210 
THE SILURIAN VOLCANOES 
BOOK IV 
stratified among the tuffs. Sir Andrew Eainsay has referred with justice to 
the difficulty of always discriminating in the field between the fine tuffs and 
■ some of the lavas.^ Yet I am compelled to admit that, if the ground were 
to he re-mapped now, the area represented as eoverec^ by fragmental rocks 
would be considerably restricted. Mr. Marker is undoubtedly correct when 
he lemaiks that, taken “as a whole, the Bala volcanic series of Caer- 
narvonshire is rather remarkable for the paucity of genuine ashes and 
agglomerates.”^ 
'I he lavas of the Bala volcanic group, like those of the Arenig series, 
were ^mapped by the Survey as “ porphyries,” “ felstones,” or “ felspathie 
traps.” They were shown to be acid-lavas, having often a well-developed 
flow-structure comparable with that of olisidian and pitchstoue, and to 
consist of successive sheets that were poured out over the sea-floor. Their 
petrography has subsequently been studied more in detail by many observers, 
among whom I need only cite Professor Bonney, Professor Cole, Mr. Ptutley! 
Mr. Teall, and Miss Eaisin; tlie most important recent additions to our 
knowledge of this subject having been made by Mr. Marker in the Essay 
to which I have just referred. 
The great majority of these lavas are thoroughly acid rocks, and present 
close analogies of composition and structure to modern rhyolites, though I 
prefer to retain for them the old name of “felsites.” Their silica-percentage 
ranges from 75 to more than 80. To the naked eye they are externally 
pale greyish, or even white, but when broken into below the thick decom- 
posed and decoloured crust, they are bluish-grey to dark iron-grey, or even 
black. They break with a splintery or almost conchoidal Vracture, and 
show on a fresh surface an exceedingly fine - grained, tolerably uniform 
texture, with minute scattered felspans. 
One of their most striking features is the frequency and remarkable 
development of their flow-structure. Mot merely as a microscopic character 
but on such a scale as to be visible at a little distance on the face of a cliff 
or crag, this structure may be followed for some way along the crops of 
particular flows. The darker and lighter bands of devitrification, with their 
lenticular forms, rude parallelism and twisted curvature, have been com- 
pared to the structure of mica -schist and gneiss. One aspect of this 
structure, however, appears to have escaped observation, or, at least, has 
attracted less notice than it seems to me to deserve. Tn many cases it is 
not difficult to detect, from the manner in which the lenticles and strips of 
the flow-structure have been curled over and pushed onward, what was the 
direction in which the lava was moving while still a viscous mass. By 
making a sufficient number of observations of this direction, it mio-lit in 
some places be possible to ascertain the quarter from which the several 
flows proceeded. As an illustration, I would refer to one of the basement- 
felsites of Snowdon, wliioh forms a line of picturesque crags on the slope 
facing Llanberis. The layers of variously-devitrified matter curl and fold 
over each other, and have been rolled into balls, or have been broken up 
^ Oj?. cit. p. 148. ^ Bidet Volcanic Bvcl-s, p. 2S. 
