78 
THE SILURIAN VOLCANOES 
BOOK IV 
Iniportaut confirmation of this vierv of the Cambrian age of the earlier 
volcanic eruptions of the Cader Idris region has recentlj’ been obtained by 
Messrs. P. Lake and S. H. Eeynohls who, in the ground intervening between 
the lower slopes of Cader Idris and Dolgelly, have ascertained the existence 
of a marked )>and of andesitic lava traceable for some distance in the upper 
Lingula Flags. They have also observed a higher volcanic group reposing 
upon the Tremadoc strata at the top of the Cambrian system, and consisting 
of rhyolite with rhyolite-tuffs.^ 
Some of the most stupendous memorials of the earlier eruptions are 
to be seen in the huge mountain mass of Phobell Fawr (2403 feet). They 
consist mainly of agglomerates and tuffs, one of the most remarkable varieties 
of which is distinguished by its abundant scattered crystals of hornblende 
and of augite. The fragments of rock included in these rocks are scoria; 
and lumps of various lavas, especially basaltic and trachytic andesites. The 
tuffs become finer towards the top of the mountain where they are inter- 
leaved with grits. Among the pyroclastic materials occasional lavas 
(basaltic andesites) occur which may be contemporaneous streams, but 
Fig. 46. — Section across Rliobell Fa\vr.‘^ 
L L, Lingula flags ; f, tuffs and ashy slates ; .<?, slates and grits ; F F, Areiiig volcanic series ; D, dolerite. 
most of the lava-form rocks appear to be intrusive. They include dolerites 
(augite-aphanites), basaltic andesites, and trachytic andesites.^ 
The materials from the lihobell Fawr volcano are clearly distinguishable 
from those of the Arenig volcanoes in the neighbourhood. The latter begin 
to make their appearance among the black slates at the base of the northern 
declivities of Cader Idris, and extend upward through that mountain into 
the country Ijeyond. 
An upper limit to this volcanic group is not easily traceable ; partly, 
no doubt, from tlie gradual cessation of the eruptions and partly from the 
want of any marked and persistent stratigraphieal horizon near the top of 
the group. Sir Andrew Eamsay, indeed, refers to the well-known band 
of pisolitic iron-ore as lying at or near to the top of the Arenig rocks.‘‘ 
There can be no doubt, however, that the volcanic intercalations continue 
far above that liorizon in the southern part of the district. 
In spite of the extent to which the volcanic masses of the Arenig period 
have been covered by later Faltcozoic formations, it is still possible to fix 
approximately the northern, western, and southern limits of the district 
over which the ashes and lavas were distributed. These materials die out 
* QimA. Jmirn. (feol. Soc. vol. lii. (1896), p. .511. 
'■* After Messrs. Cole and Holland, (leol. Mntj. (1890), p. 450. 
“ Prof. Cole, neoh Mag. (1893), p.' 337. 
Mem. Gcol. Surrey, vol. ill. 2nd edit. jip. 249, 250. 
