Coder /dr US 
182 
THE SILURIAN VOLCANOES 
BOOK IV 
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a coarse volcanic conglomerate like that below, lying at the base of the high 
precipice of Cader Idris. Hence tliis inter- 
calated group of sedimentary strata marks a 
pause in the discharge of ashes and lavas, 
during which the peculiar conditions of sedi- 
mentation indicated by the ironstone spread 
over at least the southern part of the volcanic 
area. Some few miles to the east, where the 
ironstone has been excavated near Cross boxes, 
the band is again found lying among tufls and 
grits full of volcanic lapilli. 
Between a lower and an upper band of 
tuff in the Arenig volcanic group the Maps 
and Memoirs of the Geological Survey dis- 
tinguish a central zone of “ felspathie por- 
phyry,” which attains a maximum thickness 
of 1500 feet (see Fig. 48). From Sir Andrew 
Bamsay’s descriptions, it is clear that he 
recognized in this zone both intrusive and 
extrusive sheets, and that tlie latter, where 
thickest, were not to be regarded as one 
miglity lava-llow, Imt rather as the result of 
successive outpourings, with occasional intervals 
marked by tlie intercalation of bands of slate 
or of tuff. To a certain extent the intruded 
sheets are separated on the map from the con- 
temporaneous lavas ; but this has been done 
only in a broad and sketchy way. One of tlie 
most important, and at the same time most 
difficult, tasks yet to be accomplished in this 
district is the separation of the rocks which 
were probalily poured out at the surface from 
those that were injected underneath it. My 
own traverses of the ground have convinced 
me that good evidence of superficial outflows 
may be found in tracts which have been mapped 
as entirely intrusive ; while, on the other hand, 
some of the so-called “ lavas ” may more prob- 
ably be of the nature of sills. 
The petrography of the rocks, moreover, 
still requires much study. Among the so- 
called “ felspathie porphyries ” of the Survey 
maps a considerable variety of texture, struc- 
ture and composition will doulitless be detected. 
In the DescrijMve Ckdalogue of Rock-Sijccimens 
in the Museurn of Practiced Geology (ord edit., 1862) the rocks that form 
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