706 J. H. GARDNER THE MID-CONTINENT OIL FIELDS 



the latter portion of the time during which the Austin group was laid 

 down and extending into the period of Taylor marls deposition there was 

 considerable volcanic activity in this part of Texas. It resulted in the 

 limburgite and basalt intrusions of Pilot Knob and other districts. , At 

 this time there were flows of lava which were possibly in part pyroclastic, 

 and submarine, in nature; other igneous bodies are in part sill-like and 

 appear to have been intruded laterally into the soft, unconsolidated beds 

 of the newly deposited sediments. In the Thrall field the upper surface 

 of this igneous rock is arched so as to give the general effect of an anti- 

 cline or dome of small dimension. This condition has permitted water 

 to hive up or concentrate petroleum in the porous body of the serpentine, 

 thus presenting an occurrence of oil which is quite unusual in its-- nature. 

 While the oil and gas sands of the Cretaceous' series are practically 

 everywhere saturated with an abundance of water, this condition is not 

 so generally true of the sandstones in the Pennsylvanian series. A num- 

 ber of wells in search of petroleum in the latter strata have found the 

 salt-water saturation rather slight, and this fact, taken together with the 

 probability that the amount of the original carbonaceous matter was in 

 some territories somewhat meager, augurs badly for an abundance of 

 petroleum in the southern portion of the Pennsylvanian area in Texas. 

 Keconnaissance work by numerous petroleum geologists shows, also, tliat 

 typical oil-bearing structures are somewhat restricted in this region; the 

 sedimentary rocks are not so generally folded as those of the Mid-Conti- 

 Jient region in Kansas and Oklahoma. 



STRUCTURE 



Back of any general view of the local structural features in northern 

 Texas one must consider the broader phases of earth movements that have 

 warped the strata over an extended territory. The oil and gas fields lie 

 in a region between the Arbuckle- Wichita Mountains of southern Okla- 

 homa on the north and the Llano-Burnet uplift in central Texas on the 

 south. At several different periods of geological history strong forces 

 were brought to play on the surrounding sedimentary strata of each of 

 these areas, and it is more than likely that the periods of movement in 

 the two cases were coincident. The result of such stresses has been the 

 folding of th^ formations at different points, and where the other factors 

 of oil and gas accumulation occur in harmony with them, important pi'o- 

 ducing fields have been, and will continue to be, found. The oil field at 

 Electra, for instance, is one of the most dependable in the whole Mid- 

 (^ontinent region. The area of saturated oil sands is large, the field does 



