424 Notices of Memoirs—R. N. Worth—Geology of Plymouth. 
originating an enormous vertical and lateral displacement, the ex- 
treme thrust of which was taken by a rocky buttress occupying 
what is now the Channel area,—itself probably an active agent. 
Where these two forces more nearly approached each other, folding 
and repetition would be most marked; while the widening of the 
space between the moorland and Channel axes would be accompanied 
by an expansion of the folds, and lead after denudation to greater 
irregularity of stratigraphical relationship. It was in the Plymouth 
district that the Devonian rocks of South Devon were pinched to 
their narrowest limits; and he believed that they supplied the key 
to the area generally. 
Dividing these rocks into three series—the slates, etc., underlying 
the limestones; the limestones; and the overlying slates and sand- 
stones ;—Mr. Worth again subdivided the lower group into—(a) 
slates with metalliferous veins and elvans; which he called, from the 
locality of their greatest development, the Buckland and Bickleigh 
beds ; (6) slates with interbedded volcanic matter—the Weston and 
Compton beds; (¢) purple and green slates—the Mutley beds. All 
these, with the overlying limestones, belonged to the Middle Devonian ; 
and were to be correlated with the Ilfracombe group of North Devon. 
Except in the immediate vicinity of the limestone, none of these 
beds were fossiliferous. 
The slates overlying the limestone on the east of the Sound (and 
practically on the west also) appeared to belong to the Morthoe 
group. The Staddon and Bovisand grits and associated beds seemed 
to be faulted down and in all probability represented the Pickwell 
Down sandstones. Associated with these, but the conditions of 
association not very clearly made out, were highly fossiliferous 
slates, in some respects strongly resembling those of South Petherwin. 
Mr. Worth suggested that all these groups might be traced folding 
round the granite in South Devon and East Cornwall; and that 
detailed mapping would render the connection clear, while without 
it nothing could certainly be done. However scattered the South 
Devon limestones might appear, he believed they belonged essentially 
to one horizon. 
Going westward from Plymouth, Mr. Worth believed that there 
was a well-marked descending series, and that the east of the county 
of Cornwall only could be treated as Devonian. The existence of 
Lower Silurian rocks on the South Coast of Cornwall was admitted 
on all hands; and Mr. J. H. Collins, F.G.S., had adduced what 
seemed conclusive evidence that Upper Silurians extended across the 
county, and that the rocks in the mining district of West Cornwall 
were Cambrian or at least pre-Silurian. 
