VOL. XV. (1) THE PRESIDENTS ADDRESS II 
remainder have not yet been recognised in the pre- 
Uriconian rocks of the district. We must then conclude 
that in Shropshire the Uriconian formation is largely 
of subaqueous origin. The area was presumably occupied 
by a shallow sea with scattered islands. Volcanoes 
- ejected their contents over land and water. Waves beat- 
ing on a rocky shore ‘worked up beds of pebbles, and these 
became intercalated with finer material conveyed by cur- 
rents or showered down from the sky. The volcanic 
cones were gradually worn away by denudation at a 
subsequent epoch. Submergence took place, and the 
sediments of the Cambrian were laid down. Later on, 
great dislocations split the Cambrian and later formations 
into cracks, and along these lines of fracture the wedges of 
Archzean rock were thrust up. The forces of the atmo- 
sphere have since worn them down to their present 
rounded forms. 
To the west of Shropshire stretches a great expanse of 
Cambrian, Ordovician and Silurian strata; but Archean 
masses re-emerge at the extreme south-west and the 
extreme north-west of Wales. St. Davids is classic as 
the region where Precambrian rocks other than the 
Fundamental Gneiss were first detected in this country. 
A central axis of granite is flanked on either side by 
volcanic strata, which Dr Hicks called Pebidian. They 
are, I believe, contemporaneous with the Uriconian, and as 
they were described at an earlier date, the term “ Pebidian ” 
has priority. However, as the evidence for Precambrian 
age is clearer in Shropshire than at St Davids, geologists 
have usually adopted my nomenclature for these rocks in 
the Midland counties. At St Davids the volcanic series 
consists predominantly of tuffs of greenish and reddish 
tints, varying in texture from coarse breccias, through many 
intermediate gradations, to fine silky schists, which are 
fine-grained ashes which have undergone metamorphism. 
