THE LAURENTIAN AND HURONIAN PERIODS. 6/ 



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. If a 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 : (i) 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- 

 fore, by analogy, the strongest 

 possible ground for believing 

 that the vast beds of Lauren- 

 tian limestone have been ori- 

 ginally organic in their origin, 

 and primitively composed, in the main, of the calcareous skele- 



Fig. 21. Section of Lower Laurentian 

 Limestone from Hull, Ottawa; enlarged 

 five diameters. The rock is very highly 

 crystalline, and contains mica and other 

 minerals. The irregular black masses in 

 it are graphite. (Original.) 



