THE LAURENTIAN AND HURONIAN PERIODS. 67 
ancient metamorphic rocks of Bohemia and Bavaria may be 
regarded as being approximately of the same age. 
By some geological writers the ancient and highly meta- 
morphosed sediments of the Laurentian and the succeeding 
Huronian series have been spoken of as the “ Azoic rocks” 
(Gr. a, without ; zoe, life) ; but even if we were wholly destitute 
of any evidence of life during these periods, this name would be 
objectionable upon theoretical grounds. Ifa general name be 
needed, that of ‘ Eozoic” (Gr. eos, dawn ; zoe, life), proposed 
by Principal Dawson, is the most appropriate. Owing to their 
metamorphic condition, geologists long despaired of ever de- 
tecting any traces of life in the vast pile of strata which con- 
stitute the Laurentian System. Even before any direct traces 
were discovered, it was, however, pointed out that there were 
good reasons for believing that the Laurentian seas had been 
tenanted by an abundance of living beings. These reasons 
are briefly as follows :—(1) Firstly, the Laurentian series con- 
sists, beyond question, of marine sediments which originally 
differed in no essential respect from those which were subse- 
quently laid down in the Cambrian or Silurian periods. (2) 
In all formations later than the Laurentian, any limestones 
which are present can be shown, with few exceptions, to be 
organic rocks, and to be more or less largely made up of the 
comminuted debris of marine or fresh-water animals. The 
Laurentian limestones, in consequence of the metamorphism 
to which they have been subjected, are so highly crystalline 
(fig. 21) that the microscope fails to detect any organic struc- 
ture in the rock, and no fos- 
sils beyond those which will 
be spoken of immediately have 
as yet been discovered in 
them. We know, however, of 
numerous cases in which lime- 
stones, of later age, and un- 
doubtedly organic to begin 
with, have been rendered so 
intensely crystalline by meta- 
morphic action that all traces 
of organic structure have been ? 
obliterated. We have there- eZ 
fore, by analogy, the strongest __ Fig. 21.—Section of Lower Laurentian 
possible ground for believing Fingone from Hull, Ottawa; enlarged 
that the vast beds of Lauren- crystalline, and contains mica and other 
tian limestone have been ori- ea eeittal Only acne 
ginally organic in their origin, 
and primitively composed, in the main, of the calcareous skele- 
