THE LAVAS OF THE GLYDER FAWR, NORTH WALES. 509 
or denticulate boundaries, as though their configuration had been 
affected by the crystallization of the surrounding matter. These 
may be considered spherulitic rhyolites. If this assumption be 
true, the spherules, or pseudo-spherules, must be regarded as 
crystalline structures emanating from separate points of devitrifi- 
cation. In the rocks of this kind which I have examined I have 
met with no trace of perlitic structure. Other felstones from the 
Llanberis neighbourhood give evidence of fluxion by the presence of 
irregular bands, which merely differ from the surrounding matter 
in texture, and simply imply a difference in the character of two 
imperfectly incorporated glasses of different densities, a difference 
which is still demonstrated by variation in the texture or grain of 
the devitrification products. Some of the felstones of the Glyders 
contain spheroidal bodies, occasionally as large as filberts. These 
are not true spherulites. I believe that they are now being examined 
and will shortly be described by Prof. Bonney; and since they are 
in such able hands, I leave them alone, resting assured that due 
justice will be done to them. In conclusion I have merely to 
remark that what I have often expressed as a belief is now, to some 
small extent, demonstrated—namely, that some of the Welsh fel- 
stones are the representatives of formerly vitreous rocks. 
It is interesting to find that, from the Lower Silurian period until 
the present day, time has failed to efface from some of these lavas 
the structural features which characterize rocks of a vitreous nature, 
although they have long ceased to possess the physical characters 
by which vitreous rocks are commonly distinguished. 
It is only by the recognition of structural peculiarities that we 
can hope to demonstrate the original character of some of our most 
interesting paleozoic lavas. 
Devitrification has, no doubt, often obliterated these structures. 
We then have no clue whatever to the conditions under which such 
rocks haye been erupted; and in view of this fact we can hardly 
look forward to the day when felstones will be struck off the roll of 
normal eruptive rocks. In many cases felstones are essentially 
devitrified hyaline rhyolites ; but it seems impossible to demonstrate 
that they are so in all instances. At the present time the felstones 
offer some of the greatest difficulties and the greatest attractions to 
the student of petrology. 
Discussion. 
Prof. Bonnny stated that there could be no doubt the Wrekin 
rocks were much older than those of the Snowdonian district. He 
quite agreed with the author that this specimen was a true perlite. 
He mentioned the fact that the lavas of Bala age in Wales were 
generally vitreous, and instanced some remarkable cases of sphe- 
rulitic structure from that district. 
Prof. Jupp stated that among the most ancient rocks of the north- 
west of Scotland were lavas showing spherulitic and fluidal struc- 
ture. These were also common in the Old Red Sandstone lavas. He 
thought that as the spherulitic, perlitic, and fluidal structures were, 
