64 EVOLUTION OF THE EARTH 



order and possibly more prevalent on the lands than in the 

 oceans. 



Then follows the second era of geologic time, the Protero- 

 zoic, with a thickness in south-central Canada of nearly 14 

 miles of coarse sediments almost devoid of fossils, and 4 miles 

 of lavas. Probably more than three-fourths of this mass is 

 of fresh-water and volcanic origin, and less than one-fourth 

 may be of marine origin. In the Rocky Mountains there are 

 upwards of 7 miles of less coarse sediments that include about 

 1.5 miles of possibly marine limestones. In this connection it 

 should also be noted that one of the remarkable recent dis- 

 coveries in geology is that we know very little of marine Pro- 

 terozoic sediments (in the sense of marine and fossiliferous 

 Paleozoic formations) and that there is as yet even no hint 

 as to where geologists are to look for them. 



The Archeozoic and Proterozoic eras have the most ancient 

 strata of the earth and their combined thickness is 32 miles as 

 against about 21 miles for all subsequent, fossiliferous strata. 

 In this we discern the astounding fact that at least one-half 

 of geological time and the greater part of the earth's history 

 had passed before organisms became sufficiently endowed with 

 hard parts to be abundantly preserved as fossils in the sedi- 

 mentary rocks. It also means that the evolution of life for an 

 incredibly long time was exceedingly slow in rising from the 

 simple unicellular forms into the multicellular forms of plants 

 and animals. It may be that the Archeozoic was entirely 

 occupied in originating only the unicellular plants and animals. 

 In the Proterozoic, organic differentiation appears to have 

 gone on faster, because at the very beginning of the Paleozoic 

 era are found all of the main kinds of marine animals other 

 than fishes. The subject of the origin and progress of life 

 from the unicellular forms to the most complex modern verte- 

 brates will be developed by Professors Woodruff and Lull in 

 subsequent lectures. 



