been affected by the move- 
ments which produced the anticlinal and synclinal folds so marked: 
in the Cambrian regions. Cleavage, moreover, is for the most part 
absent in Silurian formations. There is, however, some little 
difficulty here, for the Denbighshire grits appear to have been affected 
by movements which have also affected the Cambrian, and in the 
Lake District a similar condition of things has been observed. In 
all probability these facts are to be explained by a second series of 
earth movements, of Post-Silurian date, which affected the already 
disturbed Cambrian rocks, and also the comparatively undisturbed 
Silurian rocks. One bit of evidence of great importance in determin- 
ing the date of the earth movements we are considering, and the 
metamorphism which seems to have accompanied them in the 
region now occupied by the Highlands of Scotland and the North 
Western Highlands of Ireland, is to be found on the shores of 
Killery Harbour, and in the Eriff Valley on the borders of Counties: 
Galway and Mayo (see Geology and Physical Geography of Ireland, 
by Prof. Hull). Here Silurian rocks of the Llandovery age rest 
