508  F. RUTLEY ON PERLITIC AND SPHERULITIC STRUCTURES IN 
38. On Periitic and SpHeruriric Structures in the Lavas of the 
GiypER Fawr, Norte Watzs. By Franx Rurtey, Esq., F.G.S8., 
H.M. Geological Survey. (Read March 12, 1879.) 
(Communicated by permission of the Director-General of the Geological 
Survey.) 
Amone the specimens belonging to the rock-collection in Jermyn 
Street of the various rocks which constitute the Glyders on the 
north side of the Pass of Llanberis, and which have been mapped 
by the Geological Survey as felstones and felspathic traps, and 
described by Professor Ramsay as lava-flows, I have discovered one 
which presents unquestionable perlitic structure. (The specimen is 
marked 376, wall-case 41 in the published catalogue.) The rock is 
associated with Bala-beds, and is overlain by shales containing 
fossils similar to those which occur in the Bala Limestone. 
It presents the appearance of a felstone to the naked eye, and by 
polarized light, under the microscope, it shows the microcrystalline 
structure which felstones so commonly exhibit. It is now practi- 
cally a felstone ; once it was a vitreous lava. Portions of the spe- 
cimen show a coarsely vesicular structure ; and the vesicles are, for 
the most part, filled by crystalline aggregates of quartz. 
The above observations perfectly confirm the early conclusions 
formed by the Survey, which express as much truth as it was pos- 
sible. to arrive at in the absence of microscopic investigation. 
The only additional information now procured is that the lava in 
question was once vitreous, and that it still shows the perlitic struc- 
ture as clearly as the perlites of Saxony, which are of Tertiary age. 
The accompanying figure will sufficiently demonstrate this fact. 
Some of the other felstones of the Glyders show spherulitic 
structure ; and in some cases, indeed, they consist almost exclusively 
of little bodies which frequently present ill-defined or even serrated 
