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contemporaneously with it. So far no iron pyrites have been found in the rock to sustain the first of these theories, while the second is untenable from the fact that it occurs remote from faulting contacts. The third theory, on the other hand, is made plausible by the occurrence of minute specks, which in thin slices of the rock, as seen under the microscope, appear to be decomposing olivine or other igneous matter breaking down into serpentine and iron oxides. This point has not been finally determined, however.In places the Shoal Creek limestone is decomposing and crumbling, while everywhere it is much jointed and faulted. The fossils contained therein are interesting, but have been as yet but little studied. The top surface of this limestone has been corroded and waterworn, and deposited unconformably upon it can be seen the radically different sub-littoral unconsolidated clays of the basal Upper series.
THE DENISON BEDS.
This abrupt conclusion of the Lower Cretaceous, together with the Shoal Creek limestone at Austin, is local, and, as will be shown later, due to the peculiar igneous disturbances that prevailed in this vicinity. To the northward, where these disturbing conditions were not present, the final termination of the Comanche series is quite different, as seen at Denison, for instance, where the Washita limestone, as seen two and one-half miles north of the city, is succeeded by shallowing alternations of clays and impure limestones containing an abundant littoral fauna. The details of these beds have not been as yet accurately determined, but further field work will soon be undertaken in that region.
Dr. G. G. Shumard partially described these beds, as follows:
Marly Clay or Red River Group.
—Of these he says: "This member immediately underlies the fish bed of the arenaceous group (Lower Cross Timbers), and is described by Dr. G. G. Shumard as 'a blue marly clay, occasionally variegated with red and brown, and with thin bands of sandstone interstratified. The clay contains crystals of selenite, flattened nodules of compact brown and blue limestone, and septariæ of compact blue limestone, reticulated with brown, yellow, and purple spar. The nodules occur in the upper part and the septariæ towards the base of the formation. The best exposures of the group are in Grayson, on Post Oak, Choctaw, and Big Mineral creeks, where sections of from fifty to sixty feet have been measured. It occurs also on Red River, in Fannin and Lamar counties. The estimated thickness of the group in this part of the State is about one hundred and fifty feet; but we have not seen the base of the formation.'
B. F. Shumard: "Observations upon the Cretaceous Strata of Texas." Trans., Acad. Science of St. Louis, Vol. I 1856-60.