136 CRETACEOUS ROCKS OF TEXAS. 



The aggregate thickness of the Comanche and Black Prairie series approxi- 

 mates nearly four thousand feet of strata, and they play a most important 

 part in the geologic history of the United States. It is not our purpose at 

 the present time, however, to enter into the philosophic aspects of these strata, 

 leaving for the future the interpretation of the effect these two profound sub- 

 sidences have had in the building up of our continent and its history, and the 

 interpretation of the ancient and beautiful forms of life imbedded in the 

 strata. It is sufficient to say that the day has come when it is not essential 

 to discuss together the Comanche series and the Upper Cretaceous series, so 

 different are they in geologic age, although they possess the following simi- 

 larities: (1) Each is composed of sediments laid down upon a slowly sub- 

 siding and rising sea bottom, thus recording all the different ocean depths, 

 from littoral or shore condition to deep sea. (2) The general strike and dip 

 of their rocks are in the same direction. (3) Bach is characterized more or 

 less throughout by an excess of lime — usually in the. form of chalky calcium 

 carbonate, pure, or mixed in every imaginable proportion with sand or clay. 



Notwithstanding these resemblances, the separate identity of the two 

 series is shown by (1) the absolute stratigraphic break between them, as can 

 be seen in numerous contacts in the city of Austin and elsewhere ; (2) the rad- 

 ical change in character of sediments, as seen along the partings of the Lower 

 Cross Timbers and the Comanche series; (3) the absolute change of life in 

 the two formations, not a single species, as far as known, passing from the 

 Lower series into the Upper, thus indicating a lapse of time between them 

 sufficiently long for an almost complete change of specific characters in the 

 ocean's inhabitants. They are as distinct from each other in origin and oc- 

 currence as they are from the rocks of the overlying Tertiary and Quaternary 

 systems, and hence it is necessary to describe them separately. 



These ancient ocean bottoms have undergone wonderful transitions. Origi- 

 nally the mud and ooze beneath hundreds of fathoms of ocean water, they 

 now form an elevated, healthy land. The noiseless life of their depths is re- 

 placed by a busy population of intelligence, who build, plow, and quarry, and 

 in a thousand ways utilize the rocks of the former sea bottom. Vegetation 

 finds in the decay of these reclaimed ocean beds a matrix for its rootlets, 

 which are nourished and sustained by the rich remains of the ancient life 

 imbedded in them. Mankind, with the ever-growing art of agriculture, im- 

 proves upon this natural vegetation a hundred fold, and civilization grows 

 where archaic silence once reigned supreme. If the transition from the past 

 to the present has been so great under our pioneer methods, what does the 

 future of science, with its experiments and constantly increasing knowledge, 

 portend? 



