THE SILURIAN VOLCANOES 
BOOK IV 
184 
The slopes of Cader Idris are partly obscured with debris, from above 
which rises the great precipitous face formed by the escarpment of 
“ porphyry,” here intrusively interposed among the Arenig volcanic rocks. 
I'his enormous sill will be referred to a little further on in connection 
with the other intrusive sheets of the region. 
The remarkably cellular rock which forms the peak of Cader Idris is 
coloured on the Survey map as an intrusive sill of “greenstone,” which in 
the Memoir is said to alter the contiguous slates and to appear to cut 
across them diagonally. I am disposed, however, to think that these 
ajipearances of intrusion are deceptive. On the southern declivity of the 
mountain this rock presents one of the most curious structures to be seen 
in the whole district. Its surface displays a mass of sjiheroidal or pillow- 
shaped blocks aggregated together, each having a tendency to divide 
internally into prisms which diverge from the outside towards the centre.^’ 
Some portions are extremely slaggy, and round these more solid portions finely 
crystalline parts are drawn, suggestive rather of free motion at the surface 
than of the conditions under which a siibterranean sill must be formed. 
The idea occurred to me on the ground that while the band of rock marked 
as “greenstone” on the map is proljably, in the main, an interstratified 
lava, there may nevertheless he basic intrusions along its course, as in the 
lower part of the mountain. The minute structure of this amygdaloid, 
as revealed by the microscope, shows it to be an epidiorite wherein the 
hornblende, paramorphic after augite, has been again partially altered along 
the margins into chlorite. 
The highest lavas of Cader Idris, forming the ridge to the south of 
Llyn Can, are separated from the amygdaloid just described by a thick 
zone of black slate with thin ashy intercalations, beyond which comes the 
coarse volcanic agglomerate already referred to as containing blocks of 
felsite a yard or more in diameter. These lavas are true felsites, sometimes 
lieautifully sphcrulitic and exhiliiting abundant fiow- structure, like some 
of the felsites of the next or Bala volcanic period.'^ The petrography of 
these rocks still remains to be worked out. 
The volcanic series of Cader Idris sweeps northward through the chain 
of Aran and Arenig, and then curves westward through the group of 
Manod and Moelwyn, beyond which it rapidly dies out. In its course of 
’ This jieculkr structure of the more ba.sio Arenig lavas, where the rock looks as if built up 
of irregularly-spheroidal, sack-like or pillow-shaped blocks, will ho again referred to in connection 
with the Arenig (and Llaudeilo) lavas of Scotland and Ireland. It ajipears to he widely distributed, 
and especially in connection with the occurrence of radiolaiian cherts. The black .slate above the 
Cader Idris amygdaloid would, in a similar position in Scotland, be associated with such cherts, 
but these have not yet lieen noticed at thus locality. With the spheroidal internally-radiating 
Iirismatio structure of the Cader Idris rock, compare that of the lava at Acicastello already- 
noticed on p. 2ti, 
2 Messrs. Cole and Jennings, Quart. Jouru. Geot. Soe. vol. xlv. (1889), p. 430. From the examina- 
tion of slices prepared from a few of the felsites of the Dolgelly district. Dr. Hatch observed a 
“striking dilferonee between their characters and those of the Cambrian felsites of Caernarvonshire. 
The porphyritic constituent is now no longer (piartz, but felspar (plagioelase), and the rocks 
belong, not to the rhyolitic, but rather to the less acid trachytes, perhaps even to the andesites.” 
