Vice-President’s Address. 303 
those originally described by Sir Archibald Geikie in con- 
nection with the Lanarkian and Caledonian Old Reds; and, 
still more to the point, of the same nature as those I 
described in my paper “On the Stratigraphical Relations 
of the Skidda Slates,” in which rocks of Bala age have been 
made to lie upon the upturned edges of the Tremadoc and 
older rocks of the northern side of the English Lake District; 
and, again, in the case of the Bala Rocks in Ireland, which 
repose upon the upturned ends of the Cambrians. I do not 
therefore consider that we are fully justified in counting the 
unconformity specially under notice, as it is probably, in 
part at least, contemporaneous in time with the Bala Rocks 
themselves. 
Time required for the Formation of the Older Ordovician 
Rocks.—Still confining our attention to the Ordovician Rocks 
of the Longmynd area, we find that the collective thickness 
of the Llandeilo? and Arenig Rocks there is, according to the 
Survey “ Horizontal” Section, fully 16,000 feet. Here again 
the rocks consist largely of marine strata, amongst which 
graptolitiferous mudstones bulk largely. This is especially 
true of the upper part of the series. The lower part, still 
marine, includes a large percentage of volcanic material, and, 
moreover, contains quartz-particles in abundance. But if we 
consider that the more rapid formation of the lower strata is 
compensated, or more than compensated, by the slow rate of 
accumulation of the upper, as evidenced by both its mineral 
character and the biological changes of which its fossils 
present a record, we should be well within the mark if we 
estimate that the formation of these Lower Ordovician Rocks, 
as a whole, went on at the average rate here assumed, 
2.€., 1 foot in 3000 years. At this computation these rocks 
add 33,000,000 years to our roll. 
Time implied by the Cambrian Period.—In general, the 
Lower Ordovician Rocks pass down conformably into the 
underlying Cambrians. In North Wales, near Harlech, 
1 Proc. Geol. Assoc. 
2? I employ the term Llandeilo for those strata which occur below the 
pre-Bala unconformity, and which conformably succeed the rocks of Arenig 
age. 
