The Era of Invertebrates 



entirely before we are half-way toward the 

 lower fossil-bearing rocks, while reptiles and 

 amphibians are found wanting farther down. 

 But at the very beginning of things the animals 

 that have actually come to light are somewhat 

 higher than many living to-day, and hence it is 

 inferred that these must have been pi-eceded 

 by other and still lower creatures. These 

 su23posititious, minute, single-celled plants and 

 animals that marked the very commencement 

 of life would, from their very nature, have left 

 no traces of their presence, and many of the 

 rocky pages in which the history of the past is 

 contained are blank, not because there were no 

 livino; thins^s, but because these have left no 

 record of their existence. The existence of life 

 at an early date is also inferred by some from 

 the presence, in those ancient rocks called Lau- 

 rentian, of strata of limestone, beds of grajjhite, 

 layers of shale and slate containing carbona- 

 ceous material, all of which may be organic prod- 

 ucts, that is, the results of the action of plant 

 and animal life, as well as the result of chem- 

 ical action. 



59 



