The source of the Pecos River is in the Pre-Cambrian rocks of the Sangre de Cristo Mountains, New Mexico. From the mountains the course of the river is to the south-southeast, where it flows over successively younger exposures of Pennsylvanian, Permian, and Triassic formations. The sediments in the tributaries of Rio Hondo and Rio Felix are of both igneous and sedimentary origin. The maximum size of the sands is in the 1/2 to 1/4 mm. range and sands in this range increase from 34 per cent of the total at Dilia to 64 per cent at Carlsbad. The heavy minerals transported by the river are magnetite, ilmenite, garnet, epidote, hematite, rutile, purple and transparent zircon, and green, rose, black, blue, and brown tourmaline.

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