SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 89 



ores at Kiruna are of the nature of large dikes between igneous 

 rocks. The largest, that of Kurunavaara, is 5 kilometers long and 

 from 50 to 164 meters thick, and lies between a quartz porphyry 

 hanging wall and syenite porphyry foot. The ore has a dense, 

 steely appearance, and consists of magnetite with a little apatite. 

 They are probably igneous in origin because of the igneous charac- 

 ter of the wall rocks, the igneous texture of the ore, the presence of 

 apatite dikes in the ore and wall rock, and the existence of apophyses 

 of ore in the footwall and the surrounding porphyry. 



Sederholm^ beheves that certain pre-Cambrian graphitic schists 

 and limestones indicate the existence of progonozoique forms. 

 The progonozoique formations alluded to are the formations which 

 contain vestiges of the ancestral forms of the life which suddenly 

 appears in the Cambrian. 



SoUas^ ascribes the scarcity of fossils in the pre-Cambrian to a 

 feeble development of hard parts in the organisms of that period. 

 From the rough approximation that " the ontogeny of the individual 

 repeats the phylogeny of the race," he infers that the organisms of 

 the pre-Cambrian were similar to the larval forms of the existing 

 invertebrata. Such larval forms are not known to have been pre- 

 served, notwithstanding that some are endowed with calcareous 

 skeletons. 



Walcott^ accounts for the sudden appearance of abundant 

 marine life with the Cambrian by assuming that marine life first 

 developed in the open ocean and did not migrate to the shore zones 

 until the Algonkian upKft preceding the Cambrian had largely 

 excluded the seas from the continents. 



Walther^ ascribes the scarcity of fossils in certain pre-Cambrian 

 formations immediately below the Cambrian to their deposition 



^ J. J. Sederholm, "Sur les vestiges de la vie dans les formations progonozoique," 

 Compte Rendu, XI. Congres Geologique International, 1910, pp. 515-24. 



^ W. J. SoUas, "The Fauna of the Protaeon," Compte Rendu, XI. Congres Geolo- 

 gique International, 1910, Vol. I, pp. 499-501. 



3 C. D. Walcott, "Abrupt Appearance of the Cambrian Fauna on the North 

 American Continent," Smithsonian Misc. Coll., LVII, No. i (1910), pp. 1-16, i pi., 

 I fig. 



-•T. Walther, "Die lithologischen Eigenschaften der Gesteine im Liegenden der 

 kambrischen Formations," Compte Rendu, XI. Congres Geologique International, 

 1910, Vol. I, pp. 511-513- 



