748 GEOLOGY 



formations predominate in the Colorado series as a whole, l>ut there 

 are beds of chalk comparable to those of Europe, in Texas, Kan 

 Iowa, Nebraska, and South Dakota. Occasional beds of coal are 

 present, probably formed about the borders of the sea, or about the 

 islands which stood above it. Charred wood and even charcoal in 



Fig. 506. A group of concretions weathered out from the Diikota sand- 

 stone. Near Minneapolis, Kan. (Schaffner.) 



the series point to the existence of fires during the epoch. The 

 aggregate thickness of the series is locally as much as 3,000 feet, as 

 strata are measured, though its average thickness is niudi less. 

 The earlier formations of the Gulf region are probably not older 

 than the Colorado series of the western interior. 



The origin of the chalk. 1 There has been much difference of 

 opinion concerning the origin of chalk. Its resemblance to the 

 foraminiferal ooze of the deep seas long since led to the belief that 

 it was a deep-sea deposit; but closer examination has tin-own 

 doubt on this conclusion, for the differences between the chalk and 

 foraminiferal ooze are as striking as their likenesses. Both consist 



1 Wallace, Island Life. Also Chamberlin and Salisbury, Vol. Ill, p. 1 !'.. 



