THE LOWER OR COMANCHE SERIES. 131 



STRATIGRAPHY OF THE COMANCHE SERIES IN GENERAL. 



From the foregoing facts it is evident that the Comanche series possesses 

 a well defined lithologic and stratigraphic history. Its lower division is 

 essentially sandy, but becomes less and less so and more calcareous as the 

 bottom upon which it was laid down subsided. 



The lithology of the Comanche series is predominantly calcareous and is 

 marked by several essentially chalky horizons. There are also magnesian 

 and arenaceous beds, but these are modified in color and appearance by the 

 predominance of the accessory chalky matter. In color the tint is chalk 

 white, yellow, cream-colored, and occasionally the white rock weathers into a 

 dark grey, and not even in a single case are these rocks concretionary as 

 recently recorded, unless it is in a few feet of the Denison beds above 

 mentioned. 



Portions of the section are stratified into bands of one foot or more, but a 

 large majority of the strata are massive, while the whole series, except a few 

 alternating marls and layers of the Trinity, are remarkably free from lami- 

 nation. 



The alternating beds of the Basal subdivision of the Fredericksburg clearly 

 show a deeper sea condition of origin than the Trinity, but not as deep as the 

 chalk of the Comanche Peak and Caprina limestone subdivisions, which were 

 deposited in very deep and quiet waters. After the latter there is a hiatus in 

 our knowledge, but the Washita division reveals an elevation of the ocean's 

 bottom as slow and positive as is the subsidence recorded in the other basal 

 divisions. In brief, there is recorded (1) a long continued subsidence, dur- 

 ing which nearly one thousand feet of deepening uniform sediments were 

 laid down over vast areas; (2) a long continued deep sea condition, in which 

 four or five hundred feet or more of chalks were deposited; (3) an elevation 

 in which from three hundred to five hundred feet of shallowing sediments 

 were deposited (the Washita division). 



There can be but little doubt that the rocks now composing the Comanche 

 series were elevated into dry land, that the succeeding land epoch continued 

 probably as long as the time of deposition of either of the including series, 

 and that the rocks of the Upper series were largely derived from the under- 

 lying Comanche strata, and laid down during an entirely different and later 

 oceanic subsidence. 



It is also evident that the Comanche series thickens to the southwestward 

 and thins northward. Mr. Taffs measurements along the Colorado section 

 make it 1500 feet, but both the top and basal beds are locally curtailed there. 



Throughout its extent the rocks are jointed, and along their eastern margin 

 greatly broken and faulted parallel to their strike, while occasional areas of 

 igneous rocks protrude through the series southwest of Austin. 



