54 



HISTORICAL GEOLOGY 



Montana. " 1 More and still older forms are quite likely to be 

 discovered, though the remains thus far found are those of very 

 thin-shelled animals and hence not so favorable for fossilization. 

 Most animals of the time were probably without shells or other 

 hard parts. 



Very recently Walcott has described a number of species of 

 calcareous Algae from the Belt series (Algonkian) of Montana 



Fig. 25 



Part of a pre-Cambrian (Huronian) Alga. This is one of the oldest 

 known definitely determinable fossils. After Geol. Sur. Canada. 

 (From Schuchert's "Historical Geology," permission of John 

 Wiley and Sons.) 



and one (discovered by Lawson) from a Huronian limestone of 

 western Ontario, this latter being the oldest known well preserved 

 fossil. These Algae were very simple plants (Thallophytes) which 

 lived in water. They were hemispherical or cylindrical bodies 

 which secreted crudely concentric laj^ers of carbonate of lime 

 from 1 to 15 inches in diameter. They occur in distinct beds 

 through hundreds or even thousands of feet of Algonkian lime- 

 stones. 



Graphite and carbonaceous material so often disseminated 

 through the shales and schists almost certainly indicate the ex- 

 istence of life. Likewise beds of limestone (even near the base of 



1 C. D. Walcott: Jour. Geol, Vol. 17, 1909, p. 196. 



