REPORT OF THE STATE GEOLOGIST. xlv 



Black Prairie and the Lower or Comanche. While the difference in the 

 rocks of the two series is very marked, both are especially character- 

 ized by the amount of limestone and chalky material contained in 

 them. These limy materials are found mixed with larger or smaller 

 quantities of sand or clay. 



Chalk is essentially a deposit formed in deep seas, and usually con- 

 tains remains of minute forms of animal life which existed in the 

 water. These deposits may retain their original character or be hard- 

 ened by various agencies into limestones. Limestones are also formed 

 of calcareous material worn away from the exposed land surface of 

 other limestones through atmospheric influences, and carried out by 

 various rivers until, on reaching less rapid water, they are gradually de- 

 posited, mixed with more or less sand or clay as they are nearer to or 

 farther from the shore ; at other times they owe their origin to the 

 great shell beds, which remain to tell us of the former inhabitants of 

 the seas. Thus we may judge from the character of the limestones the 

 way in which they were formed. 



These two series of Cretaceous rocks differ from those of the Ter- 

 tiary in the manner of their formation and in their composition. The 

 Tertiary strata were deposited somewhat after the manner of the beds 

 now making along our seacoast, and the present manner is in reality a 

 direct continuation of that older one, while the Cretaceous deposits, 

 with their alternations of sands to clays and limestones and back to 

 clays and sands again, as well as by the character of their fossils, show 

 us clearly that the lower series represents a series in which the sea 

 gradually grew deeper and deeper, and then again became shallower 

 and shallower, until an emergence took place and there was dry land 

 for ages. This was followed by a second period like the first, and the 

 sandy beds, which were the last deposit previous to its final emergence 

 before the beginning of the Tertiary period, became the shore line 

 against and upon which the waters of the shallow seas, bays, and 

 lagoons deposited the sediments which form the beds of that system. 



UPPER CRETACEOUS SERIES. 



The Upper, which is called the Black Prairie Series, from the Great 

 Black Waxy Prairie, which is the representative of its most prominent 

 member, is composed of the Glauconitic Beds (sands), the Ponderosa 

 Marls (clays), the Austin Chalk (chalk and limestone), the Eagle Ford 

 Clays, and the Lower Cross Timber Sands. 



