The Story of the Uocks 



117 



while the physicist is yet more extravagant, asking in the light 

 of recent experiments on radium 1,600,000,000 years for the 

 age of the earth since it attained its present diameter. 



So in rehearsing briefly the story of the rocks perhaps we 

 cannot do better than employ the words of the old story books 

 and say that "once upon 

 a time" when the waters 

 covered most of North 

 America and the earliest 

 Laurentian rocks of 

 northeastern Canada 

 were beginning to be 

 lifted up, above the sur- 

 face of the sea, life prob- 

 ably came upon the earth 

 in the form of unicellular 

 plants and animals. But 

 regarding the birth of 

 life the rocks are mute. 

 We have no record of its 

 advent or its cradle. 



Its earliest remains 





I 



A. TBILOBITE 



Original photograph from a specimen 

 in the geological collection of the Uni- 

 versity of North Dakota. 



Courtesy of Dr. A. G. Leonard. 



known are those of the 

 Huronian period, where 

 buried beneath rock strata 



several miles in thickness are marine alg*, radiolarians and 

 the tubes and burrows of annelid worms. Following these 

 there appeared in the Cambrian period all the principal 



branches of inverte- 

 brate animals, with 

 the trilobites, the cu- 

 rious crustacean fos- 

 sils resembling the 

 modern king crabs, 

 and so named from 

 the two longitudinal 

 grooves which divide 

 the body into three 

 parallel lobes, occu- 

 pying the dominant 

 place. Hence this 

 period is known as 

 the age of trilobites. 



A KING CRAB 



A living ' ' fossil, ' ' related to the trilobite 

 on the one hand, and, according to Professor 

 Patten of Dartmouth, to the vertebrate, on 

 the other. Photo by Elwin R. Sanborn. 

 Courtesy of the New York ZoiJlog-ical Society. 



riod, which succeeded the Cambrian, 



The ' ' Ordovician " pe- 

 witnessed the rise of 



land plants and corals, the marvelous nautilids, with their 

 chambered shells, and the armored "fishes" or ostracoderms. 



