The purpose of this paper is to summarize the available data derived from surface exposures and drilling records as to the extent of early Paleozoic seas in Texas. The first Paleozoic sea to invade Texas, so far as known, was that of Upper Cambrian time. This sea spread widely across central Texas, occupying a seaway extending from Oklahoma southwestward to El Paso. The Lower Ordovician sea occupied a similar seaway and may have extended somewhat more widely to the northwest. The Middle Ordovician sea covered at least the southern part of the Permian basin and probably connected through the Llanoria geosyncline with the Ouachita region of Oklahoma. The Upper Ordovician sea occupied the Llanoria geosyncline and a large part of trans-Pecos Texas. A Silurian sea is believed o have extended into northeast Texas, lying in the Llanoria geosyncline. A Silurian sea likewise submerged the Diablo Plateau-El Paso region of trans-Pecos Texas. Evidence is lacking to prove the connection of the two seas across Texas. A Devonian sea is believed to have extended across Texas, occupying the Llanoria geosyncline.

Starting with an emergent condition at the beginning of Paleozoic time, the major changes of Paleozoic time in the Texas region are: submergence through Cambrian, with maximum submergence in Lower Ordovician; emergence through Middle and Upper Ordovician, with maximum emergence in Silurian; submergence in Devonian and Mississippian, with maximum submergence about Upper Pennsylvanian; emergence reaching maximum at close of Permian or in Triassic.

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