The Day dome, 1 mi west of Madisonville, Madison County, Texas, is in the southerly extension of the East Texas basin. Caprock (anhydrite) is present at an average depth of 2,600 ft and rock salt at a depth of about 3,100 ft. The dome was formed at the northern end of a NNW-SSE-trending, troughlike depression. This trough, about 12-13 mi long, is flanked on each side by a ridge, the Madisonville field being on the western one.

The sedimentary beds covering the salt core were uplifted slightly, but the roof later collapsed and now forms a local central depression on top of the salt dome. The sedimentary beds on the flanks of the dome show very little or no upturn (drag) against the salt core; this fact seems to be associated with the mode of salt emplacement.

The geologic setting suggests that the present trough formerly was occupied by a salt anticline which apparently furnished the salt for the formation of the Day dome. To explain the absence of drag in the flanking sedimentary beds and the presence of a relatively thick anhydrite caprock, it is concluded that the growth of the dome was mainly by downbuilding and by extrusive flow of salt, the top of the salt core remaining continuously near the depositional surface, at least until the time of Wilcox deposition. As evidenced by the domed roof and also by the presence of a truncation unconformity formed before deposition of the Sparta sands, some minor upward growth took place after Wilcox deposition, but was slowed and finally stopped by increased and more rapid sedimentary loading, sal solution, and diminishing salt supply. The growth history of the Day dome appears to have been controlled to a considerable degree by the strength of the surrounding sedimentary rocks. The stronger, more competent Mesozoic formations surrounding the dome permitted very little intrusive movement of salt, and their presence seems to explain the absence of drag.