From the TSHA: Mineral Rights and Royalties

An interesting aspect of land rights. 

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Private title to all land in Texas emanates from a grant by the sovereign of the soil (successively, Spain, Mexico, the Republic of Texas, and the state of Texas). Under the laws of Spain and Mexico, mines and their metals or minerals did not pass by the ordinary grant of the land without express words of designation. In one of the earliest acts of the Congress of the Republic of Texas, this rule was adopted, and it was continued in force after Texas had become a state. A grantee of land before 1866 therefore had no interest in the minerals in the land unless that interest was expressly granted. By a provision of the state Constitution of 1866, which was carried over in substantially the same language into the constitutions of 1869 and 1876, the state released to the owner of the soil all mines and mineral substances therein. This constitutional provision had retrospective effect; the landowner was given complete ownership of the minerals in all lands that passed from the sovereign before the effective date of the Constitution of 1876.