266 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1923 



where there are other bauxite deposits; a third is at Magnet Cove, 

 Hot Spring County, from which beautiful mineral specimens have 

 found their way into most of the museums of the world; and a 

 fourth is at Potash Sulphur Springs, Garland County. In addition 

 to these areas, hundreds of dikes of igneous rock are found here and 

 there through much of the eastern half of the Ouachita Mountain 

 region. 



AGE OF THE PERIDOTITE 



The intrusion of the peridotite probably accompanied the dias- 

 trophic movements that produced the down warping of the Mis- 

 sissippi embayment early in Upper Cretaceous time. 



Although the rocks of the four different exposures mentioned are 

 not connected at the surface, the similarity of the material makes it 

 seem very probable that they are of common origin. The three types 

 of peridotite were apparently formed by three distinct volcanic out- 

 bursts, but they are so closely related that they probably mark suc- 

 cessive stages in a single period of volcanic activity. The evidence 

 so far obtained indicates that the igneous history of the region was 

 probably as follows: 



First, the massive peridotite, probably accompanied by explosions, 

 was intruded into the Carboniferous and Lower Cretaceous rocks. 

 Next, volcanic explosions broke into small fragments not only much 

 of the massive peridotite but some of the Carboniferous shale and 

 sandstone. In the area of peridotite near Prairie Creek the frag- 

 ments ejected into the air were deposited in inclined layers which, 

 in hardening, have formed a breccia. The layers dip 30° W. in the 

 Ozark mine (pi. 2, fig. 2), 20°-30° SW. at the apex of the Mauney 

 mine, and 50° or more toward the north near the south side of the 

 Arkansas mine. The fact that the layers thus dip toward the center 

 of the exposure of peridotite near Prairie Creek strongly suggests 

 that the vent or vents from which the fragments were ejected were 

 near the center of the Prairie Creek area and that the fragments 

 were deposited within the crater of a volcano. 



A second group of explosions probably formed the tuffs of the 

 area near Prairie Creek. 



The peridotite is younger than the Trinity formation, which is 

 of Lower Cretaceous age, as is shown by the high dip of the contacts 

 between the peridotite and the nearly horizontal beds of the Trinity, 

 by the metamorphism of the clay of the Trinity adjacent to its 

 contact with the peridotite, and by the occurrence in the peridotite 

 of fragments of clay and pebbles derived from the Trinity. 



The peridotite is probably of the same age as the Bingen forma- 

 tion (Upper Cretaceous), though it may be older. This is shown 

 by the fact that the lower beds of the Bingen contain at places 



