120 
PRE-CAMBRIAN VOLCANOES 
BOOK 11 
dykes ; and these movements, in turn, had long been accomplished before 
the Torridon Sandstone was laid down, for the dykes, with their abundant 
deformation, run up to and pass beneath the sandstone which buries them 
and all the rocks with which they are associated. Though later than the 
original fundamental complex, the dykes have become so integral and 
essential a part of the gneiss as it now exists that they must be unhesitat- 
ingly grouped with it. 
With so wide an extension of the subterranean relics of volcanic 
energy, it is sui’ely not too much to hope that somewhere there may have 
been preserved, and may still be discovered, proofs that these eruptive rocks 
opened a connection with the surface, and that we may thus recognize 
vestiges of the superficial products of actual Archaean volcanoes. Among 
the pebbles in the conglomerates of the Torridon Sandstone there occur, 
indeed, fragments of felsites which possess great interest from the perfection 
with which they retain some of the characteristic features of younger 
lavas. Mr. Teall has described their minute structure. They are dark, 
purplish, compact rocks, consisting of a spherulitic micro-pogmatitic, micro- 
poikilitic or micro-crystalline groundmass, in which are imbedded porphy- 
ritic crystals or crystal-groups of felspar, often oligoclase. These spherulitic 
rocks occasionally show traces of perlitic structure. They bear a striking- 
resemblance to some of the Uriconian felsites of Shropshire, pebbles from 
which occur in the Longmynd rocks.' These fragments suggest tlie 
existence of volcanic materials at the surface when the Torridon Sandstone 
was deposited. Possibly they may represent some vanished Lewisian lavas. 
But the time between the uprise of the dykes and the formation of the 
Torridonian series was vast enough for the advent of many successive 
volcanic episodes. The pebbles may therefore be the relics of eruptions 
that took place long after the j^eriod of the dykes. 
Among the Torridonian strata no undoubted trace of any contempor- 
aneous volcanic eruptions has been met with.'^ The only relics of volcanic 
rocks in this enormous accumulation of sediments are the pebldes just 
referred to, which may be referable to a time long anterior to the very 
oldest parts of the Torridonian series. 
That Archaean time witnessed volcanic eruptions on a considerable scale, 
and with great variety of petrographical material, has recently been sliown 
in detail by Mr. Otto Nordeiiskjbld from a study of the rocks of Smaland 
in Sweden. He has described a series of acid outbursts, including masses 
of rhyolite and dacite, together with agglomerates and tuffs, likewise basic 
eruptions, with dioritic rocks, augite-porphyrite and breccia. He refers 
these rocks to the same age as most of the Scandinavian gneisses, and re- 
marks that though they have undergone much mechanical deformation and 
metamorphism, they have yet here and there retained some of their dis- 
tinctive volcanic structures, such as the spherulitic.^ When the large 
^ Annml Report of GcolnrjieeU Sm-rcy for 1895, p. 21 of reprint. 
2 Tlio supposed tuff referred to in Quart. Journ. Geol. Sue. vol. xlviii. (1892), p. 168, is 
probably not of truly volcanic origin. 
“ liber Arcliieischc Krgussgesteine aus Smaland,” Sveriges Geot. Undertsokn, No. 135 (1894). 
