264 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



stones, which were compressed into close east-west folds near the 

 middle of the Pennsylvanian epoch. Between this epoch and the 

 Lower Cretaceous epoch they were greatly eroded and a peneplain 

 was formed which beveled their upturned edges. This peneplain 

 was submerged near Murfreesboro as well as in other areas during 

 the Lower Cretaceous epoch, and upon it several hundred feet of 

 gravel, sand, clay, and limestone, which comprise the Trinity forma- 

 tion, was laid down. Another period of erosion followed the Lower 

 Cretaceous epoch. In consequence of this the next younger strata, 

 the volcanic tuffs, gravels, sands, and clays of the Bingen formation, 

 of Upper Cretaceous age, rest unconformably upon the Trinity 

 formation. Both the Trinity and the Bingen have a dip of about 

 100 feet to the mile toward the south. Terrace and alluvial deposits 

 of Quaternary age which consist of gravels and silts 25 feet thick 

 or less occur along Little Missouri River and its larger tributaries. 

 Peridotite was intruded into the Carboniferous rocks and the 

 Trinity formation. The Carboniferous sandstones have been 

 changed to quartzites at places, and the clays of the Trinity forma- 

 tion have been vitrified for a few feet away from the peridotite. 



CHARACTER OF THE PERIDOTITE 



The peridotite in each of the four separate areas is similar. The 

 area of peridotite near the mouth of Prairie Creek, which so far 

 as known is larger than the others, presents much the best rock ex- 

 posures and has produced practically all of the diamonds found 

 up to the present time in Arkansas. Although this area comprises 

 about 73 acres, the exposures of the hard unweathered rock cover 

 not more than 12 acres, for the peridotite has weathered to a soft 

 earth at many places and to a fairly soft rock at others. (See 

 Pis. 1, 2, and 3.) The depth of the altered peridotite has not been 

 fully determined, though drill holes have shown that it is at some 

 places at least 205 feet. The surface soil that overlies it to a depth 

 of 1 to 4 feet is black from the presence of organic matter and is 

 known as " black ground." 



The peridotite is divisible into three rather distinct types. One 

 is a massive little- weathered rock that the miners call " hardebank," 

 which was intruded and which solidified in place. Another is vol- 

 canic breccia that was blown out of the craters by violent explosive 

 outbursts of volcanic activity. The third is a volcanic tuff of finer 

 grained material that was deposited in about the same way as the 

 breccia. 



The hard unweathered massive rock, the " hardebank," is a por- 

 phyry containing large crystals of olivine (now partly altered to ser- 

 pentine) and brown mica, inclosed in a finer grained groundmass 



