26 THE DAWN OF LIFE. 



Considering the colossal thickness given to the series (90,000 

 feet), it remains to be seen whether it may not include both the 

 Lower and Upper Laurentian, and possibly, in addition, the 

 Hnronian. 



" This third Canadian group (the Hnronian) has been shown 

 by my colleague, Mr. Murray, to be about 18,000 feet thick, 

 and to consist chiefly of quartzites, slate-conglomerates, 

 diorites, and limestones. The horizontal strata which form 

 the base of the Lower Silurian in western Canada, rest upon 

 the upturned edges of the Huronian series ; which, in its turn, 

 unconformably overlies the Lower Laurentian. The Huronian 

 is believed to be more recent than the Upper Laurentian series, 

 although the two formations have never yet been seen in con- 

 tact. 



"The united thickness of these three great series may pos- 

 sibly far surpass that of all the succeeding rocks from the 

 base of the Palaeozoic series to the present time. We are thus 

 carried back to a period so far remote, that the appearance of 

 the so-called Primordial fauna may by some be considered a 

 comparatively modern event. We, however, find that, even 

 during the Laurentian period, the same chemical and mechani- 

 cal processes which have ever since been at work disintegrat- 

 ing and reconstructing the earth's crust were in operation 

 as now. In the conglomerates of the Huronian series there 

 are enclosed boulders derived from the Laurentian, which seem 

 to show that the parent rock was altered to its present crystal- 

 line condition before the deposit of the newer formation; 

 while interstratified with the Laurentian limestones there are 

 beds of conglomerate, the pebbles of which are themselves 

 rolled fragments of a still older laminated sand-rock, and the 

 formation of these beds leads us still further into the past. 



" In both the Upper and Lower Laurentian series there are 

 several zones of limestone, each of sufficient volume to consti- 

 tute an independent formation. Of these calcareous masses 

 it has been ascertained that three, at least, belong to the 

 Lower Laurentian. But as we do not as yet know with cer- 

 tainty either the base or the summit of this series, these three 

 may be conformably followed by many more. Although the 



