The writer has recently examined a large number of thin sections3 of “basement” rocks from wells in west Texas and southeast New Mexico in connection with a current Bureau of Economic Geology research project on the “basement” rocks of that area. In several wells the rocks studied proved to be alkalic igneous rocks,4 containing distinctive sodic minerals such as analcite, aegirine-augite, and riebeckite. These rocks are very similar in both texture and mineralogy to Tertiary alkalic igneous rocks in the Big Bend area of Texas that have been described by Lonsdale (1940), whose suite of specimens was at...

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