I 10 
PRE-CAMBRIAN VOLCANOES 
KOOK II 
advent of living things on the surface of the earth. The chance discovery 
of a single fossil, wliieh might at any moment lie made, would show the 
name “ Azoic ” to he a misnomer. Other geologists, believing that, as a 
matter of fact, organic structures of low types do actually occur in them, 
have called these old rocks “ Eozoic,” to denote that they were deposited 
during the dawn of life upon our planet. But the supposed organisms 
have not been everywhere accepted as evidence of former life. By many 
aide observers they are regarded ns mere mineral aggregates. Another 
term, “Archaean,” has been proposed for the primeval ages of geological 
history, whicli are recorded in rocks that carry us as far as may ever he 
possilile towards the beginnings of that history. 
In choosing some general term to include the oldest known parts of the 
earth’s crust, geologists are a})t unconsciously to assume that the rocks thus 
classed together I’epresent a definite section of geological time, comparable, 
for instance, to that denoted by one of the Paheozoic systems. Yet it is 
olivious that, under one of these general terms of convenient classification, a 
most imdtifarious series of rocks may be included, representing not one but 
possibly many, and widely separated, periods of geological Instory. 
In many countries tlie oldest sedimentary accumulations, whether 
fossiliferous or not, are underlain by a series of crystalline rocks, which 
consist in great part of coarse massive gneisses and otlier schists. All over 
the world these rocks present a singular sameness of structure and composi- 
tion. What might be found below them no man can say. They are in 
each country the oldest rocks of which anything is yet known, and whatso- 
ever may be our theory of their origin, we must, at least for the present, 
start from them as the fundamental platform of the terrestrial crust. 
But though crystalline rocks of this persistent clinracter are widely 
distributed, both in tlie Old YArld and in the New, they in themselves 
furnish no nieams of determining their precise geological age. No method 
has yet been devised whereby the oldest gneiss of one country can be shown 
to be the true stratigraphical equivalent of the oldest gneiss of another. 
Palaiontology is here of no avail, and Petrology has not yet provided us 
with such a genetic scheme as will enable us to make use of minerals and 
rock -structures, as we do of fossils, in the determination of geological 
horizons. All that can be positively affirmed regarding the stratigraphical 
relations of the rocks in question is that they are vastly more ancient than 
tlie oldest sedimentary and fossiliferous formations in each country where 
they are found. The “ Lewisian ” gneiss of the north-west of Scotland, tlie 
“Urgneiss” of Central Europe, and the “ Lauren tian” gneiss of Canada 
occupy similar stratigrapliieal positions, and present a close resemblance in 
lithological characters. We may conveniently class them under one common 
name to denote this general relationship. But we have, as yet, no means of 
determining how for they belong to one continuous period of geological 
history. They may really be of vastly different degrees of antiquity. 
From the very nature of the case, any name by which we may choose to 
designate such ancient rooks cannot possess the precise stratigraphical value 
