[139] Quart. Journ. Geol. Soc. vol. xlv. (1889), p. 436; Geol. Mag. (1890), p. 447. Pres. Address Geol. Soc. 1890, p. 107.
Important confirmation of this view of the Cambrian age of the earlier volcanic eruptions of the Cader Idris region has recently been obtained by Messrs. P. Lake and S. H. Reynolds who, in the ground intervening between the lower slopes of Cader Idris and Dolgelly, have ascertained the existence of a marked band of andesitic lava traceable for some distance in the upper Lingula Flags. They have also observed a higher volcanic group reposing upon the Tremadoc strata at the top of the Cambrian system, and consisting of rhyolite with rhyolite-tuffs.[140]
[140] Quart. Journ. Geol. Soc. vol. lii. (1896), p. 511.
Some of the most stupendous memorials of the earlier eruptions are to be seen in the huge mountain mass of Rhobell Fawr (2403 feet). They consist mainly of agglomerates and tuffs, one of the most remarkable varieties of which is distinguished by its abundant scattered crystals of hornblende and of augite. The fragments of rock included in these rocks are scoriæ and lumps of various lavas, especially basaltic and trachytic andesites. The tuffs become finer towards the top of the mountain where they are interleaved with grits. Among the pyroclastic materials occasional lavas (basaltic andesites) occur which may be contemporaneous streams, but most of the lava-form rocks appear to be intrusive. They include dolerites (augite-aphanites), basaltic andesites, and trachytic andesites.[141]
[141] Prof. Cole, Geol. Mag. (1893), p. 337.
Fig. 46.—Section across Rhobell Fawr.[142]
L L, Lingula flags; t, tuffs and ashy slates; s, slates and grits; F F, Arenig volcanic series; D, dolerite.
[142] After Messrs. Cole and Holland, Geol. Mag. (1890), p. 450.
The materials from the Rhobell Fawr volcano are clearly distinguishable from those of the Arenig volcanoes in the neighbourhood. The latter begin to make their appearance among the black slates at the base of the northern declivities of Cader Idris, and extend upward through that mountain into the country beyond.
An upper limit to this volcanic group is not easily traceable; partly, no doubt, from the gradual cessation of the eruptions and partly from the want of any marked and persistent stratigraphical horizon near the top of the group. Sir Andrew Ramsay, indeed, refers to the well-known band of pisolitic iron-ore as lying at or near to the top of the Arenig rocks.[143] There can be no doubt, however, that the volcanic intercalations continue far above that horizon in the southern part of the district.