CHAP, vm 
URICONIAN ERUPTIONS 
131 
cliistrict, and his recognition of Cambrian fossils under the Coal-measures of 
Warwickshire, supplied valuable evidence for the discussion of the geological 
position of the older rocks of the Midlands. He has mapped in minute 
detail the rocks of tlie Wrekin, and has exhausted all the evidence that is 
at present obtainable on the subject. But unfortunately the puhlication of 
his researches is still delayed.^ 
It is now recognized that the core of the ancient ridge, extending from 
near Wellington through the Wrekin, Caer Caradoc and other lulls, until it 
sinks beneath the Upper Silurian formations, is formed of igneous rocks 
that consist partly of lavas, partly of volcanic breccias and fine tufls. The 
lavas are thoroixghly acid rocks of the felsitic or rhyolitic type. One of 
them, about 100 feet thick, which forms a prominent feature on the flanks 
and crest of Caer Caradoc, shows abundant finely-banded flow-structure, 
olten curved or on end, while its bottom and upper parts are strongly 
amygdaloidal, the cavities being occasionally pulled out in the direction of 
flow and lined with quartz or chalcedony. Some of the detached areas of 
oruptive rocks show the beautiful spherulitic and perlitic structures first 
noticed in this region by Mr. Allport. More recently the structures of 
these acid rocks have been described by Mr. F. Eutley.^ 
The breccias and tuffs appear to consist mainly of felsitic material. In 
the coarser varieties, fragments of finely-banded felsite may be noticed, while 
the finer kinds pass into a kind of hornstone (hallefliuta), which in hand- 
specimens could hai'dly be distinguished from close-grained felsite. In 
some places, these pyroclastic rocks are well stratified, but elsewhere no 
satisfactory bedding can be recognized in them. Various other rocks, which 
nre probably intrusive, occur in the ridge. At either end of the Wrekin 
there is a mass of pink microgranite, while at Caer Caradoc numerous sheets 
of “ greenstone,” intercalated in the fine tuffs, sweep across the hill. Mr. 
Hutley has published an account of these basic rocks, which he classes as 
oaelaphyres,” or altered forms of basalt or andesite.’’ That at least some 
of them are intrusive is manifest by the way in which they ramify through 
the surrounding strata. But others are so strongly amygdaloidal and slaggy 
that they may possibly be true interbedded lavas, though there may be 
some hesitation in admitting that such basic outflows could he erupted in 
the midst of thoroughly acid ejections.* Leaving these doubtful flows out 
ot account, we have here a group of undoubted volcanic rocks represented 
y acid lavas and pyroclastic materials, by intrusive bosses of acid rocks, 
and by younger basic sills. The general lithological characters of these 
masses and the sequence of their appearance thus strongly resemble those of 
subsequent I’alieozoic volcanic episodes. 
wti I'- !’• t’- !’• w®* 
' ^ -Ir. W. \V'. Watts ou the Geology of South Shropshire, Proc. Gaol. Assoc, vol. xiii. (1894) 
1>I>- 302, 335. 
Quart, ,/ourn. Geol. Soc. vol. xlvii. (1891) p. 540. Mr. Rutley more particularly describes 
«^ose of Caradoc Hill. o/cU. p. 534 '. 
able ^hfficulty, however, need not be in itself iu.superable, a.s is evident from the reinark- 
anW^ ''^**^*^^*°*^ of basic and acid lavas and tutfs in the Cambrian volcanic group of St. David’s 
in the Old Red Sandstone series of the Pentlaud Hills. 
