THE SILURIAN VOLCANOES 
BOOK IV 
1 88 
and tuffs die out laterally, and their scarcity above the lower part of the 
volcanic group, seem t< > indicate their close relationship to that group. More- 
over, that they must have been as a whole later than the main part of the lavas 
and tuffs may be inferred from their position. The molten material of which 
they were formed could hardly have forced its way between and across the 
strata unless egress to the surface had been impeded by some thick overlying 
mass. The “greenstones” may therefore be regarded as lateral emanations from 
funnels of more basic lava towards the close of the volcanic period. Possibly 
some at least of the highly slaggy and vesicular bands to which I ha^ e 
referred may represent portions of this material, which actually flowed out 
as streams of lava at the surface. 
But there is likewise evidence of extensive intrusion of more siliceous 
rocks. On the Geological Survey map, besides the numerous “ greenstones,” 
various sheets of “ felspathic porphyry ” are represented as running with the 
general strike of the region, but here and there breaking across it. One of 
the most remarkable of these acid sills is that wdiich, in the noble precipice 
of Cader Idris, has a thickness of aborrt 1500 feet and a length of three or 
four miles. It is shown on the maj) to be transgressive across other rocks, 
and, as seen on the ground, it maintains the uniformity of texture which is 
characteristic rather of sheets that have solidified ciuderne<ath than of those 
which have congealed with comparative rapidity at the surface. On a fresh 
fracture the rock presents a pale bluish -grey tint, becoming yellowish or 
brownish as the result of weathering. Its texture is finely granvdar, with 
occasional disseminated felspars. Under the microscope a section of it was 
found by Dr. Hatch to exhibit the characteristic structure of a niicrogranite, 
a confused holocrystalline aggregate of quarts! and felspar, with a few por- 
phyritic felspars. IMessrs. Cole and Jennings have proposed to revive for 
this rock Daubuisson’s name “ Eurite.” ^ 
A similar rock occurs at a lower horizon among the Lingula Elags at 
Gelli-llwyd-fawr, two miles south-west of Dolgelly,'^ and much microgranite 
has been injected along the slopes above Tyddyn-mawr. 
The chronological relation of these acid sheets and bosses to the more 
basic intrusions has not yet been definitely determined. That some of them 
may have solidified in vents and may have lieen directly connected with the 
protrusion of the later or more highly siliceous lavas is not at all improb- 
able. Others again would seem to belong to a much later geological period 
than the Arenig volcanoes. In this late series the well-known boss of 
Taii-y-gi-isiau near Eestiniog shoixld probably be included. This mass 
of eruptive material was mapped by the Geological Survey as “intrusive 
syenite.” It has been more recently examined and descrilxed by Messrs. 
Jennings and Williams as a granitite.^ These observers have noticed not 
only that it intrusively trav'erses and alters the Tremadoc group, but that 
its intrusion appears to have taken place subsequent to the cleavage which 
1 Hr. Harker speaks of the roek as a granophyre. 
- Messrs. Cole and .Teniiings, op. cit. p. 4.35. 
“ Quart. Jouni. Ifeol. Soc. vol. xlvii. (1S91), p. 379, 
