ii6 PRE-CAMBRIAN VOLCANOES book ii 
extravasations of eruptive materials, though it may not be always possible 
now to detei'inine the order in which these followed each other. In the 
feebly foliated or massive hands and bosses there is a parallel arrangement 
of their constituent minerals or of fine and coarse crystalline layers which 
recalls sometimes very strikingly the flow-structure of rliyolites and 
other lavas. This resend dance was strongly insisted on by Poulett Scrope, 
who believed that the laminar structure of sucli rocks as gneiss and mica- 
schist was best explained by the supposition of the flow of a granitic magma 
under great pressure within the earth’s crust.^ 
The conviction that these parallel structures do, in some cases, really 
represent traces of movements in the original unconsolidated igneous 
masses, not yet wholly effaced by later mechanical stresses, has been greatly 
strengthened in my mind by a recent study of the sti'uctures of various 
eruptive bosses, especially those of gahhro in the Tertiary volcanic series of 
the Inner Hebrides. The banded structure, the separation of the con- 
stituent minerals into distinct layers or zones, the alternation of markedly 
basic with more acid layers, and tlie puckering and plication of those bands, 
can be seen as perfectly among the Tertiary gabbro bosses of Skye as in 
the Lewisian gnei.ss (see Figs. 336 and 337). It cannot be contended that 
such structures in the gabbro are due to any subsequent terrestrial disturb- 
ance and consequent deformation. They must he accepted as part of tlie 
original structure of the molten magma.- It seems to me, therefore, highly 
})robable that the pai'allel banding in the uncrushed cores of the Lewisian 
gneiss reveals to us some of the movements of the original magma at the 
time of its extrusion and before it underwent those great mechanical 
stresses which have so largely contributed to the production of many of its 
most characteristic structures. 
While the material of the oldest gneiss presents many affinities to plutonic 
rocks of much younger date, a wide region of mere speculation opens out 
when we try to picture tlie conditions under which this material was 
accumulated. Some geologists have boldly advanced the doctrine that the 
Archaean gneisses represent the earliest crust that consolidated upon the 
surface of the globe. But these rocks offer no points of resemblance to the 
ordinary aspect of superficial volcanic ejections. On the contrary, the 
coarsely-crystalline condition even of those portions of the gneiss which 
seem most nearly to represent original structure, the absence of anything 
like scoriae or fragmental bands of any kind, and the resemblances which 
may be traced between parts of the gneiss and intnisive bosses of igneous 
rock compel us to seek the nearest analogies to the original gneiss in deep- 
seated masses of eruptive material. It is difficult to conceive that any 
rocks approaching in character to tlie gahbros, picrites, granulites and 
other coarsely-crystalline portions of the old gneiss could have consolidated 
at or near the surface. 
When the larger area of gneiss forming the chain of the Outer 
1 Volcanoes, ]>(]. 140, 283, 299. 
“ See A. Geilvie and .1. .1. H. Teall, Quart. Jourii. Ocol. Sue. vol. 1. (1894), p. 645. 
