8o 
THE SILURIAN VOLCANOES 
BOOK IV 
been mere gaseous explosions, with the discharge of comparatively little 
volcanic material. Many of the tuffs that are interstratified with black 
slates (? Lingula Flags) at the foot of the long northern slope of Cacler 
Idris, consist mainly of black-slate fragments like the slate underneath, 
with a variable proportion of grey volcanic dust. 
The accompanying section (Fig. 47) represents the arrangement of the 
rocks exposed at the Slate Quarry of Peurhyn Gwyn. About 50 feet of 
black slate (a) are there seen, the bedding in which 
dips S. at 20°, while the cleavage is inclined towards 
S.W. at a slightly higher angle. The next 20 feet 
of slate {b) are distinguished by many intercalations 
of slate -tuff or breccia, varying from less than an 
inch to three feet in thickness. An intrusive sheet 
of andesite (c), which varies from two or three to 
ten feet in thickness, and is strongly cellular in the 
centre, interrupts the slates and hardens them. Above 
this sill the indurated slate and tuff (d), containing 
abundant felspar crystals, pass under a flinty por- 
phyritic felsite (e) or exceedingly fine tuff, enclosing 
a band of granular tuff. Beyond this band the black 
slates with their seams of tuff continue up the hill and 
include a sheet of slaggy felsitic lava 8 or 1 0 feet thick. 
This section, affording as it does the first glimpse of the volcanic history 
of Cader Idris, indicates a continued series of feeble gaseous discharges, probably 
from one or more small vents, whei’eby the black clay on the sea-floor was 
blown out, the fragments falling back again to be covered up under a gradual 
accumulation of similar dark mud. By degrees, as the vigour of eruption 
increased, lava-dust and detached felspar crystals were ejected, and eventually 
lava rose to the surface and flowed over the sea-bottom in thin sheets. 
But elsewhere, and likewise at a later period in this same southern part 
of the district, the fragmental discharges consisted mainly of volcanic material. 
Sir Andrew Pmmsay has described the coarse conglomerates composed of 
subangular and rounded blocks of different “porphyries,” somtimes 20 
inches in diameter, embedded in a fine matrix of similar materials. The 
true nature of the component fragments in these rocks has still to be 
worked out. 
Messrs. Cole and Jennings have noticed that the grey volcanic dust of 
the older slate-tuff of Cader Idris is seen under the microscope “ to abound 
in 2 rarticles of scoriaceous andesite - glass, now converted into a green 
]mlagonite.” ' Their investigations show that while the same kinds of 
volcanic rocks continue to be met with from the bottom to the top, 
nevertheless there is an increase in the acid character of the lapilli as the 
section is ti’aced upwards. Some of the fragments consist of colourless 
de vitrified glass, with pieces of pumice, as if derived from the breaking u]) 
of previovrsly-formed tuffs. Others resemble quartz-andesites, rhyolites, or 
^ Quart Journ. GeoJ. Soc.j vol. xlv. (1889), p. 424 ; GeoL Mag. (1890), p. 447. 
Fig. 47. — Section at the Slate 
Quarry, Penrhyn (Iwyn, 
north slopes of Cacler 
Iilris. 
