It is essential that there should be a definition of the stratigraphic arrangement and composition and structure of the rocks composing the two Cretaceous series of Texas, as far as this is known, and in the following pages an attempt is made to fulfill this need. It is believed that, with a few exceptions, the general sequence of the principal features have been solved, but concerning the subdivisions and local variations very little has been recorded. The following descriptions are based upon the writer's personal study and upon an accurately measured section of the Comanche series, made under his direction, by Mr. J. A. Taff, of the State Geological Survey, the whole of which will be published later. No attempt has as yet been made accurately measure the Upper series.