ANCIENT LIFE ON THE EARTH 19 



sponge. The fauna is, therefore, an extensive and 

 varied one ; and it is evident that both Radiolarians 

 and sponges had existed for a long time when the 

 rocks of Brittany were being laid down. Even if 

 they are wrongly referred to the Huronian period, 

 this great variety of form may be taken as good 

 evidence that the ancestors of these Eadiolarians 

 and sponges existed long before the Mollusca and 

 Trilobites of the Algonkian period came into 

 existence. 



Algonkian Life. In Nevada and Utah on the 

 west, and in Vermont and New Brunswick on the 

 east of North America, as also in North-west India, 

 the Algonkian beds are overlaid conformably by the 

 Cambrian; but in all other known places, not only 

 in North America, but also in Europe, there is an 

 unconformity at the base of the Cambrian, thus 

 distinctly separating the two systems. 



In the rocks of Animikie, near Lake Superior, a 

 shell something like Lingula, as well as some obscure 

 fragments of Trilobites and worm-like tracks have 

 been found. In the rocks of the Grand Canyon of 

 Colorado, where it passes through Arizona, Dr. 

 Walcott has detected in a limestone, about 4,000 feet 

 below the base of the Cambrian, abundant fragments 

 of a genus which differs from S'tromatopora in having 

 thinner and coriaceous laminae without any connec- 

 ting pillars or pores. Four hundred and fifty feet 

 higher up he found a fragment of what seems to be 

 the pleural lobe of a segment of a Trilobite ; also a 

 minute discinoid or patelloid shell and a small 

 Lingula-like shell, possibly a HyoUthes. In North 



