Handbook of Paleontology 219 



synclines repeatedly flooded this area, and at times there 

 was a transgression of the sea from Arctic regions also. 

 It was apparently by way of this channel that communi- 

 cation was maintained with European seas. 



The physical geography of ancient periods, that is, the 

 relative distribution of land and water areas, has been 

 shown on a series of maps known as pale o geographic 

 maps (Green paleos, ancient). The determination of the 

 distribution of land areas in these ancient times is a diffi- 

 cult matter and therefore the outline of the lands, oceans 

 and seas are only approximately known. Paleogeo- 

 graphic maps are based upon the character of the deposits 

 taken into consideration with the distribution of the fossil 

 marine faunas and formations as shown on geologic maps. 



Cambrian and Ozarkian Periods 

 The name Cambrian was first proposed (1835) by the 

 Rev. Adam Sedgwick, Professor of Geology in Cam- 

 bridge University, England, for the oldest stratified rocks 

 of North Wales. The name was derived from the Roman 

 name for the region, the province of Cambria. Here the 

 Cambrian strata have an estimated thickness of 20,000 

 feet; but if translated into terms of the Cambrian as de- 

 fined in America the thickness would be much less than 

 this, and indeed it is doubtful whether even including the 

 Ozarkian the strata would be as thick as this. In 1911 

 the name Ozarkian was proposed by Ulrich (from the 

 Ozark uplift in Missouri) for a great series of forma- 

 tions previously regarded as passage beds between the 

 Cambrian and Ordovician. These formations will there- 

 fore be found variously classified by different authors. 

 By some they are accepted as the Ozarkian series and 



