Handbook of Paleontology 195 



PRECAMBRIAN ERAS 



The basal rocks of the Paleozoic era are separated 

 from the rocks below by a pronounced unconformity. 

 The underlying rocks show a smooth, well-eroded sur- 

 face which sometimes approaches a perfect plane ; and, 

 since these rocks are highly folded and metamor- 

 phosed and the Paleozoic beds above do not show 

 such a degree of distortion and metamorphism, it is 

 evident that the older rocks suffered extreme meta- 

 morphism and then were subjected to a long period 

 of erosion before the deposition of the Paleozoic sedi- 

 ments. Typical sections show that the usual relation- 

 ship of the Paleozoic and pre-Paleozoic rocks is that 

 of a bed of quartz-sandstone resting upon the crystal- 

 lines. The age of this basal sandstone has been found 

 to be Lower, Middle or Upper Cambrian etc., even 

 Ordovician in some sections, indicating overlapping 

 formations deposited by a transgressing, but not con- 

 tinuously transgressing, sea. The basal quartz-sand- 

 stone followed by limestones rather than by clay shales 

 indicates that the surface of the land had been worn 

 down to the extent now seen and the products of dis- 

 integration had been well worked over and sorted by 

 wind and water before the advance of the Cambrian 

 sea over much of North America. 



Studies of the Precambrian rocks were first made 

 by Sir William Logan, the first director of the Cana- 

 dian Geological Survey, who is known because of this 

 as the Father of Precambrian Geology. An absence 

 of fossils makes the correlation of Precambrian rocks 

 very difficult, particularly where they are in distant 

 or isolated places. Even the lithological character 



